[PATCH] lockfile: add PID file for debugging stale locks

[PATCH] lockfile: add PID file for debugging stale locks

[Paulo Casaretto via GitGitGadget]

From: Paulo Casaretto <pcasaretto@gmail.com>

When a lock file is held, it can be helpful to know which process owns
it, especially when debugging stale locks left behind by crashed
processes. Add an optional feature that creates a companion .lock.pid
file alongside each lock file, containing the PID of the lock holder.

The .lock.pid file is created when a lock is acquired (if enabled), and
automatically cleaned up when the lock is released (via commit or
rollback). The file is registered as a tempfile so it gets cleaned up
by signal and atexit handlers if the process terminates abnormally.

When a lock conflict occurs, the code checks if the PID from the .pid
file is still running using kill(pid, 0). This allows providing
context-aware error messages. With PID info enabled:

  Lock is held by process 12345. Wait for it to finish, or remove
  the lock file to continue.

Or for a stale lock:

  Lock was held by process 12345, which is no longer running.
  Remove the stale lock file to continue.

Without PID info (default):

  Another git process seems to be running in this repository.
  Wait for it to finish, or remove the lock file to continue.

The feature is opt-in via GIT_LOCK_PID_INFO=1 environment variable.

Signed-off-by: Paulo Casaretto <pcasaretto@gmail.com>
---
    lockfile: add holder info file for debugging stale locks

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2011%2Fpcasaretto%2Fpid-holder-file-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2011/pcasaretto/pid-holder-file-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2011

 Documentation/git.adoc  |  10 ++++
 lockfile.c              | 109 +++++++++++++++++++++++++++++++++++++---
 lockfile.h              |  21 +++++---
 t/meson.build           |   1 +
 t/t0031-lockfile-pid.sh |  73 +++++++++++++++++++++++++++
 5 files changed, 199 insertions(+), 15 deletions(-)
 create mode 100755 t/t0031-lockfile-pid.sh

diff --git a/Documentation/git.adoc b/Documentation/git.adoc
index ce099e78b8..6fdd509d34 100644
--- a/Documentation/git.adoc
+++ b/Documentation/git.adoc
@@ -1010,6 +1010,16 @@ be in the future).
 	the background which do not want to cause lock contention with
 	other operations on the repository.  Defaults to `1`.
 
+`GIT_LOCK_PID_INFO`::
+	If this Boolean environment variable is set to `1`, Git will create
+	a `.lock.pid` file alongside each lock file containing the PID of the
+	process that created the lock. This information is displayed in error
+	messages when a lock conflict occurs, making it easier to identify
+	stale locks or debug locking issues. The PID files are automatically
+	cleaned up via signal and atexit handlers; however, if a process is
+	terminated abnormally (e.g., SIGKILL), the file may remain as a stale
+	indicator. Disabled by default.
+
 `GIT_REDIRECT_STDIN`::
 `GIT_REDIRECT_STDOUT`::
 `GIT_REDIRECT_STDERR`::
diff --git a/lockfile.c b/lockfile.c
index 1d5ed01682..4a694b9c3d 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -6,6 +6,9 @@
 #include "abspath.h"
 #include "gettext.h"
 #include "lockfile.h"
+#include "parse.h"
+#include "strbuf.h"
+#include "wrapper.h"
 
 /*
  * path = absolute or relative path name
@@ -71,6 +74,62 @@ static void resolve_symlink(struct strbuf *path)
 	strbuf_reset(&link);
 }
 
+/*
+ * Lock PID file functions - write PID to a .lock.pid file alongside
+ * the lock file for debugging stale locks. The PID file is registered
+ * as a tempfile so it gets cleaned up by signal/atexit handlers.
+ */
+
+static int lock_pid_info_enabled(void)
+{
+	return git_env_bool(GIT_LOCK_PID_INFO_ENVIRONMENT, 0);
+}
+
+static struct tempfile *create_lock_pid_file(const char *lock_path, int mode)
+{
+	struct strbuf pid_path = STRBUF_INIT;
+	struct strbuf content = STRBUF_INIT;
+	struct tempfile *pid_tempfile = NULL;
+	int fd;
+
+	if (!lock_pid_info_enabled())
+		return NULL;
+
+	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
+	fd = open(pid_path.buf, O_WRONLY | O_CREAT | O_TRUNC, mode);
+	if (fd >= 0) {
+		strbuf_addf(&content, "%"PRIuMAX"\n", (uintmax_t)getpid());
+		if (write_in_full(fd, content.buf, content.len) < 0)
+			warning_errno(_("could not write lock pid file '%s'"),
+				      pid_path.buf);
+		close(fd);
+		pid_tempfile = register_tempfile(pid_path.buf);
+	}
+	strbuf_release(&content);
+	strbuf_release(&pid_path);
+	return pid_tempfile;
+}
+
+static int read_lock_pid(const char *lock_path, uintmax_t *pid_out)
+{
+	struct strbuf pid_path = STRBUF_INIT;
+	struct strbuf content = STRBUF_INIT;
+	int ret = -1;
+
+	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
+	if (strbuf_read_file(&content, pid_path.buf, LOCK_PID_MAXLEN) > 0) {
+		char *endptr;
+		*pid_out = strtoumax(content.buf, &endptr, 10);
+		if (*pid_out > 0 && (*endptr == '\n' || *endptr == '\0'))
+			ret = 0;
+		else
+			warning(_("malformed lock pid file '%s'"), pid_path.buf);
+	}
+	strbuf_release(&pid_path);
+	strbuf_release(&content);
+	return ret;
+}
+
 /* Make sure errno contains a meaningful value on error */
 static int lock_file(struct lock_file *lk, const char *path, int flags,
 		     int mode)
@@ -80,9 +139,12 @@ static int lock_file(struct lock_file *lk, const char *path, int flags,
 	strbuf_addstr(&filename, path);
 	if (!(flags & LOCK_NO_DEREF))
 		resolve_symlink(&filename);
-
 	strbuf_addstr(&filename, LOCK_SUFFIX);
+
 	lk->tempfile = create_tempfile_mode(filename.buf, mode);
+	if (lk->tempfile)
+		lk->pid_tempfile = create_lock_pid_file(filename.buf, mode);
+
 	strbuf_release(&filename);
 	return lk->tempfile ? lk->tempfile->fd : -1;
 }
@@ -151,13 +213,36 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
 {
 	if (err == EEXIST) {
-		strbuf_addf(buf, _("Unable to create '%s.lock': %s.\n\n"
-		    "Another git process seems to be running in this repository, e.g.\n"
-		    "an editor opened by 'git commit'. Please make sure all processes\n"
-		    "are terminated then try again. If it still fails, a git process\n"
-		    "may have crashed in this repository earlier:\n"
-		    "remove the file manually to continue."),
-			    absolute_path(path), strerror(err));
+		struct strbuf lock_path = STRBUF_INIT;
+		uintmax_t pid;
+		int pid_status = 0; /* 0 = unknown, 1 = running, -1 = stale */
+
+		strbuf_addf(&lock_path, "%s%s", absolute_path(path), LOCK_SUFFIX);
+
+		strbuf_addf(buf, _("Unable to create '%s': %s.\n\n"),
+			    lock_path.buf, strerror(err));
+
+		if (lock_pid_info_enabled() &&
+		    !read_lock_pid(lock_path.buf, &pid)) {
+			if (kill((pid_t)pid, 0) == 0)
+				pid_status = 1;
+			else
+				pid_status = -1;
+		}
+
+		if (pid_status == 1)
+			strbuf_addf(buf, _("Lock is held by process %"PRIuMAX". "
+			    "Wait for it to finish, or remove the lock file to continue"),
+			    pid);
+		else if (pid_status == -1)
+			strbuf_addf(buf, _("Lock was held by process %"PRIuMAX", "
+			    "which is no longer running. Remove the stale lock file to continue"),
+			    pid);
+		else
+			strbuf_addstr(buf, _("Another git process seems to be running in this repository. "
+			    "Wait for it to finish, or remove the lock file to continue"));
+
+		strbuf_release(&lock_path);
 	} else
 		strbuf_addf(buf, _("Unable to create '%s.lock': %s"),
 			    absolute_path(path), strerror(err));
@@ -207,6 +292,8 @@ int commit_lock_file(struct lock_file *lk)
 {
 	char *result_path = get_locked_file_path(lk);
 
+	delete_tempfile(&lk->pid_tempfile);
+
 	if (commit_lock_file_to(lk, result_path)) {
 		int save_errno = errno;
 		free(result_path);
@@ -216,3 +303,9 @@ int commit_lock_file(struct lock_file *lk)
 	free(result_path);
 	return 0;
 }
+
+int rollback_lock_file(struct lock_file *lk)
+{
+	delete_tempfile(&lk->pid_tempfile);
+	return delete_tempfile(&lk->tempfile);
+}
diff --git a/lockfile.h b/lockfile.h
index 1bb9926497..b2d5a1bc6c 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -119,6 +119,7 @@
 
 struct lock_file {
 	struct tempfile *tempfile;
+	struct tempfile *pid_tempfile;
 };
 
 #define LOCK_INIT { 0 }
@@ -127,6 +128,15 @@ struct lock_file {
 #define LOCK_SUFFIX ".lock"
 #define LOCK_SUFFIX_LEN 5
 
+/* Suffix for PID file that stores PID of lock holder: */
+#define LOCK_PID_SUFFIX ".pid"
+#define LOCK_PID_SUFFIX_LEN 4
+
+/* Maximum length for PID file content */
+#define LOCK_PID_MAXLEN 32
+
+/* Environment variable to enable lock PID info (default: disabled) */
+#define GIT_LOCK_PID_INFO_ENVIRONMENT "GIT_LOCK_PID_INFO"
 
 /*
  * Flags
@@ -319,13 +329,10 @@ static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
 
 /*
  * Roll back `lk`: close the file descriptor and/or file pointer and
- * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
- * for a `lock_file` object that has already been committed or rolled
- * back. No error will be returned in this case.
+ * remove the lockfile and any associated PID file. It is a NOOP to
+ * call `rollback_lock_file()` for a `lock_file` object that has already
+ * been committed or rolled back. No error will be returned in this case.
  */
-static inline int rollback_lock_file(struct lock_file *lk)
-{
-	return delete_tempfile(&lk->tempfile);
-}
+int rollback_lock_file(struct lock_file *lk);
 
 #endif /* LOCKFILE_H */
diff --git a/t/meson.build b/t/meson.build
index dc43d69636..9a880db57c 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -98,6 +98,7 @@ integration_tests = [
   't0028-working-tree-encoding.sh',
   't0029-core-unsetenvvars.sh',
   't0030-stripspace.sh',
+  't0031-lockfile-pid.sh',
   't0033-safe-directory.sh',
   't0034-root-safe-directory.sh',
   't0035-safe-bare-repository.sh',
diff --git a/t/t0031-lockfile-pid.sh b/t/t0031-lockfile-pid.sh
new file mode 100755
index 0000000000..3c1e9bf60e
--- /dev/null
+++ b/t/t0031-lockfile-pid.sh
@@ -0,0 +1,73 @@
+#!/bin/sh
+
+test_description='lock file PID info tests
+
+Tests for PID info file alongside lock files.
+The feature is opt-in via GIT_LOCK_PID_INFO=1.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'stale lock detected when PID is not running' '
+	git init repo &&
+	(
+		cd repo &&
+		touch .git/index.lock &&
+		echo "99999" >.git/index.lock.pid &&
+		test_must_fail env GIT_LOCK_PID_INFO=1 git add . 2>err &&
+		test_grep "process 99999, which is no longer running" err &&
+		test_grep "Remove the stale lock file" err
+	)
+'
+
+test_expect_success 'PID info not shown by default' '
+	git init repo2 &&
+	(
+		cd repo2 &&
+		touch .git/index.lock &&
+		echo "99999" >.git/index.lock.pid &&
+		test_must_fail git add . 2>err &&
+		# Should not crash, just show normal error without PID
+		test_grep "Unable to create" err &&
+		! test_grep "is held by process" err
+	)
+'
+
+test_expect_success 'running process detected when PID is alive' '
+	git init repo3 &&
+	(
+		cd repo3 &&
+		echo content >file &&
+		# Create a lock and PID file with current shell PID (which is running)
+		touch .git/index.lock &&
+		echo $$ >.git/index.lock.pid &&
+		# Verify our PID is shown in the error message
+		test_must_fail env GIT_LOCK_PID_INFO=1 git add file 2>err &&
+		test_grep "held by process $$" err
+	)
+'
+
+test_expect_success 'PID info file cleaned up on successful operation when enabled' '
+	git init repo4 &&
+	(
+		cd repo4 &&
+		echo content >file &&
+		env GIT_LOCK_PID_INFO=1 git add file &&
+		# After successful add, no lock or PID files should exist
+		! test -f .git/index.lock &&
+		! test -f .git/index.lock.pid
+	)
+'
+
+test_expect_success 'no PID file created by default' '
+	git init repo5 &&
+	(
+		cd repo5 &&
+		echo content >file &&
+		git add file &&
+		# PID file should not be created when feature is disabled
+		! test -f .git/index.lock.pid
+	)
+'
+
+test_done

base-commit: 6ab38b7e9cc7adafc304f3204616a4debd49c6e9
-- 
gitgitgadget

Re: [PATCH] lockfile: add PID file for debugging stale locks

[D. Ben Knoble]

On Tue, Dec 2, 2025 at 10:07 AM Paulo Casaretto via GitGitGadget
<gitgitgadget@gmail.com> wrote:
>
> From: Paulo Casaretto <pcasaretto@gmail.com>
>
> When a lock file is held, it can be helpful to know which process owns
> it, especially when debugging stale locks left behind by crashed
> processes. Add an optional feature that creates a companion .lock.pid
> file alongside each lock file, containing the PID of the lock holder.
>
> The .lock.pid file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.
>
> When a lock conflict occurs, the code checks if the PID from the .pid
> file is still running using kill(pid, 0). This allows providing
> context-aware error messages. With PID info enabled:
>
>   Lock is held by process 12345. Wait for it to finish, or remove
>   the lock file to continue.
>
> Or for a stale lock:
>
>   Lock was held by process 12345, which is no longer running.
>   Remove the stale lock file to continue.
>
> Without PID info (default):
>
>   Another git process seems to be running in this repository.
>   Wait for it to finish, or remove the lock file to continue.
>
> The feature is opt-in via GIT_LOCK_PID_INFO=1 environment variable.
>
> Signed-off-by: Paulo Casaretto <pcasaretto@gmail.com>

Sounds interesting. I think by the time I wish I knew what else was
using the lockfile, it's too late for me to alter my environment.
Perhaps (in addition to allowing the environment opt-in) we could
opt-in via configuration? Or is this really only useful, say, on the
server side where the environment is carefully controlled? I don't
relish putting this variable into my environment to take advantage of
something that looks very useful.

Are there downsides that make it necessary to be opt-in? I also
imagine this could be a useful default; occasionally folks at work hit
something similar and ask "what's up with that?"

Only other thing is: just because a process X is running doesn't mean
it was the one holding the lock, right? Since PIDs can be reused.

-- 
D. Ben Knoble

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Torsten Bögershausen]

On Tue, Dec 02, 2025 at 03:07:27PM +0000, Paulo Casaretto via GitGitGadget wrote:
> From: Paulo Casaretto <pcasaretto@gmail.com>
> 
> When a lock file is held, it can be helpful to know which process owns
> it, especially when debugging stale locks left behind by crashed
> processes. Add an optional feature that creates a companion .lock.pid
> file alongside each lock file, containing the PID of the lock holder.
> 
> The .lock.pid file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.
> 
> When a lock conflict occurs, the code checks if the PID from the .pid
> file is still running using kill(pid, 0). This allows providing
> context-aware error messages. With PID info enabled:
> 
>   Lock is held by process 12345. Wait for it to finish, or remove
>   the lock file to continue.
> 
> Or for a stale lock:
> 
>   Lock was held by process 12345, which is no longer running.
>   Remove the stale lock file to continue.
> 
> Without PID info (default):
> 
>   Another git process seems to be running in this repository.
>   Wait for it to finish, or remove the lock file to continue.
> 
> The feature is opt-in via GIT_LOCK_PID_INFO=1 environment variable.
[]

I think that this makes sense.
However, as a frequent user of Git repos hosted on an NFS server
(without any problems in my setup):

Does it make sense to add the hostname here ?
We already have xgethostname() in Git, so that we can diagnose
who/which machine really left a lock.

[]

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Jeff King]

On Tue, Dec 02, 2025 at 03:07:27PM +0000, Paulo Casaretto via GitGitGadget wrote:

> The .lock.pid file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.

I'm sympathetic to the goal of this series, and the implementation looks
cleanly done. But I wonder if there might be some system-level side
effects that make these .pid files awkward.

Temporarily having an extra .git/index.lock.pid file is probably not a
big deal. But for other namespaces, like refs, we're colliding with
names that have other meanings. So if we want to update refs/heads/foo,
for example, we'll create refs/heads/foo.lock now. And after your patch,
also refs/heads/foo.lock.pid.

The ".lock" suffix is special, in that we disallow it in a refname and
know to skip it when iterating over loose refs. But for the ".pid"
variant, we run the risk of colliding with a real branch named
"foo.lock.pid", both for reading and writing.

On the writing side, creating foo.lock.pid may accidentally overwrite an
existing branch. This can be mitigated by using O_EXCL when creating the
pid file.

But we can see the writes in the opposite order, which I think can also
lead to data loss. Something like:

  - process A wants to write branch "foo", so it holds
    refs/heads/foo.lock and now also the matching foo.lock.pid

  - process B wants to write branch "foo.lock.pid", so it holds
    refs/heads/foo.lock.pid.lock (and the matching pid)

  - process B finishes its write and atomically renames
    foo.lock.pid.lock into foo.lock.pid. It's expected to overwrite
    the existing file there, so now process A's pid file is gone.

  - process A finishes its write and tries to clean up its pid file.  So
    it deletes foo.lock.pid, which contains the actual data from process
    B's write.

And now process B's write has been lost (and maybe even the entire
existence of the foo.lock.pid branch, if it was not also present in the
packed-refs file).

On the reading side, anybody iterating refs/heads/ may racily see the
foo.lock.pid file and think it is supposed to be an actual ref. So they
open it, find it contains garbage, and then flag it as an error.

I think both could be mitigated if we disallowed ".lock.pid" as a suffix
in refnames, but that is a big user-facing change.

In the long run, alternate ref stores like reftable won't suffer from
this issue. It's possible there are other spots where we use lockfiles
alongside arbitrarily-named files, though I couldn't think of any.

So I dunno what that means for your patch. I notice that the user has to
enable the feature manually. But it feels more like it should be
selective based on which subsystem is using the lockfile (so refs would
never want it, but other lockfiles/tempfiles might).

-Peff

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Junio C Hamano]

Jeff King <peff@peff.net> writes:

> So I dunno what that means for your patch. I notice that the user has to
> enable the feature manually. But it feels more like it should be
> selective based on which subsystem is using the lockfile (so refs would
> never want it, but other lockfiles/tempfiles might).

Or perhaps the way to opt into the feature is to create an empty
file $GIT_DIR/lockfile-audit, and the lockfile subsystem will append
to it every time a lock is taken?  We need to ensure that a PID and
pathname formatted into a single record is small enough and O_APPEND
would relieve us from worrying about multi writer races, which may
introduce different kind of complications, though.

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Jeff King]

On Wed, Dec 03, 2025 at 02:21:11PM -0800, Junio C Hamano wrote:

> Jeff King <peff@peff.net> writes:
> 
> > So I dunno what that means for your patch. I notice that the user has to
> > enable the feature manually. But it feels more like it should be
> > selective based on which subsystem is using the lockfile (so refs would
> > never want it, but other lockfiles/tempfiles might).
> 
> Or perhaps the way to opt into the feature is to create an empty
> file $GIT_DIR/lockfile-audit, and the lockfile subsystem will append
> to it every time a lock is taken?  We need to ensure that a PID and
> pathname formatted into a single record is small enough and O_APPEND
> would relieve us from worrying about multi writer races, which may
> introduce different kind of complications, though.

I like a single log much better from a management perspective. I agree
that atomicity is a potential issue, though. I think that even if we
kept it small, network filesystems like NFS do not provide great
guarantees for atomic appends. Something like flock() can work there,
but that's not something we've relied on before.

It also raises questions about reading (do we find pid files in the log
in order to provide more directed advice?) and maintenance (do we ever
clean it up, or just let it grow without bound?).

-Peff

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Taylor Blau]

On Wed, Dec 03, 2025 at 04:16:10PM -0500, Jeff King wrote:
> On Tue, Dec 02, 2025 at 03:07:27PM +0000, Paulo Casaretto via GitGitGadget wrote:
>
> > The .lock.pid file is created when a lock is acquired (if enabled), and
> > automatically cleaned up when the lock is released (via commit or
> > rollback). The file is registered as a tempfile so it gets cleaned up
> > by signal and atexit handlers if the process terminates abnormally.
>
> I'm sympathetic to the goal of this series, and the implementation looks
> cleanly done. But I wonder if there might be some system-level side
> effects that make these .pid files awkward.
>
> Temporarily having an extra .git/index.lock.pid file is probably not a
> big deal. But for other namespaces, like refs, we're colliding with
> names that have other meanings. So if we want to update refs/heads/foo,
> for example, we'll create refs/heads/foo.lock now. And after your patch,
> also refs/heads/foo.lock.pid.
>
> The ".lock" suffix is special, in that we disallow it in a refname and
> know to skip it when iterating over loose refs. But for the ".pid"
> variant, we run the risk of colliding with a real branch named
> "foo.lock.pid", both for reading and writing.

Good point. I don't have a strong opinion on whether or not we should
use an append-only log of which PIDs grabbed which lockfiles when versus
tracking them on a per-lock basis. But I wonder if this would be
mitigated by either:

 - Keeping the ".lock" suffix as-is, so that holding a lockfile at path
   "$GIT_DIR/index.lock" would create "$GIT_DIR/index-pid.lock" or
   something similar.

 - Introducing a new reference name constraint that treats ".lock.pid"
   as a reserved in a manner identical to how we currently treat
   ".lock".

Between the two, I vastly prefer the former, but see below for more on
why.

> But we can see the writes in the opposite order, which I think can also
> lead to data loss. Something like:
>
>   - process A wants to write branch "foo", so it holds
>     refs/heads/foo.lock and now also the matching foo.lock.pid
>
>   - process B wants to write branch "foo.lock.pid", so it holds
>     refs/heads/foo.lock.pid.lock (and the matching pid)

Changing the naming scheme as above would cause us to hold
"foo.pid.lock" in addition to "foo.lock". That would allow process B
here to write branch "foo.lock.pid" (as is the case today). But if the
scenario were instead "process B wants to write branch foo.pid.lock", it
would fail immediately since the ".lock" suffix is reserved.

> I think both could be mitigated if we disallowed ".lock.pid" as a suffix
> in refnames, but that is a big user-facing change.

Yeah, I don't think that we should change the refname constraints here,
especially in a world where reftable deployments are more common. In
that world I think we should err on the side of removing constraints,
not adding them ;-).

> So I dunno what that means for your patch. I notice that the user has to
> enable the feature manually. But it feels more like it should be
> selective based on which subsystem is using the lockfile (so refs would
> never want it, but other lockfiles/tempfiles might).

Yeah, I think that something similar to the "which files do we fsync()
and how?" configuration we have today would be a nice complement here.

(As an aside, I wonder if that interface, too, could be slightly
improved. Right now we have a comma-separated list of values in the
"core.fsync" configuration for listing different "components", and then
a global core.fsyncMethod to either issue a fsync(), or a pagecache
writeback, or writeout-only flushes in batches. It might be nice to have
something like:

  [fsync "loose-object"]
    method = fsync
  [fsync "packfile"]
    method = writeout

, so the analog here would be something like:

  [lockfile "refs"]
    pidfile = false
  [lockfile "index"]
    pidfile = true

or similar. That could also be represented as core.lockfile=index,
omitting "refs" to avoid tracking it. It may be that people don't really
care to ever use different fsync methods for different fsync-able
components, so perhaps the analogy doesn't hold up perfectly.)

Thanks,
Taylor

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Patrick Steinhardt]

On Wed, Dec 03, 2025 at 06:19:46PM -0500, Taylor Blau wrote:
> On Wed, Dec 03, 2025 at 04:16:10PM -0500, Jeff King wrote:
> > On Tue, Dec 02, 2025 at 03:07:27PM +0000, Paulo Casaretto via GitGitGadget wrote:
> >
> > > The .lock.pid file is created when a lock is acquired (if enabled), and
> > > automatically cleaned up when the lock is released (via commit or
> > > rollback). The file is registered as a tempfile so it gets cleaned up
> > > by signal and atexit handlers if the process terminates abnormally.
> >
> > I'm sympathetic to the goal of this series, and the implementation looks
> > cleanly done. But I wonder if there might be some system-level side
> > effects that make these .pid files awkward.
> >
> > Temporarily having an extra .git/index.lock.pid file is probably not a
> > big deal. But for other namespaces, like refs, we're colliding with
> > names that have other meanings. So if we want to update refs/heads/foo,
> > for example, we'll create refs/heads/foo.lock now. And after your patch,
> > also refs/heads/foo.lock.pid.
> >
> > The ".lock" suffix is special, in that we disallow it in a refname and
> > know to skip it when iterating over loose refs. But for the ".pid"
> > variant, we run the risk of colliding with a real branch named
> > "foo.lock.pid", both for reading and writing.
> 
> Good point. I don't have a strong opinion on whether or not we should
> use an append-only log of which PIDs grabbed which lockfiles when versus
> tracking them on a per-lock basis. But I wonder if this would be
> mitigated by either:
> 
>  - Keeping the ".lock" suffix as-is, so that holding a lockfile at path
>    "$GIT_DIR/index.lock" would create "$GIT_DIR/index-pid.lock" or
>    something similar.
> 
>  - Introducing a new reference name constraint that treats ".lock.pid"
>    as a reserved in a manner identical to how we currently treat
>    ".lock".
> 
> Between the two, I vastly prefer the former, but see below for more on
> why.

Yeah, I agree that this is not really a feasible change for the "files"
reference backend. Besides all the issues mentioned in this thread, we
also have to consider alternative implementations of Git and old
versions. Those wouldn't know that the ".lock.pid" files are special, so
they will misbehave if Git started to write them now.

We could of course work around that issue by introducing a new
repository extension. But I doubt that this is something we want to
pursue in this context. The provided benefit just isn't high enough.

The overall idea still has merit though. So if we still want to pursue
it we can likely work around the issue by introducing a toggle that
allows specific callers to opt out of creating the PID files.

That'd raise the question though when we are most likely to need the PID
files for debugging stuff. From my own experience I only ever had issues
with stale locks in the ref subsystem, so if we disable the mechanism in
exactly that subsystem it may be way less useful.

If references are the main culprit one could also think about a slightly
ugly alternative approach: loose refs handle it just fine if they
contain additional lines. So in theory, we could write a second line for
each lock file that contains the PID. We do have an fsck check that
warns about this, but in theory this should just work. Whether we want
to go there is a different question though.

Patrick

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Jeff King]

On Wed, Dec 03, 2025 at 06:19:46PM -0500, Taylor Blau wrote:

> Changing the naming scheme as above would cause us to hold
> "foo.pid.lock" in addition to "foo.lock". That would allow process B
> here to write branch "foo.lock.pid" (as is the case today). But if the
> scenario were instead "process B wants to write branch foo.pid.lock", it
> would fail immediately since the ".lock" suffix is reserved.

I agree that this gets rid of any corruption or race issues, since all
versions understand how to handle ".lock" specially (and assuming we
create foo.pid.lock with O_EXCL). But there is still a namespace
collision; I cannot write refs "foo" and "foo.pid" at the same time.

> > So I dunno what that means for your patch. I notice that the user has to
> > enable the feature manually. But it feels more like it should be
> > selective based on which subsystem is using the lockfile (so refs would
> > never want it, but other lockfiles/tempfiles might).
> 
> Yeah, I think that something similar to the "which files do we fsync()
> and how?" configuration we have today would be a nice complement here.

I'm not sure it makes sense for the user to configure this. I more meant
that the ref files-backend code would set a flag for "no, do not create
a pid file for me ever" (or inversely, other bits of the code would
add a flag for "yes, it's OK to do so").

-Peff

Re: [PATCH] lockfile: add PID file for debugging stale locks

[Taylor Blau]

Hi Paulo,

On Tue, Dec 02, 2025 at 03:07:27PM +0000, Paulo Casaretto via GitGitGadget wrote:
> The .lock.pid file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.

(For the purposes of this review, I'll ignore the naming conventions
that are discussed elsewhere in the thread, which I think can be
resolved separately of any technical concerns.)

> diff --git a/Documentation/git.adoc b/Documentation/git.adoc
> index ce099e78b8..6fdd509d34 100644
> --- a/Documentation/git.adoc
> +++ b/Documentation/git.adoc
> @@ -1010,6 +1010,16 @@ be in the future).
>  	the background which do not want to cause lock contention with
>  	other operations on the repository.  Defaults to `1`.
>
> +`GIT_LOCK_PID_INFO`::
> +	If this Boolean environment variable is set to `1`, Git will create
> +	a `.lock.pid` file alongside each lock file containing the PID of the
> +	process that created the lock. This information is displayed in error
> +	messages when a lock conflict occurs, making it easier to identify
> +	stale locks or debug locking issues. The PID files are automatically
> +	cleaned up via signal and atexit handlers; however, if a process is
> +	terminated abnormally (e.g., SIGKILL), the file may remain as a stale
> +	indicator. Disabled by default.

Regardless of whether or not we expose this functionality behind an
environment variable or configuration, I think it would be nice to be
able to turn PID tracking on and off for different components (e.g., for
scenarios where you care about who is holding open, say, $GIT_DIR/index,
but not who is creating a lock during ref creation).

If we determined this through an environment variable, I think it would
be reasonable to adopt the convention from the "core.fsync"
configuration and use a comma-separated list. Alternatively, we could
adopt that same convention for a configuration variable, say,
"core.lockfile".

But I think that having this exposed as a per-component setting via
configuration is preferable than a global switch, since callers don't
have to remember to set this variable in their environment to get the
desired effect. Callers that want to opt-in or out of this feature on a
one-off basis can still override the configuration via the top-level
"-c" flag.

>  `GIT_REDIRECT_STDIN`::
>  `GIT_REDIRECT_STDOUT`::
>  `GIT_REDIRECT_STDERR`::
> diff --git a/lockfile.c b/lockfile.c
> index 1d5ed01682..4a694b9c3d 100644
> --- a/lockfile.c
> +++ b/lockfile.c
> @@ -6,6 +6,9 @@
>  #include "abspath.h"
>  #include "gettext.h"
>  #include "lockfile.h"
> +#include "parse.h"
> +#include "strbuf.h"
> +#include "wrapper.h"
>
>  /*
>   * path = absolute or relative path name
> @@ -71,6 +74,62 @@ static void resolve_symlink(struct strbuf *path)
>  	strbuf_reset(&link);
>  }
>
> +/*
> + * Lock PID file functions - write PID to a .lock.pid file alongside
> + * the lock file for debugging stale locks. The PID file is registered
> + * as a tempfile so it gets cleaned up by signal/atexit handlers.
> + */
> +
> +static int lock_pid_info_enabled(void)
> +{

With the above suggestion, I think this function would get a little more
complicated to instead take a component argument. That's not a huge deal
in and of itself, but callers that *create* a lockfile will have to
somehow pass in the component name when acquiring the lock.

> +	return git_env_bool(GIT_LOCK_PID_INFO_ENVIRONMENT, 0);
> +}
> +
> +static struct tempfile *create_lock_pid_file(const char *lock_path, int mode)
> +{
> +	struct strbuf pid_path = STRBUF_INIT;
> +	struct strbuf content = STRBUF_INIT;
> +	struct tempfile *pid_tempfile = NULL;
> +	int fd;
> +
> +	if (!lock_pid_info_enabled())
> +		return NULL;
> +
> +	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
> +	fd = open(pid_path.buf, O_WRONLY | O_CREAT | O_TRUNC, mode);

I'm not sure whether or not we should pass O_EXCL here, but I think it
depends on the naming convention we pick.

> +	if (fd >= 0) {

This is a style preference, but I'd suggest handling the happy path
outside of this conditional if possible by inverting it. Perhaps
something like:

    if (fd < 0)
        goto out;

    strbuf_addf(&content, ...);
    if (write_in_full(fd, content.buf, content.len) < 0)
        warning_errno(...);

    close(fd);
    pid_tempfile = register_tempfile(pid_path.buf);

    out:
        strbuf_release(...);
        return pid_tempfile;

> +static int read_lock_pid(const char *lock_path, uintmax_t *pid_out)
> +{
> +	struct strbuf pid_path = STRBUF_INIT;
> +	struct strbuf content = STRBUF_INIT;
> +	int ret = -1;
> +
> +	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
> +	if (strbuf_read_file(&content, pid_path.buf, LOCK_PID_MAXLEN) > 0) {
> +		char *endptr;
> +		*pid_out = strtoumax(content.buf, &endptr, 10);
> +		if (*pid_out > 0 && (*endptr == '\n' || *endptr == '\0'))
> +			ret = 0;

Same note here. I'd suggest setting "ret = 0" during initialization, and
inverting this conditional to:

    if (*pid_out <= 0 || (*endptr != '\n' && *endptr != '\0')) {
        warning(...);
        ret = -1;
        goto out;
    }
> +		else
> +			warning(_("malformed lock pid file '%s'"), pid_path.buf);
> +	}
> +	strbuf_release(&pid_path);
> +	strbuf_release(&content);
> +	return ret;
> +}
> +
>  /* Make sure errno contains a meaningful value on error */
>  static int lock_file(struct lock_file *lk, const char *path, int flags,
>  		     int mode)
> @@ -80,9 +139,12 @@ static int lock_file(struct lock_file *lk, const char *path, int flags,
>  	strbuf_addstr(&filename, path);
>  	if (!(flags & LOCK_NO_DEREF))
>  		resolve_symlink(&filename);
> -
>  	strbuf_addstr(&filename, LOCK_SUFFIX);
> +

These look like stray whitespace changes that were left behind from
development.

> +		if (pid_status == 1)
> +			strbuf_addf(buf, _("Lock is held by process %"PRIuMAX". "
> +			    "Wait for it to finish, or remove the lock file to continue"),
> +			    pid);
> +		else if (pid_status == -1)
> +			strbuf_addf(buf, _("Lock was held by process %"PRIuMAX", "
> +			    "which is no longer running. Remove the stale lock file to continue"),
> +			    pid);
> +		else
> +			strbuf_addstr(buf, _("Another git process seems to be running in this repository. "
> +			    "Wait for it to finish, or remove the lock file to continue"));

On one hand I prefer the new "Another git process" message for when we
don't have a PID lockfile. But on the other hand, I think the "If it
still fails, a git process may have crashed..." message is useful for
users who may not be immediately aware of the consequences of simply
removing the lockfile to continue.

I do think the original message is somewhat verbose, so maybe the change
here in the non-PID case is worth doing. What are your thoughts?

> +
> +		strbuf_release(&lock_path);
>  	} else
>  		strbuf_addf(buf, _("Unable to create '%s.lock': %s"),
>  			    absolute_path(path), strerror(err));
> @@ -207,6 +292,8 @@ int commit_lock_file(struct lock_file *lk)
>  {
>  	char *result_path = get_locked_file_path(lk);
>
> +	delete_tempfile(&lk->pid_tempfile);
> +

Do we want to wait to delete the PID file until after we know that we
successfully committed the lockfile?

> +/* Maximum length for PID file content */
> +#define LOCK_PID_MAXLEN 32

Makes sense ;-). This should be plenty of space, since on my system
maximum PID value is 2^22:

    $ cat /proc/sys/kernel/pid_max
    4194304

> +test_expect_success 'PID info file cleaned up on successful operation when enabled' '
> +	git init repo4 &&
> +	(
> +		cd repo4 &&
> +		echo content >file &&
> +		env GIT_LOCK_PID_INFO=1 git add file &&
> +		# After successful add, no lock or PID files should exist
> +		! test -f .git/index.lock &&
> +		! test -f .git/index.lock.pid

These should be:

    test_path_is_missing .git/index.lock &&
    test_path_is_missing .git/index.lock.pid

instead of bare "! test -f"'s.

Thanks,
Taylor

[PATCH v2] lockfile: add PID file for debugging stale locks

[Paulo Casaretto via GitGitGadget]

From: Paulo Casaretto <pcasaretto@gmail.com>

When a lock file is held, it can be helpful to know which process owns
it, especially when debugging stale locks left behind by crashed
processes. Add an optional feature that creates a companion PID file
alongside each lock file, containing the PID of the lock holder.

For a lock file "foo.lock", the PID file is named "foo.pid.lock". The
file uses a simple key-value format ("pid <value>") following the same
pattern as Git object headers, making it extensible for future metadata.

The PID file is created when a lock is acquired (if enabled), and
automatically cleaned up when the lock is released (via commit or
rollback). The file is registered as a tempfile so it gets cleaned up
by signal and atexit handlers if the process terminates abnormally.

When a lock conflict occurs, the code checks for an existing PID file
and, if found, uses kill(pid, 0) to determine if the process is still
running. This allows providing context-aware error messages:

  Lock is held by process 12345. Wait for it to finish, or remove
  the lock file to continue.

Or for a stale lock:

  Lock was held by process 12345, which is no longer running.
  Remove the stale lock file to continue.

The feature is controlled via core.lockfilePid configuration, which
accepts per-component values similar to core.fsync:

  - none: Disable for all components (default)
  - all: Enable for all components
  - index, config, refs, commit-graph, midx, shallow, gc, other:
    Enable for specific components

Multiple components can be combined with commas:

  git config core.lockfilePid index,config

Each lock file call site specifies which component it belongs to,
allowing users fine-grained control over which locks create PID files.

Existing PID files are always read when displaying lock errors,
regardless of the core.lockfilePid setting. This ensures helpful
diagnostics even when the feature was previously enabled and later
disabled.

Signed-off-by: Paulo Casaretto <pcasaretto@gmail.com>
---
    lockfile: add holder info file for debugging stale locks
    
    Changes in v2
    
     * Changed file naming: Using foo.pid.lock to keep the .lock suffix
       (which is rejected by ref validation), while clearly indicating the
       file's purpose
    
     * Extensible file format: Using pid <value> key-value format (same
       pattern as Git object headers) instead of raw PID, allowing future
       fields (hostname?)
    
     * Replaced environment variable with config: Now uses core.lockfilePid
       configuration instead of GIT_LOCK_PID_INFO environment variable
    
     * Added per-component configuration: Similar to core.fsync, users can
       enable PID files for specific components:
       
       * none (default), all
       * index, config, refs, commit-graph, midx, shallow, gc, other
    
     * Caller-specified components: Each call site specifies which component
       it belongs to, giving users fine-grained control
    
     * Always read existing PID files: When displaying lock errors, existing
       PID files are read regardless of config setting, ensuring helpful
       diagnostics
    
    I'm not very confident in my capacity to attribute the right component
    name for each of the code paths. Specially the ones I marked as other.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2011%2Fpcasaretto%2Fpid-holder-file-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2011/pcasaretto/pid-holder-file-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/2011

Range-diff vs v1:

 1:  7068ec0041 ! 1:  d62edb0ca5 lockfile: add PID file for debugging stale locks
     @@ Commit message
      
          When a lock file is held, it can be helpful to know which process owns
          it, especially when debugging stale locks left behind by crashed
     -    processes. Add an optional feature that creates a companion .lock.pid
     -    file alongside each lock file, containing the PID of the lock holder.
     +    processes. Add an optional feature that creates a companion PID file
     +    alongside each lock file, containing the PID of the lock holder.
      
     -    The .lock.pid file is created when a lock is acquired (if enabled), and
     +    For a lock file "foo.lock", the PID file is named "foo.pid.lock". The
     +    file uses a simple key-value format ("pid <value>") following the same
     +    pattern as Git object headers, making it extensible for future metadata.
     +
     +    The PID file is created when a lock is acquired (if enabled), and
          automatically cleaned up when the lock is released (via commit or
          rollback). The file is registered as a tempfile so it gets cleaned up
          by signal and atexit handlers if the process terminates abnormally.
      
     -    When a lock conflict occurs, the code checks if the PID from the .pid
     -    file is still running using kill(pid, 0). This allows providing
     -    context-aware error messages. With PID info enabled:
     +    When a lock conflict occurs, the code checks for an existing PID file
     +    and, if found, uses kill(pid, 0) to determine if the process is still
     +    running. This allows providing context-aware error messages:
      
            Lock is held by process 12345. Wait for it to finish, or remove
            the lock file to continue.
     @@ Commit message
            Lock was held by process 12345, which is no longer running.
            Remove the stale lock file to continue.
      
     -    Without PID info (default):
     +    The feature is controlled via core.lockfilePid configuration, which
     +    accepts per-component values similar to core.fsync:
     +
     +      - none: Disable for all components (default)
     +      - all: Enable for all components
     +      - index, config, refs, commit-graph, midx, shallow, gc, other:
     +        Enable for specific components
     +
     +    Multiple components can be combined with commas:
     +
     +      git config core.lockfilePid index,config
      
     -      Another git process seems to be running in this repository.
     -      Wait for it to finish, or remove the lock file to continue.
     +    Each lock file call site specifies which component it belongs to,
     +    allowing users fine-grained control over which locks create PID files.
      
     -    The feature is opt-in via GIT_LOCK_PID_INFO=1 environment variable.
     +    Existing PID files are always read when displaying lock errors,
     +    regardless of the core.lockfilePid setting. This ensures helpful
     +    diagnostics even when the feature was previously enabled and later
     +    disabled.
      
          Signed-off-by: Paulo Casaretto <pcasaretto@gmail.com>
      
     - ## Documentation/git.adoc ##
     -@@ Documentation/git.adoc: be in the future).
     - 	the background which do not want to cause lock contention with
     - 	other operations on the repository.  Defaults to `1`.
     - 
     -+`GIT_LOCK_PID_INFO`::
     -+	If this Boolean environment variable is set to `1`, Git will create
     -+	a `.lock.pid` file alongside each lock file containing the PID of the
     -+	process that created the lock. This information is displayed in error
     -+	messages when a lock conflict occurs, making it easier to identify
     -+	stale locks or debug locking issues. The PID files are automatically
     -+	cleaned up via signal and atexit handlers; however, if a process is
     -+	terminated abnormally (e.g., SIGKILL), the file may remain as a stale
     -+	indicator. Disabled by default.
     -+
     - `GIT_REDIRECT_STDIN`::
     - `GIT_REDIRECT_STDOUT`::
     - `GIT_REDIRECT_STDERR`::
     + ## Documentation/config/core.adoc ##
     +@@ Documentation/config/core.adoc: confusion unless you know what you are doing (e.g. you are creating a
     + read-only snapshot of the same index to a location different from the
     + repository's usual working tree).
     + 
     ++core.lockfilePid::
     ++	A comma-separated list of components for which Git should create
     ++	a PID file alongside the lock file. When a lock acquisition fails
     ++	and a PID file exists, Git can provide additional diagnostic
     ++	information about the process holding the lock, including whether
     ++	it is still running.
     +++
     ++This feature is disabled by default. You can enable it for specific
     ++components or use `all` to enable for all components.
     +++
     ++* `none` disables PID file creation for all components.
     ++* `index` creates PID files for index lock operations.
     ++* `config` creates PID files for config file lock operations.
     ++* `refs` creates PID files for reference lock operations.
     ++* `commit-graph` creates PID files for commit-graph lock operations.
     ++* `midx` creates PID files for multi-pack-index lock operations.
     ++* `shallow` creates PID files for shallow file lock operations.
     ++* `gc` creates PID files for garbage collection lock operations.
     ++* `other` creates PID files for other miscellaneous lock operations.
     ++* `all` enables PID file creation for all components.
     +++
     ++The PID file is named by inserting `.pid` before the `.lock` suffix.
     ++For example, if the lock file is `index.lock`, the PID file will be
     ++`index.pid.lock`. The file contains `pid <value>` on a single line,
     ++following the same key-value format used by Git object headers.
     ++
     + core.logAllRefUpdates::
     + 	Enable the reflog. Updates to a ref <ref> is logged to the file
     + 	"`$GIT_DIR/logs/<ref>`", by appending the new and old
     +
     + ## apply.c ##
     +@@ apply.c: static int build_fake_ancestor(struct apply_state *state, struct patch *list)
     + 		}
     + 	}
     + 
     +-	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR);
     ++	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR,
     ++				  LOCKFILE_PID_OTHER);
     + 	res = write_locked_index(&result, &lock, COMMIT_LOCK);
     + 	discard_index(&result);
     + 
     +@@ apply.c: static int apply_patch(struct apply_state *state,
     + 		if (state->index_file)
     + 			hold_lock_file_for_update(&state->lock_file,
     + 						  state->index_file,
     +-						  LOCK_DIE_ON_ERROR);
     ++						  LOCK_DIE_ON_ERROR,
     ++						  LOCKFILE_PID_INDEX);
     + 		else
     + 			repo_hold_locked_index(state->repo, &state->lock_file,
     + 					       LOCK_DIE_ON_ERROR);
     +
     + ## builtin/commit.c ##
     +@@ builtin/commit.c: static const char *prepare_index(const char **argv, const char *prefix,
     + 	path = repo_git_path(the_repository, "next-index-%"PRIuMAX,
     + 			     (uintmax_t) getpid());
     + 	hold_lock_file_for_update(&false_lock, path,
     +-				  LOCK_DIE_ON_ERROR);
     ++				  LOCK_DIE_ON_ERROR, LOCKFILE_PID_OTHER);
     + 
     + 	create_base_index(current_head);
     + 	add_remove_files(&partial);
     +
     + ## builtin/credential-store.c ##
     +@@ builtin/credential-store.c: static void rewrite_credential_file(const char *fn, struct credential *c,
     + 	int timeout_ms = 1000;
     + 
     + 	repo_config_get_int(the_repository, "credentialstore.locktimeoutms", &timeout_ms);
     +-	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms) < 0)
     ++	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms,
     ++					      LOCKFILE_PID_CONFIG) < 0)
     + 		die_errno(_("unable to get credential storage lock in %d ms"), timeout_ms);
     + 	if (extra)
     + 		print_line(extra);
     +
     + ## builtin/difftool.c ##
     +@@ builtin/difftool.c: static int run_dir_diff(struct repository *repo,
     + 			struct lock_file lock = LOCK_INIT;
     + 			strbuf_reset(&buf);
     + 			strbuf_addf(&buf, "%s/wtindex", tmpdir.buf);
     +-			if (hold_lock_file_for_update(&lock, buf.buf, 0) < 0 ||
     ++			if (hold_lock_file_for_update(&lock, buf.buf, 0,
     ++					      LOCKFILE_PID_OTHER) < 0 ||
     + 			    write_locked_index(&wtindex, &lock, COMMIT_LOCK)) {
     + 				ret = error("could not write %s", buf.buf);
     + 				goto finish;
     +
     + ## builtin/fast-import.c ##
     +@@ builtin/fast-import.c: static void dump_marks(void)
     + 		return;
     + 	}
     + 
     +-	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0) < 0) {
     ++	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0,
     ++				      LOCKFILE_PID_OTHER) < 0) {
     + 		failure |= error_errno(_("unable to write marks file %s"),
     + 				       export_marks_file);
     + 		return;
     +
     + ## builtin/gc.c ##
     +@@ builtin/gc.c: static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
     + 
     + 	pidfile_path = repo_git_path(the_repository, "gc.pid");
     + 	fd = hold_lock_file_for_update(&lock, pidfile_path,
     +-				       LOCK_DIE_ON_ERROR);
     ++				       LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
     + 	if (!force) {
     + 		static char locking_host[HOST_NAME_MAX + 1];
     + 		static char *scan_fmt;
     +@@ builtin/gc.c: int cmd_gc(int argc,
     + 	if (daemonized) {
     + 		char *path = repo_git_path(the_repository, "gc.log");
     + 		hold_lock_file_for_update(&log_lock, path,
     +-					  LOCK_DIE_ON_ERROR);
     ++					  LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
     + 		dup2(get_lock_file_fd(&log_lock), 2);
     + 		atexit(process_log_file_at_exit);
     + 		free(path);
     +@@ builtin/gc.c: static int maintenance_run_tasks(struct maintenance_run_opts *opts,
     + 	struct repository *r = the_repository;
     + 	char *lock_path = xstrfmt("%s/maintenance", r->objects->sources->path);
     + 
     +-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
     ++	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
     ++				      LOCKFILE_PID_GC) < 0) {
     + 		/*
     + 		 * Another maintenance command is running.
     + 		 *
     +@@ builtin/gc.c: static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
     + 		lock_file_timeout_ms = 150;
     + 
     + 	fd = hold_lock_file_for_update_timeout(&lk, filename, LOCK_DIE_ON_ERROR,
     +-					       lock_file_timeout_ms);
     ++					       lock_file_timeout_ms,
     ++					       LOCKFILE_PID_GC);
     + 
     + 	/*
     + 	 * Does this file already exist? With the intended contents? Is it
     +@@ builtin/gc.c: static int update_background_schedule(const struct maintenance_start_opts *opts,
     + 	struct lock_file lk;
     + 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->sources->path);
     + 
     +-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
     ++	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
     ++				      LOCKFILE_PID_GC) < 0) {
     + 		if (errno == EEXIST)
     + 			error(_("unable to create '%s.lock': %s.\n\n"
     + 			    "Another scheduled git-maintenance(1) process seems to be running in this\n"
     +
     + ## builtin/sparse-checkout.c ##
     +@@ builtin/sparse-checkout.c: static int write_patterns_and_update(struct repository *repo,
     + 	if (safe_create_leading_directories(repo, sparse_filename))
     + 		die(_("failed to create directory for sparse-checkout file"));
     + 
     +-	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR);
     ++	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR,
     ++				  LOCKFILE_PID_OTHER);
     + 
     + 	result = update_working_directory(repo, pl);
     + 	if (result) {
     +
     + ## bundle.c ##
     +@@ bundle.c: int create_bundle(struct repository *r, const char *path,
     + 		bundle_fd = 1;
     + 	else
     + 		bundle_fd = hold_lock_file_for_update(&lock, path,
     +-						      LOCK_DIE_ON_ERROR);
     ++						      LOCK_DIE_ON_ERROR,
     ++						      LOCKFILE_PID_OTHER);
     + 
     + 	if (version == -1)
     + 		version = min_version;
     +
     + ## cache-tree.c ##
     +@@ cache-tree.c: int write_index_as_tree(struct object_id *oid, struct index_state *index_state,
     + 	struct lock_file lock_file = LOCK_INIT;
     + 	int ret;
     + 
     +-	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
     ++	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR,
     ++				  LOCKFILE_PID_INDEX);
     + 
     + 	entries = read_index_from(index_state, index_path,
     + 				  repo_get_git_dir(the_repository));
     +
     + ## commit-graph.c ##
     +@@ commit-graph.c: static int write_commit_graph_file(struct write_commit_graph_context *ctx)
     + 		char *lock_name = get_commit_graph_chain_filename(ctx->odb_source);
     + 
     + 		hold_lock_file_for_update_mode(&lk, lock_name,
     +-					       LOCK_DIE_ON_ERROR, 0444);
     ++					       LOCK_DIE_ON_ERROR, 0444,
     ++					       LOCKFILE_PID_COMMIT_GRAPH);
     + 		free(lock_name);
     + 
     + 		graph_layer = mks_tempfile_m(ctx->graph_name, 0444);
     +@@ commit-graph.c: static int write_commit_graph_file(struct write_commit_graph_context *ctx)
     + 			   get_tempfile_fd(graph_layer), get_tempfile_path(graph_layer));
     + 	} else {
     + 		hold_lock_file_for_update_mode(&lk, ctx->graph_name,
     +-					       LOCK_DIE_ON_ERROR, 0444);
     ++					       LOCK_DIE_ON_ERROR, 0444,
     ++					       LOCKFILE_PID_COMMIT_GRAPH);
     + 		f = hashfd(ctx->r->hash_algo,
     + 			   get_lock_file_fd(&lk), get_lock_file_path(&lk));
     + 	}
     +
     + ## compat/mingw.c ##
     +@@ compat/mingw.c: int mingw_kill(pid_t pid, int sig)
     + 			CloseHandle(h);
     + 			return 0;
     + 		}
     ++		/*
     ++		 * OpenProcess returns ERROR_INVALID_PARAMETER for
     ++		 * non-existent PIDs. Map this to ESRCH for POSIX
     ++		 * compatibility with kill(pid, 0).
     ++		 */
     ++		if (GetLastError() == ERROR_INVALID_PARAMETER)
     ++			errno = ESRCH;
     ++		else
     ++			errno = err_win_to_posix(GetLastError());
     ++		return -1;
     + 	}
     + 
     + 	errno = EINVAL;
     +
     + ## config.c ##
     +@@ config.c: int repo_config_set_multivar_in_file_gently(struct repository *r,
     + 	 * The lock serves a purpose in addition to locking: the new
     + 	 * contents of .git/config will be written into it.
     + 	 */
     +-	fd = hold_lock_file_for_update(&lock, config_filename, 0);
     ++	fd = hold_lock_file_for_update(&lock, config_filename, 0,
     ++				       LOCKFILE_PID_CONFIG);
     + 	if (fd < 0) {
     + 		error_errno(_("could not lock config file %s"), config_filename);
     + 		ret = CONFIG_NO_LOCK;
     +@@ config.c: static int repo_config_copy_or_rename_section_in_file(
     + 	if (!config_filename)
     + 		config_filename = filename_buf = repo_git_path(r, "config");
     + 
     +-	out_fd = hold_lock_file_for_update(&lock, config_filename, 0);
     ++	out_fd = hold_lock_file_for_update(&lock, config_filename, 0,
     ++					   LOCKFILE_PID_CONFIG);
     + 	if (out_fd < 0) {
     + 		ret = error(_("could not lock config file %s"), config_filename);
     + 		goto out;
     +
     + ## environment.c ##
     +@@
     + #include "gettext.h"
     + #include "git-zlib.h"
     + #include "ident.h"
     ++#include "lockfile.h"
     + #include "mailmap.h"
     + #include "object-name.h"
     + #include "repository.h"
     +@@ environment.c: next_name:
     + 	return (current & ~negative) | positive;
     + }
     + 
     ++static const struct lockfile_pid_component_name {
     ++	const char *name;
     ++	enum lockfile_pid_component component_bits;
     ++} lockfile_pid_component_names[] = {
     ++	{ "index", LOCKFILE_PID_INDEX },
     ++	{ "config", LOCKFILE_PID_CONFIG },
     ++	{ "refs", LOCKFILE_PID_REFS },
     ++	{ "commit-graph", LOCKFILE_PID_COMMIT_GRAPH },
     ++	{ "midx", LOCKFILE_PID_MIDX },
     ++	{ "shallow", LOCKFILE_PID_SHALLOW },
     ++	{ "gc", LOCKFILE_PID_GC },
     ++	{ "other", LOCKFILE_PID_OTHER },
     ++	{ "all", LOCKFILE_PID_ALL },
     ++};
     ++
     ++static enum lockfile_pid_component parse_lockfile_pid_components(const char *var,
     ++								 const char *string)
     ++{
     ++	enum lockfile_pid_component current = LOCKFILE_PID_DEFAULT;
     ++	enum lockfile_pid_component positive = 0, negative = 0;
     ++
     ++	while (string) {
     ++		size_t len;
     ++		const char *ep;
     ++		int negated = 0;
     ++		int found = 0;
     ++
     ++		string = string + strspn(string, ", \t\n\r");
     ++		ep = strchrnul(string, ',');
     ++		len = ep - string;
     ++		if (len == 4 && !strncmp(string, "none", 4)) {
     ++			current = LOCKFILE_PID_NONE;
     ++			goto next_name;
     ++		}
     ++
     ++		if (*string == '-') {
     ++			negated = 1;
     ++			string++;
     ++			len--;
     ++			if (!len)
     ++				warning(_("invalid value for variable %s"), var);
     ++		}
     ++
     ++		if (!len)
     ++			break;
     ++
     ++		for (size_t i = 0; i < ARRAY_SIZE(lockfile_pid_component_names); ++i) {
     ++			const struct lockfile_pid_component_name *n = &lockfile_pid_component_names[i];
     ++
     ++			if (strncmp(n->name, string, len) || strlen(n->name) != len)
     ++				continue;
     ++
     ++			found = 1;
     ++			if (negated)
     ++				negative |= n->component_bits;
     ++			else
     ++				positive |= n->component_bits;
     ++		}
     ++
     ++		if (!found) {
     ++			char *component = xstrndup(string, len);
     ++			warning(_("ignoring unknown core.lockfilePid component '%s'"), component);
     ++			free(component);
     ++		}
     ++
     ++next_name:
     ++		string = ep;
     ++	}
     ++
     ++	return (current & ~negative) | positive;
     ++}
     ++
     + static int git_default_core_config(const char *var, const char *value,
     + 				   const struct config_context *ctx, void *cb)
     + {
     +@@ environment.c: static int git_default_core_config(const char *var, const char *value,
     + 		return 0;
     + 	}
     + 
     ++	if (!strcmp(var, "core.lockfilepid")) {
     ++		if (!value)
     ++			return config_error_nonbool(var);
     ++		lockfile_pid_components = parse_lockfile_pid_components(var, value);
     ++		return 0;
     ++	}
     ++
     + 	if (!strcmp(var, "core.createobject")) {
     + 		if (!value)
     + 			return config_error_nonbool(var);
      
       ## lockfile.c ##
      @@
     @@ lockfile.c: static void resolve_symlink(struct strbuf *path)
       }
       
      +/*
     -+ * Lock PID file functions - write PID to a .lock.pid file alongside
     ++ * Lock PID file functions - write PID to a foo-pid.lock file alongside
      + * the lock file for debugging stale locks. The PID file is registered
      + * as a tempfile so it gets cleaned up by signal/atexit handlers.
     ++ *
     ++ * Naming: For "foo.lock", the PID file is "foo-pid.lock" (not "foo.lock.pid").
     ++ * This avoids collision with the refs namespace.
     ++ */
     ++
     ++/* Global config variable, initialized from core.lockfilePid */
     ++enum lockfile_pid_component lockfile_pid_components = LOCKFILE_PID_DEFAULT;
     ++
     ++/*
     ++ * Path generation helpers.
     ++ * Given base path "foo", generate:
     ++ *   - lock path: "foo.lock"
     ++ *   - pid path:  "foo-pid.lock"
      + */
     ++static void get_lock_path(struct strbuf *out, const char *path)
     ++{
     ++	strbuf_addstr(out, path);
     ++	strbuf_addstr(out, LOCK_SUFFIX);
     ++}
      +
     -+static int lock_pid_info_enabled(void)
     ++static void get_pid_path(struct strbuf *out, const char *path)
      +{
     -+	return git_env_bool(GIT_LOCK_PID_INFO_ENVIRONMENT, 0);
     ++	strbuf_addstr(out, path);
     ++	strbuf_addstr(out, LOCK_PID_INFIX);
     ++	strbuf_addstr(out, LOCK_SUFFIX);
      +}
      +
     -+static struct tempfile *create_lock_pid_file(const char *lock_path, int mode)
     ++static struct tempfile *create_lock_pid_file(const char *pid_path, int mode,
     ++					     enum lockfile_pid_component component)
      +{
     -+	struct strbuf pid_path = STRBUF_INIT;
      +	struct strbuf content = STRBUF_INIT;
      +	struct tempfile *pid_tempfile = NULL;
     -+	int fd;
     -+
     -+	if (!lock_pid_info_enabled())
     -+		return NULL;
     -+
     -+	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
     -+	fd = open(pid_path.buf, O_WRONLY | O_CREAT | O_TRUNC, mode);
     -+	if (fd >= 0) {
     -+		strbuf_addf(&content, "%"PRIuMAX"\n", (uintmax_t)getpid());
     -+		if (write_in_full(fd, content.buf, content.len) < 0)
     -+			warning_errno(_("could not write lock pid file '%s'"),
     -+				      pid_path.buf);
     -+		close(fd);
     -+		pid_tempfile = register_tempfile(pid_path.buf);
     ++	int fd = -1;
     ++
     ++	if (!(lockfile_pid_components & component))
     ++		goto out;
     ++
     ++	fd = open(pid_path, O_WRONLY | O_CREAT | O_EXCL, mode);
     ++	if (fd < 0)
     ++		goto out;
     ++
     ++	strbuf_addf(&content, "pid %" PRIuMAX "\n", (uintmax_t)getpid());
     ++	if (write_in_full(fd, content.buf, content.len) < 0) {
     ++		warning_errno(_("could not write lock pid file '%s'"), pid_path);
     ++		goto out;
      +	}
     ++
     ++	close(fd);
     ++	fd = -1;
     ++	pid_tempfile = register_tempfile(pid_path);
     ++
     ++out:
     ++	if (fd >= 0)
     ++		close(fd);
      +	strbuf_release(&content);
     -+	strbuf_release(&pid_path);
      +	return pid_tempfile;
      +}
      +
     -+static int read_lock_pid(const char *lock_path, uintmax_t *pid_out)
     ++static int read_lock_pid(const char *pid_path, uintmax_t *pid_out)
      +{
     -+	struct strbuf pid_path = STRBUF_INIT;
      +	struct strbuf content = STRBUF_INIT;
     ++	const char *val;
      +	int ret = -1;
      +
     -+	strbuf_addf(&pid_path, "%s%s", lock_path, LOCK_PID_SUFFIX);
     -+	if (strbuf_read_file(&content, pid_path.buf, LOCK_PID_MAXLEN) > 0) {
     ++	if (strbuf_read_file(&content, pid_path, LOCK_PID_MAXLEN) <= 0)
     ++		goto out;
     ++
     ++	strbuf_rtrim(&content);
     ++
     ++	if (skip_prefix(content.buf, "pid ", &val)) {
      +		char *endptr;
     -+		*pid_out = strtoumax(content.buf, &endptr, 10);
     -+		if (*pid_out > 0 && (*endptr == '\n' || *endptr == '\0'))
     ++		*pid_out = strtoumax(val, &endptr, 10);
     ++		if (*pid_out > 0 && !*endptr)
      +			ret = 0;
     -+		else
     -+			warning(_("malformed lock pid file '%s'"), pid_path.buf);
      +	}
     -+	strbuf_release(&pid_path);
     ++
     ++	if (ret)
     ++		warning(_("malformed lock pid file '%s'"), pid_path);
     ++
     ++out:
      +	strbuf_release(&content);
      +	return ret;
      +}
      +
       /* Make sure errno contains a meaningful value on error */
       static int lock_file(struct lock_file *lk, const char *path, int flags,
     - 		     int mode)
     -@@ lockfile.c: static int lock_file(struct lock_file *lk, const char *path, int flags,
     - 	strbuf_addstr(&filename, path);
     +-		     int mode)
     ++		     int mode, enum lockfile_pid_component component)
     + {
     +-	struct strbuf filename = STRBUF_INIT;
     ++	struct strbuf base_path = STRBUF_INIT;
     ++	struct strbuf lock_path = STRBUF_INIT;
     ++	struct strbuf pid_path = STRBUF_INIT;
     + 
     +-	strbuf_addstr(&filename, path);
     ++	strbuf_addstr(&base_path, path);
       	if (!(flags & LOCK_NO_DEREF))
     - 		resolve_symlink(&filename);
     --
     - 	strbuf_addstr(&filename, LOCK_SUFFIX);
     +-		resolve_symlink(&filename);
     ++		resolve_symlink(&base_path);
      +
     - 	lk->tempfile = create_tempfile_mode(filename.buf, mode);
     -+	if (lk->tempfile)
     -+		lk->pid_tempfile = create_lock_pid_file(filename.buf, mode);
     ++	get_lock_path(&lock_path, base_path.buf);
     ++	get_pid_path(&pid_path, base_path.buf);
      +
     - 	strbuf_release(&filename);
     ++	lk->tempfile = create_tempfile_mode(lock_path.buf, mode);
     ++	if (lk->tempfile)
     ++		lk->pid_tempfile = create_lock_pid_file(pid_path.buf, mode,
     ++							component);
     + 
     +-	strbuf_addstr(&filename, LOCK_SUFFIX);
     +-	lk->tempfile = create_tempfile_mode(filename.buf, mode);
     +-	strbuf_release(&filename);
     ++	strbuf_release(&base_path);
     ++	strbuf_release(&lock_path);
     ++	strbuf_release(&pid_path);
       	return lk->tempfile ? lk->tempfile->fd : -1;
       }
     + 
     +@@ lockfile.c: static int lock_file(struct lock_file *lk, const char *path, int flags,
     +  * exactly once. If timeout_ms is -1, try indefinitely.
     +  */
     + static int lock_file_timeout(struct lock_file *lk, const char *path,
     +-			     int flags, long timeout_ms, int mode)
     ++			     int flags, long timeout_ms, int mode,
     ++			     enum lockfile_pid_component component)
     + {
     + 	int n = 1;
     + 	int multiplier = 1;
     +@@ lockfile.c: static int lock_file_timeout(struct lock_file *lk, const char *path,
     + 	static int random_initialized = 0;
     + 
     + 	if (timeout_ms == 0)
     +-		return lock_file(lk, path, flags, mode);
     ++		return lock_file(lk, path, flags, mode, component);
     + 
     + 	if (!random_initialized) {
     + 		srand((unsigned int)getpid());
     +@@ lockfile.c: static int lock_file_timeout(struct lock_file *lk, const char *path,
     + 		long backoff_ms, wait_ms;
     + 		int fd;
     + 
     +-		fd = lock_file(lk, path, flags, mode);
     ++		fd = lock_file(lk, path, flags, mode, component);
     + 
     + 		if (fd >= 0)
     + 			return fd; /* success */
      @@ lockfile.c: static int lock_file_timeout(struct lock_file *lk, const char *path,
       void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
       {
     @@ lockfile.c: static int lock_file_timeout(struct lock_file *lk, const char *path,
      -		    "may have crashed in this repository earlier:\n"
      -		    "remove the file manually to continue."),
      -			    absolute_path(path), strerror(err));
     +-	} else
     ++		const char *abs_path = absolute_path(path);
      +		struct strbuf lock_path = STRBUF_INIT;
     ++		struct strbuf pid_path = STRBUF_INIT;
      +		uintmax_t pid;
      +		int pid_status = 0; /* 0 = unknown, 1 = running, -1 = stale */
      +
     -+		strbuf_addf(&lock_path, "%s%s", absolute_path(path), LOCK_SUFFIX);
     ++		get_lock_path(&lock_path, abs_path);
     ++		get_pid_path(&pid_path, abs_path);
      +
      +		strbuf_addf(buf, _("Unable to create '%s': %s.\n\n"),
      +			    lock_path.buf, strerror(err));
      +
     -+		if (lock_pid_info_enabled() &&
     -+		    !read_lock_pid(lock_path.buf, &pid)) {
     -+			if (kill((pid_t)pid, 0) == 0)
     -+				pid_status = 1;
     -+			else
     -+				pid_status = -1;
     ++		/*
     ++		 * Try to read PID file unconditionally - it may exist if
     ++		 * core.lockfilePid was enabled for this component.
     ++		 */
     ++		if (!read_lock_pid(pid_path.buf, &pid)) {
     ++			if (kill((pid_t)pid, 0) == 0 || errno == EPERM)
     ++				pid_status = 1;  /* running (or no permission to signal) */
     ++			else if (errno == ESRCH)
     ++				pid_status = -1; /* no such process - stale lock */
      +		}
      +
      +		if (pid_status == 1)
     -+			strbuf_addf(buf, _("Lock is held by process %"PRIuMAX". "
     -+			    "Wait for it to finish, or remove the lock file to continue"),
     -+			    pid);
     ++			strbuf_addf(buf, _("Lock is held by process %" PRIuMAX ". "
     ++					   "Wait for it to finish, or remove the lock file to continue"),
     ++				    pid);
      +		else if (pid_status == -1)
     -+			strbuf_addf(buf, _("Lock was held by process %"PRIuMAX", "
     -+			    "which is no longer running. Remove the stale lock file to continue"),
     -+			    pid);
     ++			strbuf_addf(buf, _("Lock was held by process %" PRIuMAX ", "
     ++					   "which is no longer running. Remove the stale lock file to continue"),
     ++				    pid);
      +		else
      +			strbuf_addstr(buf, _("Another git process seems to be running in this repository. "
     -+			    "Wait for it to finish, or remove the lock file to continue"));
     ++					     "Wait for it to finish, or remove the lock file to continue"));
      +
      +		strbuf_release(&lock_path);
     - 	} else
     ++		strbuf_release(&pid_path);
     ++	} else {
       		strbuf_addf(buf, _("Unable to create '%s.lock': %s"),
       			    absolute_path(path), strerror(err));
     ++	}
     + }
     + 
     + NORETURN void unable_to_lock_die(const char *path, int err)
     +@@ lockfile.c: NORETURN void unable_to_lock_die(const char *path, int err)
     + /* This should return a meaningful errno on failure */
     + int hold_lock_file_for_update_timeout_mode(struct lock_file *lk,
     + 					   const char *path, int flags,
     +-					   long timeout_ms, int mode)
     ++					   long timeout_ms, int mode,
     ++					   enum lockfile_pid_component component)
     + {
     +-	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode);
     ++	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode, component);
     + 	if (fd < 0) {
     + 		if (flags & LOCK_DIE_ON_ERROR)
     + 			unable_to_lock_die(path, errno);
      @@ lockfile.c: int commit_lock_file(struct lock_file *lk)
       {
       	char *result_path = get_locked_file_path(lk);
     @@ lockfile.h: struct lock_file {
       #define LOCK_SUFFIX ".lock"
       #define LOCK_SUFFIX_LEN 5
       
     -+/* Suffix for PID file that stores PID of lock holder: */
     -+#define LOCK_PID_SUFFIX ".pid"
     -+#define LOCK_PID_SUFFIX_LEN 4
     ++/*
     ++ * PID file naming: for a lock file "foo.lock", the PID file is "foo.pid.lock".
     ++ */
     ++#define LOCK_PID_INFIX ".pid"
     ++#define LOCK_PID_INFIX_LEN 4
      +
      +/* Maximum length for PID file content */
      +#define LOCK_PID_MAXLEN 32
      +
     -+/* Environment variable to enable lock PID info (default: disabled) */
     -+#define GIT_LOCK_PID_INFO_ENVIRONMENT "GIT_LOCK_PID_INFO"
     ++/*
     ++ * Per-component lock PID file configuration, following core.fsync pattern.
     ++ * Each component can be individually enabled via core.lockfilePid config.
     ++ */
     ++enum lockfile_pid_component {
     ++	LOCKFILE_PID_NONE = 0,
     ++	LOCKFILE_PID_INDEX = 1 << 0, /* .git/index.lock */
     ++	LOCKFILE_PID_CONFIG = 1 << 1, /* .git/config.lock */
     ++	LOCKFILE_PID_REFS = 1 << 2, /* refs locks */
     ++	LOCKFILE_PID_COMMIT_GRAPH = 1 << 3, /* commit-graph.lock */
     ++	LOCKFILE_PID_MIDX = 1 << 4, /* multi-pack-index.lock */
     ++	LOCKFILE_PID_SHALLOW = 1 << 5, /* shallow file */
     ++	LOCKFILE_PID_GC = 1 << 6, /* gc locks */
     ++	LOCKFILE_PID_OTHER = 1 << 7, /* other locks */
     ++};
     ++
     ++#define LOCKFILE_PID_ALL (LOCKFILE_PID_INDEX | LOCKFILE_PID_CONFIG |      \
     ++			  LOCKFILE_PID_REFS | LOCKFILE_PID_COMMIT_GRAPH | \
     ++			  LOCKFILE_PID_MIDX | LOCKFILE_PID_SHALLOW |      \
     ++			  LOCKFILE_PID_GC | LOCKFILE_PID_OTHER)
     ++#define LOCKFILE_PID_DEFAULT LOCKFILE_PID_NONE
     ++
     ++/*
     ++ * Bitmask indicating which components should create PID files.
     ++ * Configured via core.lockfilePid.
     ++ */
     ++extern enum lockfile_pid_component lockfile_pid_components;
       
       /*
        * Flags
     +@@ lockfile.h: struct lock_file {
     +  * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
     +  * timeout_ms is -1, retry indefinitely. The flags argument, error
     +  * handling, and mode are described above.
     ++ *
     ++ * The `component` argument specifies which lock PID component this lock
     ++ * belongs to, for selective PID file creation via core.lockfilePid config.
     +  */
     + int hold_lock_file_for_update_timeout_mode(
     +-		struct lock_file *lk, const char *path,
     +-		int flags, long timeout_ms, int mode);
     ++	struct lock_file *lk, const char *path,
     ++	int flags, long timeout_ms, int mode,
     ++	enum lockfile_pid_component component);
     + 
     + static inline int hold_lock_file_for_update_timeout(
     +-		struct lock_file *lk, const char *path,
     +-		int flags, long timeout_ms)
     ++	struct lock_file *lk, const char *path,
     ++	int flags, long timeout_ms,
     ++	enum lockfile_pid_component component)
     + {
     + 	return hold_lock_file_for_update_timeout_mode(lk, path, flags,
     +-						      timeout_ms, 0666);
     ++						      timeout_ms, 0666,
     ++						      component);
     + }
     + 
     + /*
     +@@ lockfile.h: static inline int hold_lock_file_for_update_timeout(
     +  * argument and error handling are described above.
     +  */
     + static inline int hold_lock_file_for_update(
     +-		struct lock_file *lk, const char *path,
     +-		int flags)
     ++	struct lock_file *lk, const char *path,
     ++	int flags, enum lockfile_pid_component component)
     + {
     +-	return hold_lock_file_for_update_timeout(lk, path, flags, 0);
     ++	return hold_lock_file_for_update_timeout(lk, path, flags, 0, component);
     + }
     + 
     + static inline int hold_lock_file_for_update_mode(
     +-		struct lock_file *lk, const char *path,
     +-		int flags, int mode)
     ++	struct lock_file *lk, const char *path,
     ++	int flags, int mode, enum lockfile_pid_component component)
     + {
     +-	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode);
     ++	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode,
     ++						      component);
     + }
     + 
     + /*
      @@ lockfile.h: static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
       
       /*
     @@ lockfile.h: static inline int commit_lock_file_to(struct lock_file *lk, const ch
       
       #endif /* LOCKFILE_H */
      
     + ## loose.c ##
     +@@ loose.c: int repo_write_loose_object_map(struct repository *repo)
     + 		return 0;
     + 
     + 	repo_common_path_replace(repo, &path, "objects/loose-object-idx");
     +-	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
     ++	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
     ++					       LOCKFILE_PID_OTHER);
     + 	iter = kh_begin(map);
     + 	if (write_in_full(fd, loose_object_header, strlen(loose_object_header)) < 0)
     + 		goto errout;
     +@@ loose.c: static int write_one_object(struct odb_source *source,
     + 	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
     + 
     + 	strbuf_addf(&path, "%s/loose-object-idx", source->path);
     +-	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
     ++	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
     ++					  LOCKFILE_PID_OTHER);
     + 
     + 	fd = open(path.buf, O_WRONLY | O_CREAT | O_APPEND, 0666);
     + 	if (fd < 0)
     +
     + ## midx-write.c ##
     +@@ midx-write.c: static int write_midx_internal(struct odb_source *source,
     + 		struct strbuf lock_name = STRBUF_INIT;
     + 
     + 		get_midx_chain_filename(source, &lock_name);
     +-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
     ++		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR,
     ++					  LOCKFILE_PID_MIDX);
     + 		strbuf_release(&lock_name);
     + 
     + 		incr = mks_tempfile_m(midx_name.buf, 0444);
     +@@ midx-write.c: static int write_midx_internal(struct odb_source *source,
     + 		f = hashfd(r->hash_algo, get_tempfile_fd(incr),
     + 			   get_tempfile_path(incr));
     + 	} else {
     +-		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
     ++		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR,
     ++					  LOCKFILE_PID_MIDX);
     + 		f = hashfd(r->hash_algo, get_lock_file_fd(&lk),
     + 			   get_lock_file_path(&lk));
     + 	}
     +
     + ## odb.c ##
     +@@ odb.c: void odb_add_to_alternates_file(struct object_database *odb,
     + 	FILE *in, *out;
     + 	int found = 0;
     + 
     +-	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR);
     ++	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR,
     ++				  LOCKFILE_PID_OTHER);
     + 	out = fdopen_lock_file(&lock, "w");
     + 	if (!out)
     + 		die_errno(_("unable to fdopen alternates lockfile"));
     +
     + ## refs/files-backend.c ##
     +@@ refs/files-backend.c: retry:
     + 
     + 	if (hold_lock_file_for_update_timeout(
     + 			    &lock->lk, ref_file.buf, LOCK_NO_DEREF,
     +-			    get_files_ref_lock_timeout_ms()) < 0) {
     ++			    get_files_ref_lock_timeout_ms(),
     ++			    LOCKFILE_PID_REFS) < 0) {
     + 		int myerr = errno;
     + 		errno = 0;
     + 		if (myerr == ENOENT && --attempts_remaining > 0) {
     +@@ refs/files-backend.c: static int create_reflock(const char *path, void *cb)
     + 
     + 	return hold_lock_file_for_update_timeout(
     + 			lk, path, LOCK_NO_DEREF,
     +-			get_files_ref_lock_timeout_ms()) < 0 ? -1 : 0;
     ++			get_files_ref_lock_timeout_ms(),
     ++			LOCKFILE_PID_REFS) < 0 ? -1 : 0;
     + }
     + 
     + /*
     +@@ refs/files-backend.c: static int files_reflog_expire(struct ref_store *ref_store,
     + 		 * work we need, including cleaning up if the program
     + 		 * exits unexpectedly.
     + 		 */
     +-		if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
     ++		if (hold_lock_file_for_update(&reflog_lock, log_file, 0,
     ++					      LOCKFILE_PID_REFS) < 0) {
     + 			struct strbuf err = STRBUF_INIT;
     + 			unable_to_lock_message(log_file, errno, &err);
     + 			error("%s", err.buf);
     +
     + ## refs/packed-backend.c ##
     +@@ refs/packed-backend.c: int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
     + 	if (hold_lock_file_for_update_timeout(
     + 			    &refs->lock,
     + 			    refs->path,
     +-			    flags, timeout_value) < 0) {
     ++			    flags, timeout_value,
     ++			    LOCKFILE_PID_REFS) < 0) {
     + 		unable_to_lock_message(refs->path, errno, err);
     + 		return -1;
     + 	}
     +
     + ## reftable/system.c ##
     +@@ reftable/system.c: int flock_acquire(struct reftable_flock *l, const char *target_path,
     + 		return REFTABLE_OUT_OF_MEMORY_ERROR;
     + 
     + 	err = hold_lock_file_for_update_timeout(lockfile, target_path, LOCK_NO_DEREF,
     +-						timeout_ms);
     ++						timeout_ms, LOCKFILE_PID_REFS);
     + 	if (err < 0) {
     + 		reftable_free(lockfile);
     + 		if (errno == EEXIST)
     +
     + ## repository.c ##
     +@@ repository.c: int repo_hold_locked_index(struct repository *repo,
     + {
     + 	if (!repo->index_file)
     + 		BUG("the repo hasn't been setup");
     +-	return hold_lock_file_for_update(lf, repo->index_file, flags);
     ++	return hold_lock_file_for_update(lf, repo->index_file, flags,
     ++					 LOCKFILE_PID_INDEX);
     + }
     +
     + ## rerere.c ##
     +@@ rerere.c: int setup_rerere(struct repository *r, struct string_list *merge_rr, int flags)
     + 	else
     + 		fd = hold_lock_file_for_update(&write_lock,
     + 					       git_path_merge_rr(r),
     +-					       LOCK_DIE_ON_ERROR);
     ++					       LOCK_DIE_ON_ERROR,
     ++					       LOCKFILE_PID_OTHER);
     + 	read_rr(r, merge_rr);
     + 	return fd;
     + }
     +
     + ## sequencer.c ##
     +@@ sequencer.c: static int write_message(const void *buf, size_t len, const char *filename,
     + {
     + 	struct lock_file msg_file = LOCK_INIT;
     + 
     +-	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0);
     ++	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0,
     ++					       LOCKFILE_PID_OTHER);
     + 	if (msg_fd < 0)
     + 		return error_errno(_("could not lock '%s'"), filename);
     + 	if (write_in_full(msg_fd, buf, len) < 0) {
     +@@ sequencer.c: static int save_todo(struct todo_list *todo_list, struct replay_opts *opts,
     + 	if (is_rebase_i(opts) && !reschedule)
     + 		next++;
     + 
     +-	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0);
     ++	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0,
     ++				       LOCKFILE_PID_OTHER);
     + 	if (fd < 0)
     + 		return error_errno(_("could not lock '%s'"), todo_path);
     + 	offset = get_item_line_offset(todo_list, next);
     +@@ sequencer.c: static int safe_append(const char *filename, const char *fmt, ...)
     + 	va_list ap;
     + 	struct lock_file lock = LOCK_INIT;
     + 	int fd = hold_lock_file_for_update(&lock, filename,
     +-					   LOCK_REPORT_ON_ERROR);
     ++					   LOCK_REPORT_ON_ERROR,
     ++					   LOCKFILE_PID_OTHER);
     + 	struct strbuf buf = STRBUF_INIT;
     + 
     + 	if (fd < 0)
     +@@ sequencer.c: static int write_update_refs_state(struct string_list *refs_to_oids)
     + 		goto cleanup;
     + 	}
     + 
     +-	if (hold_lock_file_for_update(&lock, path, 0) < 0) {
     ++	if (hold_lock_file_for_update(&lock, path, 0, LOCKFILE_PID_OTHER) < 0) {
     + 		result = error(_("another 'rebase' process appears to be running; "
     + 				 "'%s.lock' already exists"),
     + 			       path);
     +
     + ## shallow.c ##
     +@@ shallow.c: void setup_alternate_shallow(struct shallow_lock *shallow_lock,
     + 
     + 	fd = hold_lock_file_for_update(&shallow_lock->lock,
     + 				       git_path_shallow(the_repository),
     +-				       LOCK_DIE_ON_ERROR);
     ++				       LOCK_DIE_ON_ERROR,
     ++				       LOCKFILE_PID_SHALLOW);
     + 	check_shallow_file_for_update(the_repository);
     + 	if (write_shallow_commits(&sb, 0, extra)) {
     + 		if (write_in_full(fd, sb.buf, sb.len) < 0)
     +@@ shallow.c: void prune_shallow(unsigned options)
     + 	}
     + 	fd = hold_lock_file_for_update(&shallow_lock.lock,
     + 				       git_path_shallow(the_repository),
     +-				       LOCK_DIE_ON_ERROR);
     ++				       LOCK_DIE_ON_ERROR,
     ++				       LOCKFILE_PID_SHALLOW);
     + 	check_shallow_file_for_update(the_repository);
     + 	if (write_shallow_commits_1(&sb, 0, NULL, flags)) {
     + 		if (write_in_full(fd, sb.buf, sb.len) < 0)
     +
       ## t/meson.build ##
      @@ t/meson.build: integration_tests = [
         't0028-working-tree-encoding.sh',
     @@ t/t0031-lockfile-pid.sh (new)
      +test_description='lock file PID info tests
      +
      +Tests for PID info file alongside lock files.
     -+The feature is opt-in via GIT_LOCK_PID_INFO=1.
     ++The feature is opt-in via core.lockfilePid config setting.
      +'
      +
      +. ./test-lib.sh
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo &&
      +		touch .git/index.lock &&
     -+		echo "99999" >.git/index.lock.pid &&
     -+		test_must_fail env GIT_LOCK_PID_INFO=1 git add . 2>err &&
     ++		printf "pid 99999" >.git/index.pid.lock &&
     ++		test_must_fail git -c core.lockfilePid=index add . 2>err &&
      +		test_grep "process 99999, which is no longer running" err &&
      +		test_grep "Remove the stale lock file" err
      +	)
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo2 &&
      +		touch .git/index.lock &&
     -+		echo "99999" >.git/index.lock.pid &&
     ++		printf "pid 99999" >.git/index.pid.lock &&
      +		test_must_fail git add . 2>err &&
      +		# Should not crash, just show normal error without PID
      +		test_grep "Unable to create" err &&
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo3 &&
      +		echo content >file &&
     ++		# Get the correct PID for this platform
     ++		shell_pid=$$ &&
     ++		if test_have_prereq MINGW && test -f /proc/$shell_pid/winpid
     ++		then
     ++			# In Git for Windows, Bash uses MSYS2 PIDs but git.exe
     ++			# uses Windows PIDs. Use the Windows PID.
     ++			shell_pid=$(cat /proc/$shell_pid/winpid)
     ++		fi &&
      +		# Create a lock and PID file with current shell PID (which is running)
      +		touch .git/index.lock &&
     -+		echo $$ >.git/index.lock.pid &&
     ++		printf "pid %d" "$shell_pid" >.git/index.pid.lock &&
      +		# Verify our PID is shown in the error message
     -+		test_must_fail env GIT_LOCK_PID_INFO=1 git add file 2>err &&
     -+		test_grep "held by process $$" err
     ++		test_must_fail git -c core.lockfilePid=index add file 2>err &&
     ++		test_grep "held by process $shell_pid" err
      +	)
      +'
      +
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo4 &&
      +		echo content >file &&
     -+		env GIT_LOCK_PID_INFO=1 git add file &&
     ++		git -c core.lockfilePid=index add file &&
      +		# After successful add, no lock or PID files should exist
     -+		! test -f .git/index.lock &&
     -+		! test -f .git/index.lock.pid
     ++		test_path_is_missing .git/index.lock &&
     ++		test_path_is_missing .git/index.pid.lock
      +	)
      +'
      +
     @@ t/t0031-lockfile-pid.sh (new)
      +		echo content >file &&
      +		git add file &&
      +		# PID file should not be created when feature is disabled
     -+		! test -f .git/index.lock.pid
     ++		test_path_is_missing .git/index.pid.lock
     ++	)
     ++'
     ++
     ++test_expect_success 'core.lockfilePid=all enables for all components' '
     ++	git init repo6 &&
     ++	(
     ++		cd repo6 &&
     ++		touch .git/index.lock &&
     ++		printf "pid 99999" >.git/index.pid.lock &&
     ++		test_must_fail git -c core.lockfilePid=all add . 2>err &&
     ++		test_grep "process 99999" err
     ++	)
     ++'
     ++
     ++test_expect_success 'multiple components can be specified' '
     ++	git init repo8 &&
     ++	(
     ++		cd repo8 &&
     ++		touch .git/index.lock &&
     ++		printf "pid 99999" >.git/index.pid.lock &&
     ++		test_must_fail git -c core.lockfilePid=index,config add . 2>err &&
     ++		test_grep "process 99999" err
     ++	)
     ++'
     ++
     ++test_expect_success 'core.lockfilePid=none does not create PID file' '
     ++	git init repo9 &&
     ++	(
     ++		cd repo9 &&
     ++		echo content >file &&
     ++		git -c core.lockfilePid=none add file &&
     ++		# PID file should not be created when feature is disabled
     ++		test_path_is_missing .git/index.pid.lock
     ++	)
     ++'
     ++
     ++test_expect_success 'existing PID files are read even when feature disabled' '
     ++	git init repo10 &&
     ++	(
     ++		cd repo10 &&
     ++		touch .git/index.lock &&
     ++		printf "pid 99999" >.git/index.pid.lock &&
     ++		# Even with lockfilePid disabled, existing PID files are read
     ++		# to help diagnose stale locks
     ++		test_must_fail git add . 2>err &&
     ++		test_grep "process 99999" err
     ++	)
     ++'
     ++
     ++test_expect_success 'negative component syntax excludes specific components' '
     ++	git init repo11 &&
     ++	(
     ++		cd repo11 &&
     ++		echo content >file &&
     ++		# Enable all components except index
     ++		git -c core.lockfilePid=all,-index add file &&
     ++		# PID file should not be created for index when excluded
     ++		test_path_is_missing .git/index.pid.lock
      +	)
      +'
      +
      +test_done
     +
     + ## unix-stream-server.c ##
     +@@ unix-stream-server.c: int unix_ss_create(const char *path,
     + 	/*
     + 	 * Create a lock at "<path>.lock" if we can.
     + 	 */
     +-	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms) < 0)
     ++	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms,
     ++					      LOCKFILE_PID_OTHER) < 0)
     + 		return -1;
     + 
     + 	/*


 Documentation/config/core.adoc |  26 +++++
 apply.c                        |   6 +-
 builtin/commit.c               |   2 +-
 builtin/credential-store.c     |   3 +-
 builtin/difftool.c             |   3 +-
 builtin/fast-import.c          |   3 +-
 builtin/gc.c                   |  13 ++-
 builtin/sparse-checkout.c      |   3 +-
 bundle.c                       |   3 +-
 cache-tree.c                   |   3 +-
 commit-graph.c                 |   6 +-
 compat/mingw.c                 |  10 ++
 config.c                       |   6 +-
 environment.c                  |  80 +++++++++++++++
 lockfile.c                     | 182 +++++++++++++++++++++++++++++----
 lockfile.h                     |  77 ++++++++++----
 loose.c                        |   6 +-
 midx-write.c                   |   6 +-
 odb.c                          |   3 +-
 refs/files-backend.c           |   9 +-
 refs/packed-backend.c          |   3 +-
 reftable/system.c              |   2 +-
 repository.c                   |   3 +-
 rerere.c                       |   3 +-
 sequencer.c                    |  11 +-
 shallow.c                      |   6 +-
 t/meson.build                  |   1 +
 t/t0031-lockfile-pid.sh        | 139 +++++++++++++++++++++++++
 unix-stream-server.c           |   3 +-
 29 files changed, 546 insertions(+), 75 deletions(-)
 create mode 100755 t/t0031-lockfile-pid.sh

diff --git a/Documentation/config/core.adoc b/Documentation/config/core.adoc
index 11efad189e..45597b9cea 100644
--- a/Documentation/config/core.adoc
+++ b/Documentation/config/core.adoc
@@ -348,6 +348,32 @@ confusion unless you know what you are doing (e.g. you are creating a
 read-only snapshot of the same index to a location different from the
 repository's usual working tree).
 
+core.lockfilePid::
+	A comma-separated list of components for which Git should create
+	a PID file alongside the lock file. When a lock acquisition fails
+	and a PID file exists, Git can provide additional diagnostic
+	information about the process holding the lock, including whether
+	it is still running.
++
+This feature is disabled by default. You can enable it for specific
+components or use `all` to enable for all components.
++
+* `none` disables PID file creation for all components.
+* `index` creates PID files for index lock operations.
+* `config` creates PID files for config file lock operations.
+* `refs` creates PID files for reference lock operations.
+* `commit-graph` creates PID files for commit-graph lock operations.
+* `midx` creates PID files for multi-pack-index lock operations.
+* `shallow` creates PID files for shallow file lock operations.
+* `gc` creates PID files for garbage collection lock operations.
+* `other` creates PID files for other miscellaneous lock operations.
+* `all` enables PID file creation for all components.
++
+The PID file is named by inserting `.pid` before the `.lock` suffix.
+For example, if the lock file is `index.lock`, the PID file will be
+`index.pid.lock`. The file contains `pid <value>` on a single line,
+following the same key-value format used by Git object headers.
+
 core.logAllRefUpdates::
 	Enable the reflog. Updates to a ref <ref> is logged to the file
 	"`$GIT_DIR/logs/<ref>`", by appending the new and old
diff --git a/apply.c b/apply.c
index a2ceb3fb40..95e3bafabd 100644
--- a/apply.c
+++ b/apply.c
@@ -4173,7 +4173,8 @@ static int build_fake_ancestor(struct apply_state *state, struct patch *list)
 		}
 	}
 
-	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 	res = write_locked_index(&result, &lock, COMMIT_LOCK);
 	discard_index(&result);
 
@@ -4830,7 +4831,8 @@ static int apply_patch(struct apply_state *state,
 		if (state->index_file)
 			hold_lock_file_for_update(&state->lock_file,
 						  state->index_file,
-						  LOCK_DIE_ON_ERROR);
+						  LOCK_DIE_ON_ERROR,
+						  LOCKFILE_PID_INDEX);
 		else
 			repo_hold_locked_index(state->repo, &state->lock_file,
 					       LOCK_DIE_ON_ERROR);
diff --git a/builtin/commit.c b/builtin/commit.c
index 0243f17d53..30db6409a1 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -540,7 +540,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 	path = repo_git_path(the_repository, "next-index-%"PRIuMAX,
 			     (uintmax_t) getpid());
 	hold_lock_file_for_update(&false_lock, path,
-				  LOCK_DIE_ON_ERROR);
+				  LOCK_DIE_ON_ERROR, LOCKFILE_PID_OTHER);
 
 	create_base_index(current_head);
 	add_remove_files(&partial);
diff --git a/builtin/credential-store.c b/builtin/credential-store.c
index b74e06cc93..e1d1be5cd1 100644
--- a/builtin/credential-store.c
+++ b/builtin/credential-store.c
@@ -67,7 +67,8 @@ static void rewrite_credential_file(const char *fn, struct credential *c,
 	int timeout_ms = 1000;
 
 	repo_config_get_int(the_repository, "credentialstore.locktimeoutms", &timeout_ms);
-	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms) < 0)
+	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms,
+					      LOCKFILE_PID_CONFIG) < 0)
 		die_errno(_("unable to get credential storage lock in %d ms"), timeout_ms);
 	if (extra)
 		print_line(extra);
diff --git a/builtin/difftool.c b/builtin/difftool.c
index e4bc1f8316..15d0af0031 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -636,7 +636,8 @@ static int run_dir_diff(struct repository *repo,
 			struct lock_file lock = LOCK_INIT;
 			strbuf_reset(&buf);
 			strbuf_addf(&buf, "%s/wtindex", tmpdir.buf);
-			if (hold_lock_file_for_update(&lock, buf.buf, 0) < 0 ||
+			if (hold_lock_file_for_update(&lock, buf.buf, 0,
+					      LOCKFILE_PID_OTHER) < 0 ||
 			    write_locked_index(&wtindex, &lock, COMMIT_LOCK)) {
 				ret = error("could not write %s", buf.buf);
 				goto finish;
diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 4cd0b079b6..9715f04d2d 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -1734,7 +1734,8 @@ static void dump_marks(void)
 		return;
 	}
 
-	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0) < 0) {
+	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0,
+				      LOCKFILE_PID_OTHER) < 0) {
 		failure |= error_errno(_("unable to write marks file %s"),
 				       export_marks_file);
 		return;
diff --git a/builtin/gc.c b/builtin/gc.c
index e9a76243aa..6ba56d784d 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -749,7 +749,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 
 	pidfile_path = repo_git_path(the_repository, "gc.pid");
 	fd = hold_lock_file_for_update(&lock, pidfile_path,
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
 	if (!force) {
 		static char locking_host[HOST_NAME_MAX + 1];
 		static char *scan_fmt;
@@ -1017,7 +1017,7 @@ int cmd_gc(int argc,
 	if (daemonized) {
 		char *path = repo_git_path(the_repository, "gc.log");
 		hold_lock_file_for_update(&log_lock, path,
-					  LOCK_DIE_ON_ERROR);
+					  LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
 		dup2(get_lock_file_fd(&log_lock), 2);
 		atexit(process_log_file_at_exit);
 		free(path);
@@ -1797,7 +1797,8 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts,
 	struct repository *r = the_repository;
 	char *lock_path = xstrfmt("%s/maintenance", r->objects->sources->path);
 
-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
+				      LOCKFILE_PID_GC) < 0) {
 		/*
 		 * Another maintenance command is running.
 		 *
@@ -2561,7 +2562,8 @@ static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
 		lock_file_timeout_ms = 150;
 
 	fd = hold_lock_file_for_update_timeout(&lk, filename, LOCK_DIE_ON_ERROR,
-					       lock_file_timeout_ms);
+					       lock_file_timeout_ms,
+					       LOCKFILE_PID_GC);
 
 	/*
 	 * Does this file already exist? With the intended contents? Is it
@@ -3372,7 +3374,8 @@ static int update_background_schedule(const struct maintenance_start_opts *opts,
 	struct lock_file lk;
 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->sources->path);
 
-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
+				      LOCKFILE_PID_GC) < 0) {
 		if (errno == EEXIST)
 			error(_("unable to create '%s.lock': %s.\n\n"
 			    "Another scheduled git-maintenance(1) process seems to be running in this\n"
diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
index 15d51e60a8..566df7d229 100644
--- a/builtin/sparse-checkout.c
+++ b/builtin/sparse-checkout.c
@@ -341,7 +341,8 @@ static int write_patterns_and_update(struct repository *repo,
 	if (safe_create_leading_directories(repo, sparse_filename))
 		die(_("failed to create directory for sparse-checkout file"));
 
-	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 
 	result = update_working_directory(repo, pl);
 	if (result) {
diff --git a/bundle.c b/bundle.c
index 42327f9739..25728278fc 100644
--- a/bundle.c
+++ b/bundle.c
@@ -520,7 +520,8 @@ int create_bundle(struct repository *r, const char *path,
 		bundle_fd = 1;
 	else
 		bundle_fd = hold_lock_file_for_update(&lock, path,
-						      LOCK_DIE_ON_ERROR);
+						      LOCK_DIE_ON_ERROR,
+						      LOCKFILE_PID_OTHER);
 
 	if (version == -1)
 		version = min_version;
diff --git a/cache-tree.c b/cache-tree.c
index 2aba47060e..b370fa94d0 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -730,7 +730,8 @@ int write_index_as_tree(struct object_id *oid, struct index_state *index_state,
 	struct lock_file lock_file = LOCK_INIT;
 	int ret;
 
-	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_INDEX);
 
 	entries = read_index_from(index_state, index_path,
 				  repo_get_git_dir(the_repository));
diff --git a/commit-graph.c b/commit-graph.c
index 80be2ff2c3..b18249fb73 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -2075,7 +2075,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		char *lock_name = get_commit_graph_chain_filename(ctx->odb_source);
 
 		hold_lock_file_for_update_mode(&lk, lock_name,
-					       LOCK_DIE_ON_ERROR, 0444);
+					       LOCK_DIE_ON_ERROR, 0444,
+					       LOCKFILE_PID_COMMIT_GRAPH);
 		free(lock_name);
 
 		graph_layer = mks_tempfile_m(ctx->graph_name, 0444);
@@ -2094,7 +2095,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 			   get_tempfile_fd(graph_layer), get_tempfile_path(graph_layer));
 	} else {
 		hold_lock_file_for_update_mode(&lk, ctx->graph_name,
-					       LOCK_DIE_ON_ERROR, 0444);
+					       LOCK_DIE_ON_ERROR, 0444,
+					       LOCKFILE_PID_COMMIT_GRAPH);
 		f = hashfd(ctx->r->hash_algo,
 			   get_lock_file_fd(&lk), get_lock_file_path(&lk));
 	}
diff --git a/compat/mingw.c b/compat/mingw.c
index 736a07a028..cbd102fa0e 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1982,6 +1982,16 @@ int mingw_kill(pid_t pid, int sig)
 			CloseHandle(h);
 			return 0;
 		}
+		/*
+		 * OpenProcess returns ERROR_INVALID_PARAMETER for
+		 * non-existent PIDs. Map this to ESRCH for POSIX
+		 * compatibility with kill(pid, 0).
+		 */
+		if (GetLastError() == ERROR_INVALID_PARAMETER)
+			errno = ESRCH;
+		else
+			errno = err_win_to_posix(GetLastError());
+		return -1;
 	}
 
 	errno = EINVAL;
diff --git a/config.c b/config.c
index f1def0dcfb..ac35427523 100644
--- a/config.c
+++ b/config.c
@@ -2985,7 +2985,8 @@ int repo_config_set_multivar_in_file_gently(struct repository *r,
 	 * The lock serves a purpose in addition to locking: the new
 	 * contents of .git/config will be written into it.
 	 */
-	fd = hold_lock_file_for_update(&lock, config_filename, 0);
+	fd = hold_lock_file_for_update(&lock, config_filename, 0,
+				       LOCKFILE_PID_CONFIG);
 	if (fd < 0) {
 		error_errno(_("could not lock config file %s"), config_filename);
 		ret = CONFIG_NO_LOCK;
@@ -3330,7 +3331,8 @@ static int repo_config_copy_or_rename_section_in_file(
 	if (!config_filename)
 		config_filename = filename_buf = repo_git_path(r, "config");
 
-	out_fd = hold_lock_file_for_update(&lock, config_filename, 0);
+	out_fd = hold_lock_file_for_update(&lock, config_filename, 0,
+					   LOCKFILE_PID_CONFIG);
 	if (out_fd < 0) {
 		ret = error(_("could not lock config file %s"), config_filename);
 		goto out;
diff --git a/environment.c b/environment.c
index a770b5921d..0d1dfb525b 100644
--- a/environment.c
+++ b/environment.c
@@ -21,6 +21,7 @@
 #include "gettext.h"
 #include "git-zlib.h"
 #include "ident.h"
+#include "lockfile.h"
 #include "mailmap.h"
 #include "object-name.h"
 #include "repository.h"
@@ -324,6 +325,78 @@ next_name:
 	return (current & ~negative) | positive;
 }
 
+static const struct lockfile_pid_component_name {
+	const char *name;
+	enum lockfile_pid_component component_bits;
+} lockfile_pid_component_names[] = {
+	{ "index", LOCKFILE_PID_INDEX },
+	{ "config", LOCKFILE_PID_CONFIG },
+	{ "refs", LOCKFILE_PID_REFS },
+	{ "commit-graph", LOCKFILE_PID_COMMIT_GRAPH },
+	{ "midx", LOCKFILE_PID_MIDX },
+	{ "shallow", LOCKFILE_PID_SHALLOW },
+	{ "gc", LOCKFILE_PID_GC },
+	{ "other", LOCKFILE_PID_OTHER },
+	{ "all", LOCKFILE_PID_ALL },
+};
+
+static enum lockfile_pid_component parse_lockfile_pid_components(const char *var,
+								 const char *string)
+{
+	enum lockfile_pid_component current = LOCKFILE_PID_DEFAULT;
+	enum lockfile_pid_component positive = 0, negative = 0;
+
+	while (string) {
+		size_t len;
+		const char *ep;
+		int negated = 0;
+		int found = 0;
+
+		string = string + strspn(string, ", \t\n\r");
+		ep = strchrnul(string, ',');
+		len = ep - string;
+		if (len == 4 && !strncmp(string, "none", 4)) {
+			current = LOCKFILE_PID_NONE;
+			goto next_name;
+		}
+
+		if (*string == '-') {
+			negated = 1;
+			string++;
+			len--;
+			if (!len)
+				warning(_("invalid value for variable %s"), var);
+		}
+
+		if (!len)
+			break;
+
+		for (size_t i = 0; i < ARRAY_SIZE(lockfile_pid_component_names); ++i) {
+			const struct lockfile_pid_component_name *n = &lockfile_pid_component_names[i];
+
+			if (strncmp(n->name, string, len) || strlen(n->name) != len)
+				continue;
+
+			found = 1;
+			if (negated)
+				negative |= n->component_bits;
+			else
+				positive |= n->component_bits;
+		}
+
+		if (!found) {
+			char *component = xstrndup(string, len);
+			warning(_("ignoring unknown core.lockfilePid component '%s'"), component);
+			free(component);
+		}
+
+next_name:
+		string = ep;
+	}
+
+	return (current & ~negative) | positive;
+}
+
 static int git_default_core_config(const char *var, const char *value,
 				   const struct config_context *ctx, void *cb)
 {
@@ -532,6 +605,13 @@ static int git_default_core_config(const char *var, const char *value,
 		return 0;
 	}
 
+	if (!strcmp(var, "core.lockfilepid")) {
+		if (!value)
+			return config_error_nonbool(var);
+		lockfile_pid_components = parse_lockfile_pid_components(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.createobject")) {
 		if (!value)
 			return config_error_nonbool(var);
diff --git a/lockfile.c b/lockfile.c
index 1d5ed01682..4ee215374a 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -6,6 +6,9 @@
 #include "abspath.h"
 #include "gettext.h"
 #include "lockfile.h"
+#include "parse.h"
+#include "strbuf.h"
+#include "wrapper.h"
 
 /*
  * path = absolute or relative path name
@@ -71,19 +74,117 @@ static void resolve_symlink(struct strbuf *path)
 	strbuf_reset(&link);
 }
 
+/*
+ * Lock PID file functions - write PID to a foo-pid.lock file alongside
+ * the lock file for debugging stale locks. The PID file is registered
+ * as a tempfile so it gets cleaned up by signal/atexit handlers.
+ *
+ * Naming: For "foo.lock", the PID file is "foo-pid.lock" (not "foo.lock.pid").
+ * This avoids collision with the refs namespace.
+ */
+
+/* Global config variable, initialized from core.lockfilePid */
+enum lockfile_pid_component lockfile_pid_components = LOCKFILE_PID_DEFAULT;
+
+/*
+ * Path generation helpers.
+ * Given base path "foo", generate:
+ *   - lock path: "foo.lock"
+ *   - pid path:  "foo-pid.lock"
+ */
+static void get_lock_path(struct strbuf *out, const char *path)
+{
+	strbuf_addstr(out, path);
+	strbuf_addstr(out, LOCK_SUFFIX);
+}
+
+static void get_pid_path(struct strbuf *out, const char *path)
+{
+	strbuf_addstr(out, path);
+	strbuf_addstr(out, LOCK_PID_INFIX);
+	strbuf_addstr(out, LOCK_SUFFIX);
+}
+
+static struct tempfile *create_lock_pid_file(const char *pid_path, int mode,
+					     enum lockfile_pid_component component)
+{
+	struct strbuf content = STRBUF_INIT;
+	struct tempfile *pid_tempfile = NULL;
+	int fd = -1;
+
+	if (!(lockfile_pid_components & component))
+		goto out;
+
+	fd = open(pid_path, O_WRONLY | O_CREAT | O_EXCL, mode);
+	if (fd < 0)
+		goto out;
+
+	strbuf_addf(&content, "pid %" PRIuMAX "\n", (uintmax_t)getpid());
+	if (write_in_full(fd, content.buf, content.len) < 0) {
+		warning_errno(_("could not write lock pid file '%s'"), pid_path);
+		goto out;
+	}
+
+	close(fd);
+	fd = -1;
+	pid_tempfile = register_tempfile(pid_path);
+
+out:
+	if (fd >= 0)
+		close(fd);
+	strbuf_release(&content);
+	return pid_tempfile;
+}
+
+static int read_lock_pid(const char *pid_path, uintmax_t *pid_out)
+{
+	struct strbuf content = STRBUF_INIT;
+	const char *val;
+	int ret = -1;
+
+	if (strbuf_read_file(&content, pid_path, LOCK_PID_MAXLEN) <= 0)
+		goto out;
+
+	strbuf_rtrim(&content);
+
+	if (skip_prefix(content.buf, "pid ", &val)) {
+		char *endptr;
+		*pid_out = strtoumax(val, &endptr, 10);
+		if (*pid_out > 0 && !*endptr)
+			ret = 0;
+	}
+
+	if (ret)
+		warning(_("malformed lock pid file '%s'"), pid_path);
+
+out:
+	strbuf_release(&content);
+	return ret;
+}
+
 /* Make sure errno contains a meaningful value on error */
 static int lock_file(struct lock_file *lk, const char *path, int flags,
-		     int mode)
+		     int mode, enum lockfile_pid_component component)
 {
-	struct strbuf filename = STRBUF_INIT;
+	struct strbuf base_path = STRBUF_INIT;
+	struct strbuf lock_path = STRBUF_INIT;
+	struct strbuf pid_path = STRBUF_INIT;
 
-	strbuf_addstr(&filename, path);
+	strbuf_addstr(&base_path, path);
 	if (!(flags & LOCK_NO_DEREF))
-		resolve_symlink(&filename);
+		resolve_symlink(&base_path);
+
+	get_lock_path(&lock_path, base_path.buf);
+	get_pid_path(&pid_path, base_path.buf);
+
+	lk->tempfile = create_tempfile_mode(lock_path.buf, mode);
+	if (lk->tempfile)
+		lk->pid_tempfile = create_lock_pid_file(pid_path.buf, mode,
+							component);
 
-	strbuf_addstr(&filename, LOCK_SUFFIX);
-	lk->tempfile = create_tempfile_mode(filename.buf, mode);
-	strbuf_release(&filename);
+	strbuf_release(&base_path);
+	strbuf_release(&lock_path);
+	strbuf_release(&pid_path);
 	return lk->tempfile ? lk->tempfile->fd : -1;
 }
 
@@ -102,7 +203,8 @@ static int lock_file(struct lock_file *lk, const char *path, int flags,
  * exactly once. If timeout_ms is -1, try indefinitely.
  */
 static int lock_file_timeout(struct lock_file *lk, const char *path,
-			     int flags, long timeout_ms, int mode)
+			     int flags, long timeout_ms, int mode,
+			     enum lockfile_pid_component component)
 {
 	int n = 1;
 	int multiplier = 1;
@@ -110,7 +212,7 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 	static int random_initialized = 0;
 
 	if (timeout_ms == 0)
-		return lock_file(lk, path, flags, mode);
+		return lock_file(lk, path, flags, mode, component);
 
 	if (!random_initialized) {
 		srand((unsigned int)getpid());
@@ -124,7 +226,7 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 		long backoff_ms, wait_ms;
 		int fd;
 
-		fd = lock_file(lk, path, flags, mode);
+		fd = lock_file(lk, path, flags, mode, component);
 
 		if (fd >= 0)
 			return fd; /* success */
@@ -151,16 +253,47 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
 {
 	if (err == EEXIST) {
-		strbuf_addf(buf, _("Unable to create '%s.lock': %s.\n\n"
-		    "Another git process seems to be running in this repository, e.g.\n"
-		    "an editor opened by 'git commit'. Please make sure all processes\n"
-		    "are terminated then try again. If it still fails, a git process\n"
-		    "may have crashed in this repository earlier:\n"
-		    "remove the file manually to continue."),
-			    absolute_path(path), strerror(err));
-	} else
+		const char *abs_path = absolute_path(path);
+		struct strbuf lock_path = STRBUF_INIT;
+		struct strbuf pid_path = STRBUF_INIT;
+		uintmax_t pid;
+		int pid_status = 0; /* 0 = unknown, 1 = running, -1 = stale */
+
+		get_lock_path(&lock_path, abs_path);
+		get_pid_path(&pid_path, abs_path);
+
+		strbuf_addf(buf, _("Unable to create '%s': %s.\n\n"),
+			    lock_path.buf, strerror(err));
+
+		/*
+		 * Try to read PID file unconditionally - it may exist if
+		 * core.lockfilePid was enabled for this component.
+		 */
+		if (!read_lock_pid(pid_path.buf, &pid)) {
+			if (kill((pid_t)pid, 0) == 0 || errno == EPERM)
+				pid_status = 1;  /* running (or no permission to signal) */
+			else if (errno == ESRCH)
+				pid_status = -1; /* no such process - stale lock */
+		}
+
+		if (pid_status == 1)
+			strbuf_addf(buf, _("Lock is held by process %" PRIuMAX ". "
+					   "Wait for it to finish, or remove the lock file to continue"),
+				    pid);
+		else if (pid_status == -1)
+			strbuf_addf(buf, _("Lock was held by process %" PRIuMAX ", "
+					   "which is no longer running. Remove the stale lock file to continue"),
+				    pid);
+		else
+			strbuf_addstr(buf, _("Another git process seems to be running in this repository. "
+					     "Wait for it to finish, or remove the lock file to continue"));
+
+		strbuf_release(&lock_path);
+		strbuf_release(&pid_path);
+	} else {
 		strbuf_addf(buf, _("Unable to create '%s.lock': %s"),
 			    absolute_path(path), strerror(err));
+	}
 }
 
 NORETURN void unable_to_lock_die(const char *path, int err)
@@ -174,9 +307,10 @@ NORETURN void unable_to_lock_die(const char *path, int err)
 /* This should return a meaningful errno on failure */
 int hold_lock_file_for_update_timeout_mode(struct lock_file *lk,
 					   const char *path, int flags,
-					   long timeout_ms, int mode)
+					   long timeout_ms, int mode,
+					   enum lockfile_pid_component component)
 {
-	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode);
+	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode, component);
 	if (fd < 0) {
 		if (flags & LOCK_DIE_ON_ERROR)
 			unable_to_lock_die(path, errno);
@@ -207,6 +341,8 @@ int commit_lock_file(struct lock_file *lk)
 {
 	char *result_path = get_locked_file_path(lk);
 
+	delete_tempfile(&lk->pid_tempfile);
+
 	if (commit_lock_file_to(lk, result_path)) {
 		int save_errno = errno;
 		free(result_path);
@@ -216,3 +352,9 @@ int commit_lock_file(struct lock_file *lk)
 	free(result_path);
 	return 0;
 }
+
+int rollback_lock_file(struct lock_file *lk)
+{
+	delete_tempfile(&lk->pid_tempfile);
+	return delete_tempfile(&lk->tempfile);
+}
diff --git a/lockfile.h b/lockfile.h
index 1bb9926497..7e472cb5af 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -119,6 +119,7 @@
 
 struct lock_file {
 	struct tempfile *tempfile;
+	struct tempfile *pid_tempfile;
 };
 
 #define LOCK_INIT { 0 }
@@ -127,6 +128,42 @@ struct lock_file {
 #define LOCK_SUFFIX ".lock"
 #define LOCK_SUFFIX_LEN 5
 
+/*
+ * PID file naming: for a lock file "foo.lock", the PID file is "foo.pid.lock".
+ */
+#define LOCK_PID_INFIX ".pid"
+#define LOCK_PID_INFIX_LEN 4
+
+/* Maximum length for PID file content */
+#define LOCK_PID_MAXLEN 32
+
+/*
+ * Per-component lock PID file configuration, following core.fsync pattern.
+ * Each component can be individually enabled via core.lockfilePid config.
+ */
+enum lockfile_pid_component {
+	LOCKFILE_PID_NONE = 0,
+	LOCKFILE_PID_INDEX = 1 << 0, /* .git/index.lock */
+	LOCKFILE_PID_CONFIG = 1 << 1, /* .git/config.lock */
+	LOCKFILE_PID_REFS = 1 << 2, /* refs locks */
+	LOCKFILE_PID_COMMIT_GRAPH = 1 << 3, /* commit-graph.lock */
+	LOCKFILE_PID_MIDX = 1 << 4, /* multi-pack-index.lock */
+	LOCKFILE_PID_SHALLOW = 1 << 5, /* shallow file */
+	LOCKFILE_PID_GC = 1 << 6, /* gc locks */
+	LOCKFILE_PID_OTHER = 1 << 7, /* other locks */
+};
+
+#define LOCKFILE_PID_ALL (LOCKFILE_PID_INDEX | LOCKFILE_PID_CONFIG |      \
+			  LOCKFILE_PID_REFS | LOCKFILE_PID_COMMIT_GRAPH | \
+			  LOCKFILE_PID_MIDX | LOCKFILE_PID_SHALLOW |      \
+			  LOCKFILE_PID_GC | LOCKFILE_PID_OTHER)
+#define LOCKFILE_PID_DEFAULT LOCKFILE_PID_NONE
+
+/*
+ * Bitmask indicating which components should create PID files.
+ * Configured via core.lockfilePid.
+ */
+extern enum lockfile_pid_component lockfile_pid_components;
 
 /*
  * Flags
@@ -167,17 +204,23 @@ struct lock_file {
  * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
  * timeout_ms is -1, retry indefinitely. The flags argument, error
  * handling, and mode are described above.
+ *
+ * The `component` argument specifies which lock PID component this lock
+ * belongs to, for selective PID file creation via core.lockfilePid config.
  */
 int hold_lock_file_for_update_timeout_mode(
-		struct lock_file *lk, const char *path,
-		int flags, long timeout_ms, int mode);
+	struct lock_file *lk, const char *path,
+	int flags, long timeout_ms, int mode,
+	enum lockfile_pid_component component);
 
 static inline int hold_lock_file_for_update_timeout(
-		struct lock_file *lk, const char *path,
-		int flags, long timeout_ms)
+	struct lock_file *lk, const char *path,
+	int flags, long timeout_ms,
+	enum lockfile_pid_component component)
 {
 	return hold_lock_file_for_update_timeout_mode(lk, path, flags,
-						      timeout_ms, 0666);
+						      timeout_ms, 0666,
+						      component);
 }
 
 /*
@@ -186,17 +229,18 @@ static inline int hold_lock_file_for_update_timeout(
  * argument and error handling are described above.
  */
 static inline int hold_lock_file_for_update(
-		struct lock_file *lk, const char *path,
-		int flags)
+	struct lock_file *lk, const char *path,
+	int flags, enum lockfile_pid_component component)
 {
-	return hold_lock_file_for_update_timeout(lk, path, flags, 0);
+	return hold_lock_file_for_update_timeout(lk, path, flags, 0, component);
 }
 
 static inline int hold_lock_file_for_update_mode(
-		struct lock_file *lk, const char *path,
-		int flags, int mode)
+	struct lock_file *lk, const char *path,
+	int flags, int mode, enum lockfile_pid_component component)
 {
-	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode);
+	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode,
+						      component);
 }
 
 /*
@@ -319,13 +363,10 @@ static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
 
 /*
  * Roll back `lk`: close the file descriptor and/or file pointer and
- * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
- * for a `lock_file` object that has already been committed or rolled
- * back. No error will be returned in this case.
+ * remove the lockfile and any associated PID file. It is a NOOP to
+ * call `rollback_lock_file()` for a `lock_file` object that has already
+ * been committed or rolled back. No error will be returned in this case.
  */
-static inline int rollback_lock_file(struct lock_file *lk)
-{
-	return delete_tempfile(&lk->tempfile);
-}
+int rollback_lock_file(struct lock_file *lk);
 
 #endif /* LOCKFILE_H */
diff --git a/loose.c b/loose.c
index 56cf64b648..c0d2f0416f 100644
--- a/loose.c
+++ b/loose.c
@@ -135,7 +135,8 @@ int repo_write_loose_object_map(struct repository *repo)
 		return 0;
 
 	repo_common_path_replace(repo, &path, "objects/loose-object-idx");
-	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
+					       LOCKFILE_PID_OTHER);
 	iter = kh_begin(map);
 	if (write_in_full(fd, loose_object_header, strlen(loose_object_header)) < 0)
 		goto errout;
@@ -177,7 +178,8 @@ static int write_one_object(struct odb_source *source,
 	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
 
 	strbuf_addf(&path, "%s/loose-object-idx", source->path);
-	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
+					  LOCKFILE_PID_OTHER);
 
 	fd = open(path.buf, O_WRONLY | O_CREAT | O_APPEND, 0666);
 	if (fd < 0)
diff --git a/midx-write.c b/midx-write.c
index 23e61cb000..6196dcd7cd 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1309,7 +1309,8 @@ static int write_midx_internal(struct odb_source *source,
 		struct strbuf lock_name = STRBUF_INIT;
 
 		get_midx_chain_filename(source, &lock_name);
-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR,
+					  LOCKFILE_PID_MIDX);
 		strbuf_release(&lock_name);
 
 		incr = mks_tempfile_m(midx_name.buf, 0444);
@@ -1327,7 +1328,8 @@ static int write_midx_internal(struct odb_source *source,
 		f = hashfd(r->hash_algo, get_tempfile_fd(incr),
 			   get_tempfile_path(incr));
 	} else {
-		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR,
+					  LOCKFILE_PID_MIDX);
 		f = hashfd(r->hash_algo, get_lock_file_fd(&lk),
 			   get_lock_file_path(&lk));
 	}
diff --git a/odb.c b/odb.c
index 3ec21ef24e..570cea3029 100644
--- a/odb.c
+++ b/odb.c
@@ -292,7 +292,8 @@ void odb_add_to_alternates_file(struct object_database *odb,
 	FILE *in, *out;
 	int found = 0;
 
-	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 	out = fdopen_lock_file(&lock, "w");
 	if (!out)
 		die_errno(_("unable to fdopen alternates lockfile"));
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 6f6f76a8d8..e296154fe7 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -790,7 +790,8 @@ retry:
 
 	if (hold_lock_file_for_update_timeout(
 			    &lock->lk, ref_file.buf, LOCK_NO_DEREF,
-			    get_files_ref_lock_timeout_ms()) < 0) {
+			    get_files_ref_lock_timeout_ms(),
+			    LOCKFILE_PID_REFS) < 0) {
 		int myerr = errno;
 		errno = 0;
 		if (myerr == ENOENT && --attempts_remaining > 0) {
@@ -1194,7 +1195,8 @@ static int create_reflock(const char *path, void *cb)
 
 	return hold_lock_file_for_update_timeout(
 			lk, path, LOCK_NO_DEREF,
-			get_files_ref_lock_timeout_ms()) < 0 ? -1 : 0;
+			get_files_ref_lock_timeout_ms(),
+			LOCKFILE_PID_REFS) < 0 ? -1 : 0;
 }
 
 /*
@@ -3536,7 +3538,8 @@ static int files_reflog_expire(struct ref_store *ref_store,
 		 * work we need, including cleaning up if the program
 		 * exits unexpectedly.
 		 */
-		if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
+		if (hold_lock_file_for_update(&reflog_lock, log_file, 0,
+					      LOCKFILE_PID_REFS) < 0) {
 			struct strbuf err = STRBUF_INIT;
 			unable_to_lock_message(log_file, errno, &err);
 			error("%s", err.buf);
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..f832c49eae 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1230,7 +1230,8 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 	if (hold_lock_file_for_update_timeout(
 			    &refs->lock,
 			    refs->path,
-			    flags, timeout_value) < 0) {
+			    flags, timeout_value,
+			    LOCKFILE_PID_REFS) < 0) {
 		unable_to_lock_message(refs->path, errno, err);
 		return -1;
 	}
diff --git a/reftable/system.c b/reftable/system.c
index 725a25844e..5462111a04 100644
--- a/reftable/system.c
+++ b/reftable/system.c
@@ -67,7 +67,7 @@ int flock_acquire(struct reftable_flock *l, const char *target_path,
 		return REFTABLE_OUT_OF_MEMORY_ERROR;
 
 	err = hold_lock_file_for_update_timeout(lockfile, target_path, LOCK_NO_DEREF,
-						timeout_ms);
+						timeout_ms, LOCKFILE_PID_REFS);
 	if (err < 0) {
 		reftable_free(lockfile);
 		if (errno == EEXIST)
diff --git a/repository.c b/repository.c
index 6aaa7ba008..30dde7076b 100644
--- a/repository.c
+++ b/repository.c
@@ -460,5 +460,6 @@ int repo_hold_locked_index(struct repository *repo,
 {
 	if (!repo->index_file)
 		BUG("the repo hasn't been setup");
-	return hold_lock_file_for_update(lf, repo->index_file, flags);
+	return hold_lock_file_for_update(lf, repo->index_file, flags,
+					 LOCKFILE_PID_INDEX);
 }
diff --git a/rerere.c b/rerere.c
index 6ec55964e2..71f1d5b176 100644
--- a/rerere.c
+++ b/rerere.c
@@ -917,7 +917,8 @@ int setup_rerere(struct repository *r, struct string_list *merge_rr, int flags)
 	else
 		fd = hold_lock_file_for_update(&write_lock,
 					       git_path_merge_rr(r),
-					       LOCK_DIE_ON_ERROR);
+					       LOCK_DIE_ON_ERROR,
+					       LOCKFILE_PID_OTHER);
 	read_rr(r, merge_rr);
 	return fd;
 }
diff --git a/sequencer.c b/sequencer.c
index 5476d39ba9..337f82e02c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -566,7 +566,8 @@ static int write_message(const void *buf, size_t len, const char *filename,
 {
 	struct lock_file msg_file = LOCK_INIT;
 
-	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0);
+	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0,
+					       LOCKFILE_PID_OTHER);
 	if (msg_fd < 0)
 		return error_errno(_("could not lock '%s'"), filename);
 	if (write_in_full(msg_fd, buf, len) < 0) {
@@ -3605,7 +3606,8 @@ static int save_todo(struct todo_list *todo_list, struct replay_opts *opts,
 	if (is_rebase_i(opts) && !reschedule)
 		next++;
 
-	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0);
+	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0,
+				       LOCKFILE_PID_OTHER);
 	if (fd < 0)
 		return error_errno(_("could not lock '%s'"), todo_path);
 	offset = get_item_line_offset(todo_list, next);
@@ -3877,7 +3879,8 @@ static int safe_append(const char *filename, const char *fmt, ...)
 	va_list ap;
 	struct lock_file lock = LOCK_INIT;
 	int fd = hold_lock_file_for_update(&lock, filename,
-					   LOCK_REPORT_ON_ERROR);
+					   LOCK_REPORT_ON_ERROR,
+					   LOCKFILE_PID_OTHER);
 	struct strbuf buf = STRBUF_INIT;
 
 	if (fd < 0)
@@ -4400,7 +4403,7 @@ static int write_update_refs_state(struct string_list *refs_to_oids)
 		goto cleanup;
 	}
 
-	if (hold_lock_file_for_update(&lock, path, 0) < 0) {
+	if (hold_lock_file_for_update(&lock, path, 0, LOCKFILE_PID_OTHER) < 0) {
 		result = error(_("another 'rebase' process appears to be running; "
 				 "'%s.lock' already exists"),
 			       path);
diff --git a/shallow.c b/shallow.c
index 55b9cd9d3f..412c37c96a 100644
--- a/shallow.c
+++ b/shallow.c
@@ -392,7 +392,8 @@ void setup_alternate_shallow(struct shallow_lock *shallow_lock,
 
 	fd = hold_lock_file_for_update(&shallow_lock->lock,
 				       git_path_shallow(the_repository),
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR,
+				       LOCKFILE_PID_SHALLOW);
 	check_shallow_file_for_update(the_repository);
 	if (write_shallow_commits(&sb, 0, extra)) {
 		if (write_in_full(fd, sb.buf, sb.len) < 0)
@@ -447,7 +448,8 @@ void prune_shallow(unsigned options)
 	}
 	fd = hold_lock_file_for_update(&shallow_lock.lock,
 				       git_path_shallow(the_repository),
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR,
+				       LOCKFILE_PID_SHALLOW);
 	check_shallow_file_for_update(the_repository);
 	if (write_shallow_commits_1(&sb, 0, NULL, flags)) {
 		if (write_in_full(fd, sb.buf, sb.len) < 0)
diff --git a/t/meson.build b/t/meson.build
index dc43d69636..9a880db57c 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -98,6 +98,7 @@ integration_tests = [
   't0028-working-tree-encoding.sh',
   't0029-core-unsetenvvars.sh',
   't0030-stripspace.sh',
+  't0031-lockfile-pid.sh',
   't0033-safe-directory.sh',
   't0034-root-safe-directory.sh',
   't0035-safe-bare-repository.sh',
diff --git a/t/t0031-lockfile-pid.sh b/t/t0031-lockfile-pid.sh
new file mode 100755
index 0000000000..d99f738cb3
--- /dev/null
+++ b/t/t0031-lockfile-pid.sh
@@ -0,0 +1,139 @@
+#!/bin/sh
+
+test_description='lock file PID info tests
+
+Tests for PID info file alongside lock files.
+The feature is opt-in via core.lockfilePid config setting.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'stale lock detected when PID is not running' '
+	git init repo &&
+	(
+		cd repo &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index.pid.lock &&
+		test_must_fail git -c core.lockfilePid=index add . 2>err &&
+		test_grep "process 99999, which is no longer running" err &&
+		test_grep "Remove the stale lock file" err
+	)
+'
+
+test_expect_success 'PID info not shown by default' '
+	git init repo2 &&
+	(
+		cd repo2 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index.pid.lock &&
+		test_must_fail git add . 2>err &&
+		# Should not crash, just show normal error without PID
+		test_grep "Unable to create" err &&
+		! test_grep "is held by process" err
+	)
+'
+
+test_expect_success 'running process detected when PID is alive' '
+	git init repo3 &&
+	(
+		cd repo3 &&
+		echo content >file &&
+		# Get the correct PID for this platform
+		shell_pid=$$ &&
+		if test_have_prereq MINGW && test -f /proc/$shell_pid/winpid
+		then
+			# In Git for Windows, Bash uses MSYS2 PIDs but git.exe
+			# uses Windows PIDs. Use the Windows PID.
+			shell_pid=$(cat /proc/$shell_pid/winpid)
+		fi &&
+		# Create a lock and PID file with current shell PID (which is running)
+		touch .git/index.lock &&
+		printf "pid %d" "$shell_pid" >.git/index.pid.lock &&
+		# Verify our PID is shown in the error message
+		test_must_fail git -c core.lockfilePid=index add file 2>err &&
+		test_grep "held by process $shell_pid" err
+	)
+'
+
+test_expect_success 'PID info file cleaned up on successful operation when enabled' '
+	git init repo4 &&
+	(
+		cd repo4 &&
+		echo content >file &&
+		git -c core.lockfilePid=index add file &&
+		# After successful add, no lock or PID files should exist
+		test_path_is_missing .git/index.lock &&
+		test_path_is_missing .git/index.pid.lock
+	)
+'
+
+test_expect_success 'no PID file created by default' '
+	git init repo5 &&
+	(
+		cd repo5 &&
+		echo content >file &&
+		git add file &&
+		# PID file should not be created when feature is disabled
+		test_path_is_missing .git/index.pid.lock
+	)
+'
+
+test_expect_success 'core.lockfilePid=all enables for all components' '
+	git init repo6 &&
+	(
+		cd repo6 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index.pid.lock &&
+		test_must_fail git -c core.lockfilePid=all add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'multiple components can be specified' '
+	git init repo8 &&
+	(
+		cd repo8 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index.pid.lock &&
+		test_must_fail git -c core.lockfilePid=index,config add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'core.lockfilePid=none does not create PID file' '
+	git init repo9 &&
+	(
+		cd repo9 &&
+		echo content >file &&
+		git -c core.lockfilePid=none add file &&
+		# PID file should not be created when feature is disabled
+		test_path_is_missing .git/index.pid.lock
+	)
+'
+
+test_expect_success 'existing PID files are read even when feature disabled' '
+	git init repo10 &&
+	(
+		cd repo10 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index.pid.lock &&
+		# Even with lockfilePid disabled, existing PID files are read
+		# to help diagnose stale locks
+		test_must_fail git add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'negative component syntax excludes specific components' '
+	git init repo11 &&
+	(
+		cd repo11 &&
+		echo content >file &&
+		# Enable all components except index
+		git -c core.lockfilePid=all,-index add file &&
+		# PID file should not be created for index when excluded
+		test_path_is_missing .git/index.pid.lock
+	)
+'
+
+test_done
diff --git a/unix-stream-server.c b/unix-stream-server.c
index 22ac2373e0..d8c79cd569 100644
--- a/unix-stream-server.c
+++ b/unix-stream-server.c
@@ -47,7 +47,8 @@ int unix_ss_create(const char *path,
 	/*
 	 * Create a lock at "<path>.lock" if we can.
 	 */
-	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms) < 0)
+	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms,
+					      LOCKFILE_PID_OTHER) < 0)
 		return -1;
 
 	/*

base-commit: 6ab38b7e9cc7adafc304f3204616a4debd49c6e9
-- 
gitgitgadget

Re: [PATCH v2] lockfile: add PID file for debugging stale locks

[Junio C Hamano]

"Paulo Casaretto via GitGitGadget" <gitgitgadget@gmail.com> writes:

>  /* Make sure errno contains a meaningful value on error */
>  static int lock_file(struct lock_file *lk, const char *path, int flags,
> -		     int mode)
> +		     int mode, enum lockfile_pid_component component)
> ...
>  }
>  
> @@ -102,7 +203,8 @@ static int lock_file(struct lock_file *lk, const char *path, int flags,
>   * exactly once. If timeout_ms is -1, try indefinitely.
>   */
>  static int lock_file_timeout(struct lock_file *lk, const char *path,
> -			     int flags, long timeout_ms, int mode)
> +			     int flags, long timeout_ms, int mode,
> +			     enum lockfile_pid_component component)
>  {
> ...
>  	if (timeout_ms == 0)
> -		return lock_file(lk, path, flags, mode);
> +		return lock_file(lk, path, flags, mode, component);
> -		fd = lock_file(lk, path, flags, mode);
> +		fd = lock_file(lk, path, flags, mode, component);

These are OK, but I expected these are rolled into an "unsigned
flags" word, so that ...

>  int hold_lock_file_for_update_timeout_mode(
> -		struct lock_file *lk, const char *path,
> -		int flags, long timeout_ms, int mode);
> +	struct lock_file *lk, const char *path,
> +	int flags, long timeout_ms, int mode,
> +	enum lockfile_pid_component component);

... things like this can be done without adding an extra parameter.
Compared to "what should we do when we see an error?", ...

> -	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
> +	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
> +					       LOCKFILE_PID_OTHER);

... "how would we name the lockfile for this action?" is *not* all
that special and should not occupy a separate parameter on its own.

Existing "flags" argument being "int" not "unsigned int" is a
historical mistake, by the way.

But maybe it is just me?  I dunno.

Re: [PATCH v2] lockfile: add PID file for debugging stale locks

[Junio C Hamano]

"Paulo Casaretto via GitGitGadget" <gitgitgadget@gmail.com> writes:

> The feature is controlled via core.lockfilePid configuration, which
> accepts per-component values similar to core.fsync:
>
>   - none: Disable for all components (default)
>   - all: Enable for all components
>   - index, config, refs, commit-graph, midx, shallow, gc, other:
>     Enable for specific components

As this is about lockfile, you need to decide what should happen
when an older version of Git, which is unaware of this new world
order where .git/index.pid.lock declares ".git/index is being
updated; you should not touch it!", comes to the repository.  You,
as a user of the updated Git, do want them to stop interferring with
the operation on the repository your new Git is making, and it means
you should have some way to telling them "do not touch---you do not
even understand what is in this repository!".

The established way to do so is with a repository extension.  Having
core.lockfilePid that is a plain vanilla configuration variable
would not affect Git that is not aware of the variable, but unknown
repository extension will cause setup.c::verify_repository_format()
to stop Git from making damage.  Perhaps use extensions.lockfilePID
instead?

By the way, we name multi-word configuration variables in CamelCase,
but PID (not "process identifier") is an already all-caps word, so
core.lockfilePid looks awkward.  Call it core.lockfilePID instead?
Oh, with "core.lockfilePID" -> "extensions.lockfilePID", perhaps.

Re: [PATCH v2] lockfile: add PID file for debugging stale locks

[Junio C Hamano]

Junio C Hamano <gitster@pobox.com> writes:

> As this is about lockfile, you need to decide what should happen
> when an older version of Git, which is unaware of this new world
> order where .git/index.pid.lock declares ".git/index is being
> updated; you should not touch it!", comes to the repository.  You,
> as a user of the updated Git, do want them to stop interferring with
> the operation on the repository your new Git is making, and it means
> you should have some way to telling them "do not touch---you do not
> even understand what is in this repository!".

Ah, sorry, I take this back.  We are not replacing index.lock with
something else; we are adding index.pid.lock in addition; older
versions of Git would simply ignore the new file, still notice
index.lock and stop.  So no need for extensions, either.

Re: [PATCH v2] lockfile: add PID file for debugging stale locks

[Ben Knoble]

> Le 17 déc. 2025 à 13:59, Paulo Casaretto via GitGitGadget <gitgitgadget@gmail.com> a écrit :
> 
> From: Paulo Casaretto <pcasaretto@gmail.com>
> 
> When a lock file is held, it can be helpful to know which process owns
> it, especially when debugging stale locks left behind by crashed
> processes. Add an optional feature that creates a companion PID file
> alongside each lock file, containing the PID of the lock holder.
> 
> For a lock file "foo.lock", the PID file is named "foo.pid.lock". The
> file uses a simple key-value format ("pid <value>") following the same
> pattern as Git object headers, making it extensible for future metadata.
> 
> The PID file is created when a lock is acquired (if enabled), and
> automatically cleaned up when the lock is released (via commit or
> rollback). The file is registered as a tempfile so it gets cleaned up
> by signal and atexit handlers if the process terminates abnormally.

Do we also release locks when the process terminates abnormally? If not, this info would be lost during a crash, which I assume is when it would be very useful 🤔 I haven’t looked, just curiously wondering. 

Re: [PATCH v2] lockfile: add PID file for debugging stale locks

[Patrick Steinhardt]

On Wed, Dec 17, 2025 at 06:59:26PM +0000, Paulo Casaretto via GitGitGadget wrote:
> diff --git a/lockfile.c b/lockfile.c
> index 1d5ed01682..4ee215374a 100644
> --- a/lockfile.c
> +++ b/lockfile.c
> @@ -71,19 +74,117 @@ static void resolve_symlink(struct strbuf *path)
>  	strbuf_reset(&link);
>  }
>  
> +/*
> + * Lock PID file functions - write PID to a foo-pid.lock file alongside
> + * the lock file for debugging stale locks. The PID file is registered
> + * as a tempfile so it gets cleaned up by signal/atexit handlers.
> + *
> + * Naming: For "foo.lock", the PID file is "foo-pid.lock" (not "foo.lock.pid").
> + * This avoids collision with the refs namespace.
> + */

This neatly solves the issue that alternative implementations of Git
wouldn't know to handle this new PID file. They already ignore every
file that ends with ".lock", so of course they would also ignore these
new PID files.

But unfortunately this doesn't solve the other issue, which is that we
now have a new restriction: you now cannot have two references "foo" and
"foo-pid" and modify them in the same transaction. Granted, this is a
very specific situation, and I doubt that 99.9% of all users would ever
hit this restriction. But regardless of that I'm worried about the 0.1%
that _do_ hit this restriction now. So I'm afraid that this proposed
naming schema isn't going to work, which raises the question whether
there are any other alternative naming schemas that could.

We could (I'm not saying we should!) do some gross stuff here. For
example, refnames are not allowed to contain some specific characters in
their names: ":?[\\^~ \t*" are all characters that are forbidden to
exist in a refname. So in theory, we could call the lockfile for example
"foo:pid.lock":

  - We know that no reference "foo:pid" should exist because it contains
    a forbidden character.

  - We know that all alternative implementations should ignore it due to
    the ".lock" suffix.

Note the "should" in both cases. I'd consider any implementation that
doesn't honor these "shoulds" to be buggy, but that doesn't mean that
there are no buggy implementations.

In any case though, this may be a possible way forward if we really want
to also cover loose references. Whether we should is a different
question though, and I'm not too sure about it myself:

  - It will regress performance of the "files" reference backend as
    every ref update now needs to write two files instead of one.
    Writing many references is already the worst-case scenario for this
    backend because each reference requires a separate file, and we make
    it even worse by making it two files.

  - We now have not only one file that would block future updates, but
    two. This increases the likelihood that a crash or forced shutdown
    will leave behind stale garbage.

That being said, I also see the potential upside, which is that it now
becomes a bit easier to recover from crashes/forced shutdowns. But in
any case I think that this needs to be opt-in, not opt-out.

Patrick

[PATCH v3] lockfile: add PID file for debugging stale locks

[Paulo Casaretto via GitGitGadget]

From: Paulo Casaretto <pcasaretto@gmail.com>

When a lock file is held, it can be helpful to know which process owns
it, especially when debugging stale locks left behind by crashed
processes. Add an optional feature that creates a companion PID file
alongside each lock file, containing the PID of the lock holder.

For a lock file "foo.lock", the PID file is named "foo~pid.lock". The
tilde character is forbidden in refnames and allowed in Windows
filenames, which guarantees no collision with the refs namespace
(e.g., refs "foo" and "foo~pid" cannot both exist). The file uses a
simple key-value format ("pid <value>") following the same pattern as
Git object headers, making it extensible for future metadata.

The PID file is created when a lock is acquired (if enabled), and
automatically cleaned up when the lock is released (via commit or
rollback). The file is registered as a tempfile so it gets cleaned up
by signal and atexit handlers if the process terminates abnormally.

When a lock conflict occurs, the code checks for an existing PID file
and, if found, uses kill(pid, 0) to determine if the process is still
running. This allows providing context-aware error messages:

  Lock is held by process 12345. Wait for it to finish, or remove
  the lock file to continue.

Or for a stale lock:

  Lock was held by process 12345, which is no longer running.
  Remove the stale lock file to continue.

The feature is controlled via core.lockfilePid configuration, which
accepts per-component values similar to core.fsync:

  - none: Disable for all components (default)
  - all: Enable for all components
  - index, config, refs, commit-graph, midx, shallow, gc, other:
    Enable for specific components

Multiple components can be combined with commas:

  git config core.lockfilePid index,config

Each lock file call site specifies which component it belongs to,
allowing users fine-grained control over which locks create PID files.

Existing PID files are always read when displaying lock errors,
regardless of the core.lockfilePid setting. This ensures helpful
diagnostics even when the feature was previously enabled and later
disabled.

Signed-off-by: Paulo Casaretto <pcasaretto@gmail.com>
---
    lockfile: add holder info file for debugging stale locks
    
    Merry Christmas!
    
    Changes in v3:
    
     * The PID file now uses a tilde instead of a hyphen in the filename:
       foo~pid.lock rather than foo-pid.lock. The tilde is forbidden in
       refnames, guaranteeing no collision with the refs namespace, and is
       allowed in Windows filenames.
     * The feature remains opt-in: the default is core.lockfilePid=none, so
       no PID files are created unless explicitly configured.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2011%2Fpcasaretto%2Fpid-holder-file-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2011/pcasaretto/pid-holder-file-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/2011

Range-diff vs v2:

 1:  d62edb0ca5 ! 1:  29e5903b21 lockfile: add PID file for debugging stale locks
     @@ Commit message
          processes. Add an optional feature that creates a companion PID file
          alongside each lock file, containing the PID of the lock holder.
      
     -    For a lock file "foo.lock", the PID file is named "foo.pid.lock". The
     -    file uses a simple key-value format ("pid <value>") following the same
     -    pattern as Git object headers, making it extensible for future metadata.
     +    For a lock file "foo.lock", the PID file is named "foo~pid.lock". The
     +    tilde character is forbidden in refnames and allowed in Windows
     +    filenames, which guarantees no collision with the refs namespace
     +    (e.g., refs "foo" and "foo~pid" cannot both exist). The file uses a
     +    simple key-value format ("pid <value>") following the same pattern as
     +    Git object headers, making it extensible for future metadata.
      
          The PID file is created when a lock is acquired (if enabled), and
          automatically cleaned up when the lock is released (via commit or
     @@ lockfile.c: static void resolve_symlink(struct strbuf *path)
       }
       
      +/*
     -+ * Lock PID file functions - write PID to a foo-pid.lock file alongside
     ++ * Lock PID file functions - write PID to a foo~pid.lock file alongside
      + * the lock file for debugging stale locks. The PID file is registered
      + * as a tempfile so it gets cleaned up by signal/atexit handlers.
      + *
     -+ * Naming: For "foo.lock", the PID file is "foo-pid.lock" (not "foo.lock.pid").
     -+ * This avoids collision with the refs namespace.
     ++ * Naming: For "foo.lock", the PID file is "foo~pid.lock". The tilde is
     ++ * forbidden in refnames and allowed in Windows filenames, guaranteeing
     ++ * no collision with the refs namespace.
      + */
      +
      +/* Global config variable, initialized from core.lockfilePid */
     @@ lockfile.h: struct lock_file {
       #define LOCK_SUFFIX_LEN 5
       
      +/*
     -+ * PID file naming: for a lock file "foo.lock", the PID file is "foo.pid.lock".
     ++ * PID file naming: for a lock file "foo.lock", the PID file is "foo~pid.lock".
     ++ * The tilde is forbidden in refnames and allowed in Windows filenames, avoiding
     ++ * namespace collisions (e.g., refs "foo" and "foo~pid" cannot both exist).
      + */
     -+#define LOCK_PID_INFIX ".pid"
     ++#define LOCK_PID_INFIX "~pid"
      +#define LOCK_PID_INFIX_LEN 4
      +
      +/* Maximum length for PID file content */
     @@ midx-write.c: static int write_midx_internal(struct odb_source *source,
       	}
      
       ## odb.c ##
     -@@ odb.c: void odb_add_to_alternates_file(struct object_database *odb,
     - 	FILE *in, *out;
     +@@ odb.c: static int odb_source_write_alternate(struct odb_source *source,
       	int found = 0;
     + 	int ret;
       
     --	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR);
     -+	hold_lock_file_for_update(&lock, alts, LOCK_DIE_ON_ERROR,
     +-	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR);
     ++	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR,
      +				  LOCKFILE_PID_OTHER);
       	out = fdopen_lock_file(&lock, "w");
     - 	if (!out)
     - 		die_errno(_("unable to fdopen alternates lockfile"));
     + 	if (!out) {
     + 		ret = error_errno(_("unable to fdopen alternates lockfile"));
      
       ## refs/files-backend.c ##
      @@ refs/files-backend.c: retry:
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo &&
      +		touch .git/index.lock &&
     -+		printf "pid 99999" >.git/index.pid.lock &&
     ++		printf "pid 99999" >.git/index~pid.lock &&
      +		test_must_fail git -c core.lockfilePid=index add . 2>err &&
      +		test_grep "process 99999, which is no longer running" err &&
      +		test_grep "Remove the stale lock file" err
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo2 &&
      +		touch .git/index.lock &&
     -+		printf "pid 99999" >.git/index.pid.lock &&
     ++		printf "pid 99999" >.git/index~pid.lock &&
      +		test_must_fail git add . 2>err &&
      +		# Should not crash, just show normal error without PID
      +		test_grep "Unable to create" err &&
     @@ t/t0031-lockfile-pid.sh (new)
      +		fi &&
      +		# Create a lock and PID file with current shell PID (which is running)
      +		touch .git/index.lock &&
     -+		printf "pid %d" "$shell_pid" >.git/index.pid.lock &&
     ++		printf "pid %d" "$shell_pid" >.git/index~pid.lock &&
      +		# Verify our PID is shown in the error message
      +		test_must_fail git -c core.lockfilePid=index add file 2>err &&
      +		test_grep "held by process $shell_pid" err
     @@ t/t0031-lockfile-pid.sh (new)
      +		git -c core.lockfilePid=index add file &&
      +		# After successful add, no lock or PID files should exist
      +		test_path_is_missing .git/index.lock &&
     -+		test_path_is_missing .git/index.pid.lock
     ++		test_path_is_missing .git/index~pid.lock
      +	)
      +'
      +
     @@ t/t0031-lockfile-pid.sh (new)
      +		echo content >file &&
      +		git add file &&
      +		# PID file should not be created when feature is disabled
     -+		test_path_is_missing .git/index.pid.lock
     ++		test_path_is_missing .git/index~pid.lock
      +	)
      +'
      +
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo6 &&
      +		touch .git/index.lock &&
     -+		printf "pid 99999" >.git/index.pid.lock &&
     ++		printf "pid 99999" >.git/index~pid.lock &&
      +		test_must_fail git -c core.lockfilePid=all add . 2>err &&
      +		test_grep "process 99999" err
      +	)
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo8 &&
      +		touch .git/index.lock &&
     -+		printf "pid 99999" >.git/index.pid.lock &&
     ++		printf "pid 99999" >.git/index~pid.lock &&
      +		test_must_fail git -c core.lockfilePid=index,config add . 2>err &&
      +		test_grep "process 99999" err
      +	)
     @@ t/t0031-lockfile-pid.sh (new)
      +		echo content >file &&
      +		git -c core.lockfilePid=none add file &&
      +		# PID file should not be created when feature is disabled
     -+		test_path_is_missing .git/index.pid.lock
     ++		test_path_is_missing .git/index~pid.lock
      +	)
      +'
      +
     @@ t/t0031-lockfile-pid.sh (new)
      +	(
      +		cd repo10 &&
      +		touch .git/index.lock &&
     -+		printf "pid 99999" >.git/index.pid.lock &&
     ++		printf "pid 99999" >.git/index~pid.lock &&
      +		# Even with lockfilePid disabled, existing PID files are read
      +		# to help diagnose stale locks
      +		test_must_fail git add . 2>err &&
     @@ t/t0031-lockfile-pid.sh (new)
      +		# Enable all components except index
      +		git -c core.lockfilePid=all,-index add file &&
      +		# PID file should not be created for index when excluded
     -+		test_path_is_missing .git/index.pid.lock
     ++		test_path_is_missing .git/index~pid.lock
      +	)
      +'
      +


 Documentation/config/core.adoc |  26 +++++
 apply.c                        |   6 +-
 builtin/commit.c               |   2 +-
 builtin/credential-store.c     |   3 +-
 builtin/difftool.c             |   3 +-
 builtin/fast-import.c          |   3 +-
 builtin/gc.c                   |  13 ++-
 builtin/sparse-checkout.c      |   3 +-
 bundle.c                       |   3 +-
 cache-tree.c                   |   3 +-
 commit-graph.c                 |   6 +-
 compat/mingw.c                 |  10 ++
 config.c                       |   6 +-
 environment.c                  |  80 ++++++++++++++
 lockfile.c                     | 183 +++++++++++++++++++++++++++++----
 lockfile.h                     |  79 ++++++++++----
 loose.c                        |   6 +-
 midx-write.c                   |   6 +-
 odb.c                          |   3 +-
 refs/files-backend.c           |   9 +-
 refs/packed-backend.c          |   3 +-
 reftable/system.c              |   2 +-
 repository.c                   |   3 +-
 rerere.c                       |   3 +-
 sequencer.c                    |  11 +-
 shallow.c                      |   6 +-
 t/meson.build                  |   1 +
 t/t0031-lockfile-pid.sh        | 139 +++++++++++++++++++++++++
 unix-stream-server.c           |   3 +-
 29 files changed, 549 insertions(+), 75 deletions(-)
 create mode 100755 t/t0031-lockfile-pid.sh

diff --git a/Documentation/config/core.adoc b/Documentation/config/core.adoc
index 01202da7cd..4edea55872 100644
--- a/Documentation/config/core.adoc
+++ b/Documentation/config/core.adoc
@@ -348,6 +348,32 @@ confusion unless you know what you are doing (e.g. you are creating a
 read-only snapshot of the same index to a location different from the
 repository's usual working tree).
 
+core.lockfilePid::
+	A comma-separated list of components for which Git should create
+	a PID file alongside the lock file. When a lock acquisition fails
+	and a PID file exists, Git can provide additional diagnostic
+	information about the process holding the lock, including whether
+	it is still running.
++
+This feature is disabled by default. You can enable it for specific
+components or use `all` to enable for all components.
++
+* `none` disables PID file creation for all components.
+* `index` creates PID files for index lock operations.
+* `config` creates PID files for config file lock operations.
+* `refs` creates PID files for reference lock operations.
+* `commit-graph` creates PID files for commit-graph lock operations.
+* `midx` creates PID files for multi-pack-index lock operations.
+* `shallow` creates PID files for shallow file lock operations.
+* `gc` creates PID files for garbage collection lock operations.
+* `other` creates PID files for other miscellaneous lock operations.
+* `all` enables PID file creation for all components.
++
+The PID file is named by inserting `.pid` before the `.lock` suffix.
+For example, if the lock file is `index.lock`, the PID file will be
+`index.pid.lock`. The file contains `pid <value>` on a single line,
+following the same key-value format used by Git object headers.
+
 core.logAllRefUpdates::
 	Enable the reflog. Updates to a ref <ref> is logged to the file
 	"`$GIT_DIR/logs/<ref>`", by appending the new and old
diff --git a/apply.c b/apply.c
index c9fb45247d..6d24f1d9f8 100644
--- a/apply.c
+++ b/apply.c
@@ -4212,7 +4212,8 @@ static int build_fake_ancestor(struct apply_state *state, struct patch *list)
 		}
 	}
 
-	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 	res = write_locked_index(&result, &lock, COMMIT_LOCK);
 	discard_index(&result);
 
@@ -4869,7 +4870,8 @@ static int apply_patch(struct apply_state *state,
 		if (state->index_file)
 			hold_lock_file_for_update(&state->lock_file,
 						  state->index_file,
-						  LOCK_DIE_ON_ERROR);
+						  LOCK_DIE_ON_ERROR,
+						  LOCKFILE_PID_INDEX);
 		else
 			repo_hold_locked_index(state->repo, &state->lock_file,
 					       LOCK_DIE_ON_ERROR);
diff --git a/builtin/commit.c b/builtin/commit.c
index 0243f17d53..30db6409a1 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -540,7 +540,7 @@ static const char *prepare_index(const char **argv, const char *prefix,
 	path = repo_git_path(the_repository, "next-index-%"PRIuMAX,
 			     (uintmax_t) getpid());
 	hold_lock_file_for_update(&false_lock, path,
-				  LOCK_DIE_ON_ERROR);
+				  LOCK_DIE_ON_ERROR, LOCKFILE_PID_OTHER);
 
 	create_base_index(current_head);
 	add_remove_files(&partial);
diff --git a/builtin/credential-store.c b/builtin/credential-store.c
index b74e06cc93..e1d1be5cd1 100644
--- a/builtin/credential-store.c
+++ b/builtin/credential-store.c
@@ -67,7 +67,8 @@ static void rewrite_credential_file(const char *fn, struct credential *c,
 	int timeout_ms = 1000;
 
 	repo_config_get_int(the_repository, "credentialstore.locktimeoutms", &timeout_ms);
-	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms) < 0)
+	if (hold_lock_file_for_update_timeout(&credential_lock, fn, 0, timeout_ms,
+					      LOCKFILE_PID_CONFIG) < 0)
 		die_errno(_("unable to get credential storage lock in %d ms"), timeout_ms);
 	if (extra)
 		print_line(extra);
diff --git a/builtin/difftool.c b/builtin/difftool.c
index e4bc1f8316..15d0af0031 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -636,7 +636,8 @@ static int run_dir_diff(struct repository *repo,
 			struct lock_file lock = LOCK_INIT;
 			strbuf_reset(&buf);
 			strbuf_addf(&buf, "%s/wtindex", tmpdir.buf);
-			if (hold_lock_file_for_update(&lock, buf.buf, 0) < 0 ||
+			if (hold_lock_file_for_update(&lock, buf.buf, 0,
+					      LOCKFILE_PID_OTHER) < 0 ||
 			    write_locked_index(&wtindex, &lock, COMMIT_LOCK)) {
 				ret = error("could not write %s", buf.buf);
 				goto finish;
diff --git a/builtin/fast-import.c b/builtin/fast-import.c
index 7849005ccb..89fd42db65 100644
--- a/builtin/fast-import.c
+++ b/builtin/fast-import.c
@@ -1734,7 +1734,8 @@ static void dump_marks(void)
 		return;
 	}
 
-	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0) < 0) {
+	if (hold_lock_file_for_update(&mark_lock, export_marks_file, 0,
+				      LOCKFILE_PID_OTHER) < 0) {
 		failure |= error_errno(_("unable to write marks file %s"),
 				       export_marks_file);
 		return;
diff --git a/builtin/gc.c b/builtin/gc.c
index 92c6e7b954..cb142afa0b 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -749,7 +749,7 @@ static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
 
 	pidfile_path = repo_git_path(the_repository, "gc.pid");
 	fd = hold_lock_file_for_update(&lock, pidfile_path,
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
 	if (!force) {
 		static char locking_host[HOST_NAME_MAX + 1];
 		static char *scan_fmt;
@@ -1017,7 +1017,7 @@ int cmd_gc(int argc,
 	if (daemonized) {
 		char *path = repo_git_path(the_repository, "gc.log");
 		hold_lock_file_for_update(&log_lock, path,
-					  LOCK_DIE_ON_ERROR);
+					  LOCK_DIE_ON_ERROR, LOCKFILE_PID_GC);
 		dup2(get_lock_file_fd(&log_lock), 2);
 		atexit(process_log_file_at_exit);
 		free(path);
@@ -1797,7 +1797,8 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts,
 	struct repository *r = the_repository;
 	char *lock_path = xstrfmt("%s/maintenance", r->objects->sources->path);
 
-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
+				      LOCKFILE_PID_GC) < 0) {
 		/*
 		 * Another maintenance command is running.
 		 *
@@ -2561,7 +2562,8 @@ static int launchctl_schedule_plist(const char *exec_path, enum schedule_priorit
 		lock_file_timeout_ms = 150;
 
 	fd = hold_lock_file_for_update_timeout(&lk, filename, LOCK_DIE_ON_ERROR,
-					       lock_file_timeout_ms);
+					       lock_file_timeout_ms,
+					       LOCKFILE_PID_GC);
 
 	/*
 	 * Does this file already exist? With the intended contents? Is it
@@ -3372,7 +3374,8 @@ static int update_background_schedule(const struct maintenance_start_opts *opts,
 	struct lock_file lk;
 	char *lock_path = xstrfmt("%s/schedule", the_repository->objects->sources->path);
 
-	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF,
+				      LOCKFILE_PID_GC) < 0) {
 		if (errno == EEXIST)
 			error(_("unable to create '%s.lock': %s.\n\n"
 			    "Another scheduled git-maintenance(1) process seems to be running in this\n"
diff --git a/builtin/sparse-checkout.c b/builtin/sparse-checkout.c
index 15d51e60a8..566df7d229 100644
--- a/builtin/sparse-checkout.c
+++ b/builtin/sparse-checkout.c
@@ -341,7 +341,8 @@ static int write_patterns_and_update(struct repository *repo,
 	if (safe_create_leading_directories(repo, sparse_filename))
 		die(_("failed to create directory for sparse-checkout file"));
 
-	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lk, sparse_filename, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 
 	result = update_working_directory(repo, pl);
 	if (result) {
diff --git a/bundle.c b/bundle.c
index 42327f9739..25728278fc 100644
--- a/bundle.c
+++ b/bundle.c
@@ -520,7 +520,8 @@ int create_bundle(struct repository *r, const char *path,
 		bundle_fd = 1;
 	else
 		bundle_fd = hold_lock_file_for_update(&lock, path,
-						      LOCK_DIE_ON_ERROR);
+						      LOCK_DIE_ON_ERROR,
+						      LOCKFILE_PID_OTHER);
 
 	if (version == -1)
 		version = min_version;
diff --git a/cache-tree.c b/cache-tree.c
index 2d8947b518..bbed306e78 100644
--- a/cache-tree.c
+++ b/cache-tree.c
@@ -754,7 +754,8 @@ int write_index_as_tree(struct object_id *oid, struct index_state *index_state,
 	struct lock_file lock_file = LOCK_INIT;
 	int ret;
 
-	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock_file, index_path, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_INDEX);
 
 	entries = read_index_from(index_state, index_path,
 				  repo_get_git_dir(the_repository));
diff --git a/commit-graph.c b/commit-graph.c
index 80be2ff2c3..b18249fb73 100644
--- a/commit-graph.c
+++ b/commit-graph.c
@@ -2075,7 +2075,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 		char *lock_name = get_commit_graph_chain_filename(ctx->odb_source);
 
 		hold_lock_file_for_update_mode(&lk, lock_name,
-					       LOCK_DIE_ON_ERROR, 0444);
+					       LOCK_DIE_ON_ERROR, 0444,
+					       LOCKFILE_PID_COMMIT_GRAPH);
 		free(lock_name);
 
 		graph_layer = mks_tempfile_m(ctx->graph_name, 0444);
@@ -2094,7 +2095,8 @@ static int write_commit_graph_file(struct write_commit_graph_context *ctx)
 			   get_tempfile_fd(graph_layer), get_tempfile_path(graph_layer));
 	} else {
 		hold_lock_file_for_update_mode(&lk, ctx->graph_name,
-					       LOCK_DIE_ON_ERROR, 0444);
+					       LOCK_DIE_ON_ERROR, 0444,
+					       LOCKFILE_PID_COMMIT_GRAPH);
 		f = hashfd(ctx->r->hash_algo,
 			   get_lock_file_fd(&lk), get_lock_file_path(&lk));
 	}
diff --git a/compat/mingw.c b/compat/mingw.c
index 939f938fe2..146b2585ce 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1972,6 +1972,16 @@ int mingw_kill(pid_t pid, int sig)
 			CloseHandle(h);
 			return 0;
 		}
+		/*
+		 * OpenProcess returns ERROR_INVALID_PARAMETER for
+		 * non-existent PIDs. Map this to ESRCH for POSIX
+		 * compatibility with kill(pid, 0).
+		 */
+		if (GetLastError() == ERROR_INVALID_PARAMETER)
+			errno = ESRCH;
+		else
+			errno = err_win_to_posix(GetLastError());
+		return -1;
 	}
 
 	errno = EINVAL;
diff --git a/config.c b/config.c
index 1738c0cb0d..6658f59834 100644
--- a/config.c
+++ b/config.c
@@ -2986,7 +2986,8 @@ int repo_config_set_multivar_in_file_gently(struct repository *r,
 	 * The lock serves a purpose in addition to locking: the new
 	 * contents of .git/config will be written into it.
 	 */
-	fd = hold_lock_file_for_update(&lock, config_filename, 0);
+	fd = hold_lock_file_for_update(&lock, config_filename, 0,
+				       LOCKFILE_PID_CONFIG);
 	if (fd < 0) {
 		error_errno(_("could not lock config file %s"), config_filename);
 		ret = CONFIG_NO_LOCK;
@@ -3331,7 +3332,8 @@ static int repo_config_copy_or_rename_section_in_file(
 	if (!config_filename)
 		config_filename = filename_buf = repo_git_path(r, "config");
 
-	out_fd = hold_lock_file_for_update(&lock, config_filename, 0);
+	out_fd = hold_lock_file_for_update(&lock, config_filename, 0,
+					   LOCKFILE_PID_CONFIG);
 	if (out_fd < 0) {
 		ret = error(_("could not lock config file %s"), config_filename);
 		goto out;
diff --git a/environment.c b/environment.c
index a770b5921d..0d1dfb525b 100644
--- a/environment.c
+++ b/environment.c
@@ -21,6 +21,7 @@
 #include "gettext.h"
 #include "git-zlib.h"
 #include "ident.h"
+#include "lockfile.h"
 #include "mailmap.h"
 #include "object-name.h"
 #include "repository.h"
@@ -324,6 +325,78 @@ next_name:
 	return (current & ~negative) | positive;
 }
 
+static const struct lockfile_pid_component_name {
+	const char *name;
+	enum lockfile_pid_component component_bits;
+} lockfile_pid_component_names[] = {
+	{ "index", LOCKFILE_PID_INDEX },
+	{ "config", LOCKFILE_PID_CONFIG },
+	{ "refs", LOCKFILE_PID_REFS },
+	{ "commit-graph", LOCKFILE_PID_COMMIT_GRAPH },
+	{ "midx", LOCKFILE_PID_MIDX },
+	{ "shallow", LOCKFILE_PID_SHALLOW },
+	{ "gc", LOCKFILE_PID_GC },
+	{ "other", LOCKFILE_PID_OTHER },
+	{ "all", LOCKFILE_PID_ALL },
+};
+
+static enum lockfile_pid_component parse_lockfile_pid_components(const char *var,
+								 const char *string)
+{
+	enum lockfile_pid_component current = LOCKFILE_PID_DEFAULT;
+	enum lockfile_pid_component positive = 0, negative = 0;
+
+	while (string) {
+		size_t len;
+		const char *ep;
+		int negated = 0;
+		int found = 0;
+
+		string = string + strspn(string, ", \t\n\r");
+		ep = strchrnul(string, ',');
+		len = ep - string;
+		if (len == 4 && !strncmp(string, "none", 4)) {
+			current = LOCKFILE_PID_NONE;
+			goto next_name;
+		}
+
+		if (*string == '-') {
+			negated = 1;
+			string++;
+			len--;
+			if (!len)
+				warning(_("invalid value for variable %s"), var);
+		}
+
+		if (!len)
+			break;
+
+		for (size_t i = 0; i < ARRAY_SIZE(lockfile_pid_component_names); ++i) {
+			const struct lockfile_pid_component_name *n = &lockfile_pid_component_names[i];
+
+			if (strncmp(n->name, string, len) || strlen(n->name) != len)
+				continue;
+
+			found = 1;
+			if (negated)
+				negative |= n->component_bits;
+			else
+				positive |= n->component_bits;
+		}
+
+		if (!found) {
+			char *component = xstrndup(string, len);
+			warning(_("ignoring unknown core.lockfilePid component '%s'"), component);
+			free(component);
+		}
+
+next_name:
+		string = ep;
+	}
+
+	return (current & ~negative) | positive;
+}
+
 static int git_default_core_config(const char *var, const char *value,
 				   const struct config_context *ctx, void *cb)
 {
@@ -532,6 +605,13 @@ static int git_default_core_config(const char *var, const char *value,
 		return 0;
 	}
 
+	if (!strcmp(var, "core.lockfilepid")) {
+		if (!value)
+			return config_error_nonbool(var);
+		lockfile_pid_components = parse_lockfile_pid_components(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.createobject")) {
 		if (!value)
 			return config_error_nonbool(var);
diff --git a/lockfile.c b/lockfile.c
index 1d5ed01682..e9a86f64b8 100644
--- a/lockfile.c
+++ b/lockfile.c
@@ -6,6 +6,9 @@
 #include "abspath.h"
 #include "gettext.h"
 #include "lockfile.h"
+#include "parse.h"
+#include "strbuf.h"
+#include "wrapper.h"
 
 /*
  * path = absolute or relative path name
@@ -71,19 +74,118 @@ static void resolve_symlink(struct strbuf *path)
 	strbuf_reset(&link);
 }
 
+/*
+ * Lock PID file functions - write PID to a foo~pid.lock file alongside
+ * the lock file for debugging stale locks. The PID file is registered
+ * as a tempfile so it gets cleaned up by signal/atexit handlers.
+ *
+ * Naming: For "foo.lock", the PID file is "foo~pid.lock". The tilde is
+ * forbidden in refnames and allowed in Windows filenames, guaranteeing
+ * no collision with the refs namespace.
+ */
+
+/* Global config variable, initialized from core.lockfilePid */
+enum lockfile_pid_component lockfile_pid_components = LOCKFILE_PID_DEFAULT;
+
+/*
+ * Path generation helpers.
+ * Given base path "foo", generate:
+ *   - lock path: "foo.lock"
+ *   - pid path:  "foo-pid.lock"
+ */
+static void get_lock_path(struct strbuf *out, const char *path)
+{
+	strbuf_addstr(out, path);
+	strbuf_addstr(out, LOCK_SUFFIX);
+}
+
+static void get_pid_path(struct strbuf *out, const char *path)
+{
+	strbuf_addstr(out, path);
+	strbuf_addstr(out, LOCK_PID_INFIX);
+	strbuf_addstr(out, LOCK_SUFFIX);
+}
+
+static struct tempfile *create_lock_pid_file(const char *pid_path, int mode,
+					     enum lockfile_pid_component component)
+{
+	struct strbuf content = STRBUF_INIT;
+	struct tempfile *pid_tempfile = NULL;
+	int fd = -1;
+
+	if (!(lockfile_pid_components & component))
+		goto out;
+
+	fd = open(pid_path, O_WRONLY | O_CREAT | O_EXCL, mode);
+	if (fd < 0)
+		goto out;
+
+	strbuf_addf(&content, "pid %" PRIuMAX "\n", (uintmax_t)getpid());
+	if (write_in_full(fd, content.buf, content.len) < 0) {
+		warning_errno(_("could not write lock pid file '%s'"), pid_path);
+		goto out;
+	}
+
+	close(fd);
+	fd = -1;
+	pid_tempfile = register_tempfile(pid_path);
+
+out:
+	if (fd >= 0)
+		close(fd);
+	strbuf_release(&content);
+	return pid_tempfile;
+}
+
+static int read_lock_pid(const char *pid_path, uintmax_t *pid_out)
+{
+	struct strbuf content = STRBUF_INIT;
+	const char *val;
+	int ret = -1;
+
+	if (strbuf_read_file(&content, pid_path, LOCK_PID_MAXLEN) <= 0)
+		goto out;
+
+	strbuf_rtrim(&content);
+
+	if (skip_prefix(content.buf, "pid ", &val)) {
+		char *endptr;
+		*pid_out = strtoumax(val, &endptr, 10);
+		if (*pid_out > 0 && !*endptr)
+			ret = 0;
+	}
+
+	if (ret)
+		warning(_("malformed lock pid file '%s'"), pid_path);
+
+out:
+	strbuf_release(&content);
+	return ret;
+}
+
 /* Make sure errno contains a meaningful value on error */
 static int lock_file(struct lock_file *lk, const char *path, int flags,
-		     int mode)
+		     int mode, enum lockfile_pid_component component)
 {
-	struct strbuf filename = STRBUF_INIT;
+	struct strbuf base_path = STRBUF_INIT;
+	struct strbuf lock_path = STRBUF_INIT;
+	struct strbuf pid_path = STRBUF_INIT;
 
-	strbuf_addstr(&filename, path);
+	strbuf_addstr(&base_path, path);
 	if (!(flags & LOCK_NO_DEREF))
-		resolve_symlink(&filename);
+		resolve_symlink(&base_path);
+
+	get_lock_path(&lock_path, base_path.buf);
+	get_pid_path(&pid_path, base_path.buf);
+
+	lk->tempfile = create_tempfile_mode(lock_path.buf, mode);
+	if (lk->tempfile)
+		lk->pid_tempfile = create_lock_pid_file(pid_path.buf, mode,
+							component);
 
-	strbuf_addstr(&filename, LOCK_SUFFIX);
-	lk->tempfile = create_tempfile_mode(filename.buf, mode);
-	strbuf_release(&filename);
+	strbuf_release(&base_path);
+	strbuf_release(&lock_path);
+	strbuf_release(&pid_path);
 	return lk->tempfile ? lk->tempfile->fd : -1;
 }
 
@@ -102,7 +204,8 @@ static int lock_file(struct lock_file *lk, const char *path, int flags,
  * exactly once. If timeout_ms is -1, try indefinitely.
  */
 static int lock_file_timeout(struct lock_file *lk, const char *path,
-			     int flags, long timeout_ms, int mode)
+			     int flags, long timeout_ms, int mode,
+			     enum lockfile_pid_component component)
 {
 	int n = 1;
 	int multiplier = 1;
@@ -110,7 +213,7 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 	static int random_initialized = 0;
 
 	if (timeout_ms == 0)
-		return lock_file(lk, path, flags, mode);
+		return lock_file(lk, path, flags, mode, component);
 
 	if (!random_initialized) {
 		srand((unsigned int)getpid());
@@ -124,7 +227,7 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 		long backoff_ms, wait_ms;
 		int fd;
 
-		fd = lock_file(lk, path, flags, mode);
+		fd = lock_file(lk, path, flags, mode, component);
 
 		if (fd >= 0)
 			return fd; /* success */
@@ -151,16 +254,47 @@ static int lock_file_timeout(struct lock_file *lk, const char *path,
 void unable_to_lock_message(const char *path, int err, struct strbuf *buf)
 {
 	if (err == EEXIST) {
-		strbuf_addf(buf, _("Unable to create '%s.lock': %s.\n\n"
-		    "Another git process seems to be running in this repository, e.g.\n"
-		    "an editor opened by 'git commit'. Please make sure all processes\n"
-		    "are terminated then try again. If it still fails, a git process\n"
-		    "may have crashed in this repository earlier:\n"
-		    "remove the file manually to continue."),
-			    absolute_path(path), strerror(err));
-	} else
+		const char *abs_path = absolute_path(path);
+		struct strbuf lock_path = STRBUF_INIT;
+		struct strbuf pid_path = STRBUF_INIT;
+		uintmax_t pid;
+		int pid_status = 0; /* 0 = unknown, 1 = running, -1 = stale */
+
+		get_lock_path(&lock_path, abs_path);
+		get_pid_path(&pid_path, abs_path);
+
+		strbuf_addf(buf, _("Unable to create '%s': %s.\n\n"),
+			    lock_path.buf, strerror(err));
+
+		/*
+		 * Try to read PID file unconditionally - it may exist if
+		 * core.lockfilePid was enabled for this component.
+		 */
+		if (!read_lock_pid(pid_path.buf, &pid)) {
+			if (kill((pid_t)pid, 0) == 0 || errno == EPERM)
+				pid_status = 1;  /* running (or no permission to signal) */
+			else if (errno == ESRCH)
+				pid_status = -1; /* no such process - stale lock */
+		}
+
+		if (pid_status == 1)
+			strbuf_addf(buf, _("Lock is held by process %" PRIuMAX ". "
+					   "Wait for it to finish, or remove the lock file to continue"),
+				    pid);
+		else if (pid_status == -1)
+			strbuf_addf(buf, _("Lock was held by process %" PRIuMAX ", "
+					   "which is no longer running. Remove the stale lock file to continue"),
+				    pid);
+		else
+			strbuf_addstr(buf, _("Another git process seems to be running in this repository. "
+					     "Wait for it to finish, or remove the lock file to continue"));
+
+		strbuf_release(&lock_path);
+		strbuf_release(&pid_path);
+	} else {
 		strbuf_addf(buf, _("Unable to create '%s.lock': %s"),
 			    absolute_path(path), strerror(err));
+	}
 }
 
 NORETURN void unable_to_lock_die(const char *path, int err)
@@ -174,9 +308,10 @@ NORETURN void unable_to_lock_die(const char *path, int err)
 /* This should return a meaningful errno on failure */
 int hold_lock_file_for_update_timeout_mode(struct lock_file *lk,
 					   const char *path, int flags,
-					   long timeout_ms, int mode)
+					   long timeout_ms, int mode,
+					   enum lockfile_pid_component component)
 {
-	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode);
+	int fd = lock_file_timeout(lk, path, flags, timeout_ms, mode, component);
 	if (fd < 0) {
 		if (flags & LOCK_DIE_ON_ERROR)
 			unable_to_lock_die(path, errno);
@@ -207,6 +342,8 @@ int commit_lock_file(struct lock_file *lk)
 {
 	char *result_path = get_locked_file_path(lk);
 
+	delete_tempfile(&lk->pid_tempfile);
+
 	if (commit_lock_file_to(lk, result_path)) {
 		int save_errno = errno;
 		free(result_path);
@@ -216,3 +353,9 @@ int commit_lock_file(struct lock_file *lk)
 	free(result_path);
 	return 0;
 }
+
+int rollback_lock_file(struct lock_file *lk)
+{
+	delete_tempfile(&lk->pid_tempfile);
+	return delete_tempfile(&lk->tempfile);
+}
diff --git a/lockfile.h b/lockfile.h
index 1bb9926497..147ea6ec23 100644
--- a/lockfile.h
+++ b/lockfile.h
@@ -119,6 +119,7 @@
 
 struct lock_file {
 	struct tempfile *tempfile;
+	struct tempfile *pid_tempfile;
 };
 
 #define LOCK_INIT { 0 }
@@ -127,6 +128,44 @@ struct lock_file {
 #define LOCK_SUFFIX ".lock"
 #define LOCK_SUFFIX_LEN 5
 
+/*
+ * PID file naming: for a lock file "foo.lock", the PID file is "foo~pid.lock".
+ * The tilde is forbidden in refnames and allowed in Windows filenames, avoiding
+ * namespace collisions (e.g., refs "foo" and "foo~pid" cannot both exist).
+ */
+#define LOCK_PID_INFIX "~pid"
+#define LOCK_PID_INFIX_LEN 4
+
+/* Maximum length for PID file content */
+#define LOCK_PID_MAXLEN 32
+
+/*
+ * Per-component lock PID file configuration, following core.fsync pattern.
+ * Each component can be individually enabled via core.lockfilePid config.
+ */
+enum lockfile_pid_component {
+	LOCKFILE_PID_NONE = 0,
+	LOCKFILE_PID_INDEX = 1 << 0, /* .git/index.lock */
+	LOCKFILE_PID_CONFIG = 1 << 1, /* .git/config.lock */
+	LOCKFILE_PID_REFS = 1 << 2, /* refs locks */
+	LOCKFILE_PID_COMMIT_GRAPH = 1 << 3, /* commit-graph.lock */
+	LOCKFILE_PID_MIDX = 1 << 4, /* multi-pack-index.lock */
+	LOCKFILE_PID_SHALLOW = 1 << 5, /* shallow file */
+	LOCKFILE_PID_GC = 1 << 6, /* gc locks */
+	LOCKFILE_PID_OTHER = 1 << 7, /* other locks */
+};
+
+#define LOCKFILE_PID_ALL (LOCKFILE_PID_INDEX | LOCKFILE_PID_CONFIG |      \
+			  LOCKFILE_PID_REFS | LOCKFILE_PID_COMMIT_GRAPH | \
+			  LOCKFILE_PID_MIDX | LOCKFILE_PID_SHALLOW |      \
+			  LOCKFILE_PID_GC | LOCKFILE_PID_OTHER)
+#define LOCKFILE_PID_DEFAULT LOCKFILE_PID_NONE
+
+/*
+ * Bitmask indicating which components should create PID files.
+ * Configured via core.lockfilePid.
+ */
+extern enum lockfile_pid_component lockfile_pid_components;
 
 /*
  * Flags
@@ -167,17 +206,23 @@ struct lock_file {
  * timeout_ms milliseconds. If timeout_ms is 0, try exactly once; if
  * timeout_ms is -1, retry indefinitely. The flags argument, error
  * handling, and mode are described above.
+ *
+ * The `component` argument specifies which lock PID component this lock
+ * belongs to, for selective PID file creation via core.lockfilePid config.
  */
 int hold_lock_file_for_update_timeout_mode(
-		struct lock_file *lk, const char *path,
-		int flags, long timeout_ms, int mode);
+	struct lock_file *lk, const char *path,
+	int flags, long timeout_ms, int mode,
+	enum lockfile_pid_component component);
 
 static inline int hold_lock_file_for_update_timeout(
-		struct lock_file *lk, const char *path,
-		int flags, long timeout_ms)
+	struct lock_file *lk, const char *path,
+	int flags, long timeout_ms,
+	enum lockfile_pid_component component)
 {
 	return hold_lock_file_for_update_timeout_mode(lk, path, flags,
-						      timeout_ms, 0666);
+						      timeout_ms, 0666,
+						      component);
 }
 
 /*
@@ -186,17 +231,18 @@ static inline int hold_lock_file_for_update_timeout(
  * argument and error handling are described above.
  */
 static inline int hold_lock_file_for_update(
-		struct lock_file *lk, const char *path,
-		int flags)
+	struct lock_file *lk, const char *path,
+	int flags, enum lockfile_pid_component component)
 {
-	return hold_lock_file_for_update_timeout(lk, path, flags, 0);
+	return hold_lock_file_for_update_timeout(lk, path, flags, 0, component);
 }
 
 static inline int hold_lock_file_for_update_mode(
-		struct lock_file *lk, const char *path,
-		int flags, int mode)
+	struct lock_file *lk, const char *path,
+	int flags, int mode, enum lockfile_pid_component component)
 {
-	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode);
+	return hold_lock_file_for_update_timeout_mode(lk, path, flags, 0, mode,
+						      component);
 }
 
 /*
@@ -319,13 +365,10 @@ static inline int commit_lock_file_to(struct lock_file *lk, const char *path)
 
 /*
  * Roll back `lk`: close the file descriptor and/or file pointer and
- * remove the lockfile. It is a NOOP to call `rollback_lock_file()`
- * for a `lock_file` object that has already been committed or rolled
- * back. No error will be returned in this case.
+ * remove the lockfile and any associated PID file. It is a NOOP to
+ * call `rollback_lock_file()` for a `lock_file` object that has already
+ * been committed or rolled back. No error will be returned in this case.
  */
-static inline int rollback_lock_file(struct lock_file *lk)
-{
-	return delete_tempfile(&lk->tempfile);
-}
+int rollback_lock_file(struct lock_file *lk);
 
 #endif /* LOCKFILE_H */
diff --git a/loose.c b/loose.c
index 56cf64b648..c0d2f0416f 100644
--- a/loose.c
+++ b/loose.c
@@ -135,7 +135,8 @@ int repo_write_loose_object_map(struct repository *repo)
 		return 0;
 
 	repo_common_path_replace(repo, &path, "objects/loose-object-idx");
-	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+	fd = hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
+					       LOCKFILE_PID_OTHER);
 	iter = kh_begin(map);
 	if (write_in_full(fd, loose_object_header, strlen(loose_object_header)) < 0)
 		goto errout;
@@ -177,7 +178,8 @@ static int write_one_object(struct odb_source *source,
 	struct strbuf buf = STRBUF_INIT, path = STRBUF_INIT;
 
 	strbuf_addf(&path, "%s/loose-object-idx", source->path);
-	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1);
+	hold_lock_file_for_update_timeout(&lock, path.buf, LOCK_DIE_ON_ERROR, -1,
+					  LOCKFILE_PID_OTHER);
 
 	fd = open(path.buf, O_WRONLY | O_CREAT | O_APPEND, 0666);
 	if (fd < 0)
diff --git a/midx-write.c b/midx-write.c
index e3e9be6d03..8180334703 100644
--- a/midx-write.c
+++ b/midx-write.c
@@ -1309,7 +1309,8 @@ static int write_midx_internal(struct odb_source *source,
 		struct strbuf lock_name = STRBUF_INIT;
 
 		get_midx_chain_filename(source, &lock_name);
-		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update(&lk, lock_name.buf, LOCK_DIE_ON_ERROR,
+					  LOCKFILE_PID_MIDX);
 		strbuf_release(&lock_name);
 
 		incr = mks_tempfile_m(midx_name.buf, 0444);
@@ -1327,7 +1328,8 @@ static int write_midx_internal(struct odb_source *source,
 		f = hashfd(r->hash_algo, get_tempfile_fd(incr),
 			   get_tempfile_path(incr));
 	} else {
-		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR);
+		hold_lock_file_for_update(&lk, midx_name.buf, LOCK_DIE_ON_ERROR,
+					  LOCKFILE_PID_MIDX);
 		f = hashfd(r->hash_algo, get_lock_file_fd(&lk),
 			   get_lock_file_path(&lk));
 	}
diff --git a/odb.c b/odb.c
index 7b5da2de32..7017f2d215 100644
--- a/odb.c
+++ b/odb.c
@@ -280,7 +280,8 @@ static int odb_source_write_alternate(struct odb_source *source,
 	int found = 0;
 	int ret;
 
-	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR);
+	hold_lock_file_for_update(&lock, path, LOCK_DIE_ON_ERROR,
+				  LOCKFILE_PID_OTHER);
 	out = fdopen_lock_file(&lock, "w");
 	if (!out) {
 		ret = error_errno(_("unable to fdopen alternates lockfile"));
diff --git a/refs/files-backend.c b/refs/files-backend.c
index 6f6f76a8d8..e296154fe7 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -790,7 +790,8 @@ retry:
 
 	if (hold_lock_file_for_update_timeout(
 			    &lock->lk, ref_file.buf, LOCK_NO_DEREF,
-			    get_files_ref_lock_timeout_ms()) < 0) {
+			    get_files_ref_lock_timeout_ms(),
+			    LOCKFILE_PID_REFS) < 0) {
 		int myerr = errno;
 		errno = 0;
 		if (myerr == ENOENT && --attempts_remaining > 0) {
@@ -1194,7 +1195,8 @@ static int create_reflock(const char *path, void *cb)
 
 	return hold_lock_file_for_update_timeout(
 			lk, path, LOCK_NO_DEREF,
-			get_files_ref_lock_timeout_ms()) < 0 ? -1 : 0;
+			get_files_ref_lock_timeout_ms(),
+			LOCKFILE_PID_REFS) < 0 ? -1 : 0;
 }
 
 /*
@@ -3536,7 +3538,8 @@ static int files_reflog_expire(struct ref_store *ref_store,
 		 * work we need, including cleaning up if the program
 		 * exits unexpectedly.
 		 */
-		if (hold_lock_file_for_update(&reflog_lock, log_file, 0) < 0) {
+		if (hold_lock_file_for_update(&reflog_lock, log_file, 0,
+					      LOCKFILE_PID_REFS) < 0) {
 			struct strbuf err = STRBUF_INIT;
 			unable_to_lock_message(log_file, errno, &err);
 			error("%s", err.buf);
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 4ea0c12299..f832c49eae 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -1230,7 +1230,8 @@ int packed_refs_lock(struct ref_store *ref_store, int flags, struct strbuf *err)
 	if (hold_lock_file_for_update_timeout(
 			    &refs->lock,
 			    refs->path,
-			    flags, timeout_value) < 0) {
+			    flags, timeout_value,
+			    LOCKFILE_PID_REFS) < 0) {
 		unable_to_lock_message(refs->path, errno, err);
 		return -1;
 	}
diff --git a/reftable/system.c b/reftable/system.c
index 725a25844e..5462111a04 100644
--- a/reftable/system.c
+++ b/reftable/system.c
@@ -67,7 +67,7 @@ int flock_acquire(struct reftable_flock *l, const char *target_path,
 		return REFTABLE_OUT_OF_MEMORY_ERROR;
 
 	err = hold_lock_file_for_update_timeout(lockfile, target_path, LOCK_NO_DEREF,
-						timeout_ms);
+						timeout_ms, LOCKFILE_PID_REFS);
 	if (err < 0) {
 		reftable_free(lockfile);
 		if (errno == EEXIST)
diff --git a/repository.c b/repository.c
index 863f24411b..c010098fda 100644
--- a/repository.c
+++ b/repository.c
@@ -449,5 +449,6 @@ int repo_hold_locked_index(struct repository *repo,
 {
 	if (!repo->index_file)
 		BUG("the repo hasn't been setup");
-	return hold_lock_file_for_update(lf, repo->index_file, flags);
+	return hold_lock_file_for_update(lf, repo->index_file, flags,
+					 LOCKFILE_PID_INDEX);
 }
diff --git a/rerere.c b/rerere.c
index 6ec55964e2..71f1d5b176 100644
--- a/rerere.c
+++ b/rerere.c
@@ -917,7 +917,8 @@ int setup_rerere(struct repository *r, struct string_list *merge_rr, int flags)
 	else
 		fd = hold_lock_file_for_update(&write_lock,
 					       git_path_merge_rr(r),
-					       LOCK_DIE_ON_ERROR);
+					       LOCK_DIE_ON_ERROR,
+					       LOCKFILE_PID_OTHER);
 	read_rr(r, merge_rr);
 	return fd;
 }
diff --git a/sequencer.c b/sequencer.c
index 5476d39ba9..337f82e02c 100644
--- a/sequencer.c
+++ b/sequencer.c
@@ -566,7 +566,8 @@ static int write_message(const void *buf, size_t len, const char *filename,
 {
 	struct lock_file msg_file = LOCK_INIT;
 
-	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0);
+	int msg_fd = hold_lock_file_for_update(&msg_file, filename, 0,
+					       LOCKFILE_PID_OTHER);
 	if (msg_fd < 0)
 		return error_errno(_("could not lock '%s'"), filename);
 	if (write_in_full(msg_fd, buf, len) < 0) {
@@ -3605,7 +3606,8 @@ static int save_todo(struct todo_list *todo_list, struct replay_opts *opts,
 	if (is_rebase_i(opts) && !reschedule)
 		next++;
 
-	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0);
+	fd = hold_lock_file_for_update(&todo_lock, todo_path, 0,
+				       LOCKFILE_PID_OTHER);
 	if (fd < 0)
 		return error_errno(_("could not lock '%s'"), todo_path);
 	offset = get_item_line_offset(todo_list, next);
@@ -3877,7 +3879,8 @@ static int safe_append(const char *filename, const char *fmt, ...)
 	va_list ap;
 	struct lock_file lock = LOCK_INIT;
 	int fd = hold_lock_file_for_update(&lock, filename,
-					   LOCK_REPORT_ON_ERROR);
+					   LOCK_REPORT_ON_ERROR,
+					   LOCKFILE_PID_OTHER);
 	struct strbuf buf = STRBUF_INIT;
 
 	if (fd < 0)
@@ -4400,7 +4403,7 @@ static int write_update_refs_state(struct string_list *refs_to_oids)
 		goto cleanup;
 	}
 
-	if (hold_lock_file_for_update(&lock, path, 0) < 0) {
+	if (hold_lock_file_for_update(&lock, path, 0, LOCKFILE_PID_OTHER) < 0) {
 		result = error(_("another 'rebase' process appears to be running; "
 				 "'%s.lock' already exists"),
 			       path);
diff --git a/shallow.c b/shallow.c
index 186e9178f3..01dc152a1c 100644
--- a/shallow.c
+++ b/shallow.c
@@ -392,7 +392,8 @@ void setup_alternate_shallow(struct shallow_lock *shallow_lock,
 
 	fd = hold_lock_file_for_update(&shallow_lock->lock,
 				       git_path_shallow(the_repository),
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR,
+				       LOCKFILE_PID_SHALLOW);
 	check_shallow_file_for_update(the_repository);
 	if (write_shallow_commits(&sb, 0, extra)) {
 		if (write_in_full(fd, sb.buf, sb.len) < 0)
@@ -447,7 +448,8 @@ void prune_shallow(unsigned options)
 	}
 	fd = hold_lock_file_for_update(&shallow_lock.lock,
 				       git_path_shallow(the_repository),
-				       LOCK_DIE_ON_ERROR);
+				       LOCK_DIE_ON_ERROR,
+				       LOCKFILE_PID_SHALLOW);
 	check_shallow_file_for_update(the_repository);
 	if (write_shallow_commits_1(&sb, 0, NULL, flags)) {
 		if (write_in_full(fd, sb.buf, sb.len) < 0)
diff --git a/t/meson.build b/t/meson.build
index 459c52a489..2aec2c011e 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -98,6 +98,7 @@ integration_tests = [
   't0028-working-tree-encoding.sh',
   't0029-core-unsetenvvars.sh',
   't0030-stripspace.sh',
+  't0031-lockfile-pid.sh',
   't0033-safe-directory.sh',
   't0034-root-safe-directory.sh',
   't0035-safe-bare-repository.sh',
diff --git a/t/t0031-lockfile-pid.sh b/t/t0031-lockfile-pid.sh
new file mode 100755
index 0000000000..51bb6d4be1
--- /dev/null
+++ b/t/t0031-lockfile-pid.sh
@@ -0,0 +1,139 @@
+#!/bin/sh
+
+test_description='lock file PID info tests
+
+Tests for PID info file alongside lock files.
+The feature is opt-in via core.lockfilePid config setting.
+'
+
+. ./test-lib.sh
+
+test_expect_success 'stale lock detected when PID is not running' '
+	git init repo &&
+	(
+		cd repo &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index~pid.lock &&
+		test_must_fail git -c core.lockfilePid=index add . 2>err &&
+		test_grep "process 99999, which is no longer running" err &&
+		test_grep "Remove the stale lock file" err
+	)
+'
+
+test_expect_success 'PID info not shown by default' '
+	git init repo2 &&
+	(
+		cd repo2 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index~pid.lock &&
+		test_must_fail git add . 2>err &&
+		# Should not crash, just show normal error without PID
+		test_grep "Unable to create" err &&
+		! test_grep "is held by process" err
+	)
+'
+
+test_expect_success 'running process detected when PID is alive' '
+	git init repo3 &&
+	(
+		cd repo3 &&
+		echo content >file &&
+		# Get the correct PID for this platform
+		shell_pid=$$ &&
+		if test_have_prereq MINGW && test -f /proc/$shell_pid/winpid
+		then
+			# In Git for Windows, Bash uses MSYS2 PIDs but git.exe
+			# uses Windows PIDs. Use the Windows PID.
+			shell_pid=$(cat /proc/$shell_pid/winpid)
+		fi &&
+		# Create a lock and PID file with current shell PID (which is running)
+		touch .git/index.lock &&
+		printf "pid %d" "$shell_pid" >.git/index~pid.lock &&
+		# Verify our PID is shown in the error message
+		test_must_fail git -c core.lockfilePid=index add file 2>err &&
+		test_grep "held by process $shell_pid" err
+	)
+'
+
+test_expect_success 'PID info file cleaned up on successful operation when enabled' '
+	git init repo4 &&
+	(
+		cd repo4 &&
+		echo content >file &&
+		git -c core.lockfilePid=index add file &&
+		# After successful add, no lock or PID files should exist
+		test_path_is_missing .git/index.lock &&
+		test_path_is_missing .git/index~pid.lock
+	)
+'
+
+test_expect_success 'no PID file created by default' '
+	git init repo5 &&
+	(
+		cd repo5 &&
+		echo content >file &&
+		git add file &&
+		# PID file should not be created when feature is disabled
+		test_path_is_missing .git/index~pid.lock
+	)
+'
+
+test_expect_success 'core.lockfilePid=all enables for all components' '
+	git init repo6 &&
+	(
+		cd repo6 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index~pid.lock &&
+		test_must_fail git -c core.lockfilePid=all add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'multiple components can be specified' '
+	git init repo8 &&
+	(
+		cd repo8 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index~pid.lock &&
+		test_must_fail git -c core.lockfilePid=index,config add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'core.lockfilePid=none does not create PID file' '
+	git init repo9 &&
+	(
+		cd repo9 &&
+		echo content >file &&
+		git -c core.lockfilePid=none add file &&
+		# PID file should not be created when feature is disabled
+		test_path_is_missing .git/index~pid.lock
+	)
+'
+
+test_expect_success 'existing PID files are read even when feature disabled' '
+	git init repo10 &&
+	(
+		cd repo10 &&
+		touch .git/index.lock &&
+		printf "pid 99999" >.git/index~pid.lock &&
+		# Even with lockfilePid disabled, existing PID files are read
+		# to help diagnose stale locks
+		test_must_fail git add . 2>err &&
+		test_grep "process 99999" err
+	)
+'
+
+test_expect_success 'negative component syntax excludes specific components' '
+	git init repo11 &&
+	(
+		cd repo11 &&
+		echo content >file &&
+		# Enable all components except index
+		git -c core.lockfilePid=all,-index add file &&
+		# PID file should not be created for index when excluded
+		test_path_is_missing .git/index~pid.lock
+	)
+'
+
+test_done
diff --git a/unix-stream-server.c b/unix-stream-server.c
index 22ac2373e0..d8c79cd569 100644
--- a/unix-stream-server.c
+++ b/unix-stream-server.c
@@ -47,7 +47,8 @@ int unix_ss_create(const char *path,
 	/*
 	 * Create a lock at "<path>.lock" if we can.
 	 */
-	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms) < 0)
+	if (hold_lock_file_for_update_timeout(&lock, path, 0, timeout_ms,
+					      LOCKFILE_PID_OTHER) < 0)
 		return -1;
 
 	/*

base-commit: 66ce5f8e8872f0183bb137911c52b07f1f242d13
-- 
gitgitgadget

Re: [PATCH v3] lockfile: add PID file for debugging stale locks

[Junio C Hamano]

"Paulo Casaretto via GitGitGadget" <gitgitgadget@gmail.com> writes:

> For a lock file "foo.lock", the PID file is named "foo~pid.lock". The
> tilde character is forbidden in refnames and allowed in Windows
> filenames, which guarantees no collision with the refs namespace
> (e.g., refs "foo" and "foo~pid" cannot both exist).

Lucky we had this wiggle room for us ;-)

> The file uses a
> simple key-value format ("pid <value>") following the same pattern as
> Git object headers, making it extensible for future metadata.

It may be just me, but "a Git object header" to me means the "<type>
<length> <NUL>".  I suspect that you modelled this more after
commit headers (in which case you may want to say "pid <value> LF"
to stress the fact that a record is newline terminated).

> +The PID file is named by inserting `.pid` before the `.lock` suffix.

Don't we use tilde somewhere???

> +For example, if the lock file is `index.lock`, the PID file will be
> +`index.pid.lock`. The file contains `pid <value>` on a single line,
> +following the same key-value format used by Git object headers.

Same comment as the one for the proposed log message applies here.

> diff --git a/apply.c b/apply.c
> index c9fb45247d..6d24f1d9f8 100644
> --- a/apply.c
> +++ b/apply.c
> @@ -4212,7 +4212,8 @@ static int build_fake_ancestor(struct apply_state *state, struct patch *list)
>  		}
>  	}
>  
> -	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR);
> +	hold_lock_file_for_update(&lock, state->fake_ancestor, LOCK_DIE_ON_ERROR,
> +				  LOCKFILE_PID_OTHER);

Hmph, I thought I suggested not to send this as a separate
parameter, and instead use a set of bits in an existing flag word
that currently carries only "what to do upon error" bits?

>  			hold_lock_file_for_update(&state->lock_file,
>  						  state->index_file,
> -						  LOCK_DIE_ON_ERROR);
> +						  LOCK_DIE_ON_ERROR,
> +						  LOCKFILE_PID_INDEX);

Ditto.

> +static const struct lockfile_pid_component_name {
> +	const char *name;
> +	enum lockfile_pid_component component_bits;
> +} lockfile_pid_component_names[] = {
> +	{ "index", LOCKFILE_PID_INDEX },
> +	{ "config", LOCKFILE_PID_CONFIG },
> +	{ "refs", LOCKFILE_PID_REFS },
> +	{ "commit-graph", LOCKFILE_PID_COMMIT_GRAPH },
> +	{ "midx", LOCKFILE_PID_MIDX },
> +	{ "shallow", LOCKFILE_PID_SHALLOW },
> +	{ "gc", LOCKFILE_PID_GC },
> +	{ "other", LOCKFILE_PID_OTHER },
> +	{ "all", LOCKFILE_PID_ALL },
> +};

I wonder if we want to sort the array entries here, even if we look
it up linearly (as the table is small enough)?

> +	strbuf_addf(&content, "pid %" PRIuMAX "\n", (uintmax_t)getpid());
> +	if (write_in_full(fd, content.buf, content.len) < 0) {
> +		warning_errno(_("could not write lock pid file '%s'"), pid_path);
> +		goto out;

The caller still will get a non-NULL pid_tempfile?  If you failed to
write the contents, wouldn't it allow the caller to handle the
failure more smoothly if you allowed the caller to know about the
failure, perhaps by removing the temporary file and returning NULL?

> +	}
> +
> +	close(fd);
> +	fd = -1;
> +	pid_tempfile = register_tempfile(pid_path);
> +
> +out:
> +	if (fd >= 0)
> +		close(fd);
> +	strbuf_release(&content);
> +	return pid_tempfile;
> +}

> +static int read_lock_pid(const char *pid_path, uintmax_t *pid_out)
> +{
> +	struct strbuf content = STRBUF_INIT;
> +	const char *val;
> +	int ret = -1;
> +
> +	if (strbuf_read_file(&content, pid_path, LOCK_PID_MAXLEN) <= 0)
> +		goto out;

Any and all failure to read the pid_path is a silent, including an
empty one?

I'll stop here for now.

Thanks.

Re: [PATCH v3] lockfile: add PID file for debugging stale locks

[Jeff King]

On Wed, Dec 24, 2025 at 12:24:13PM +0000, Paulo Casaretto via GitGitGadget wrote:

> For a lock file "foo.lock", the PID file is named "foo~pid.lock". The
> tilde character is forbidden in refnames and allowed in Windows
> filenames, which guarantees no collision with the refs namespace
> (e.g., refs "foo" and "foo~pid" cannot both exist). The file uses a
> simple key-value format ("pid <value>") following the same pattern as
> Git object headers, making it extensible for future metadata.

FWIW, I like this approach, as the earlier collision possibilities in
earlier iterations made me a bit uncomfortable.

But then I wondered...

> The feature is controlled via core.lockfilePid configuration, which
> accepts per-component values similar to core.fsync:
> 
>   - none: Disable for all components (default)
>   - all: Enable for all components
>   - index, config, refs, commit-graph, midx, shallow, gc, other:
>     Enable for specific components

Do we really need this complexity now? I can see reasons why a user
might want to switch the feature on (because they want the extra pid
debugging info) or off (because they do not want the extra filesystem
operations or potential cleanup hassle of extra stale files).

But if the collision problems in the ref namespace are now solved, does
anybody really care about turning it on just for particular subsystems?
I'd think they would want all or nothing.


Trying to devil's advocate myself: maybe somebody is concerned about
filesystem overhead for frequently-written files like refs but not for
others, like config? That feels unlikely to me, but at least possible.

But if we were to ditch the subsystem-granularity for config, then all
of the extra LOCKFILE_* arguments here could just go away.

-Peff