[PATCH v2 3/4] object-file: explicitly write ".keep" lockfile

[Justin Tobler <jltobler@gmail.com>]

The `odb_transaction_write_pack()` interface for the "files" backend
relies on git-index-pack(1) to write the specified packfile into an ODB
transaction as a packfile. To prevent a concurrent housekeeping
operation from deleting the newly written packfile prior to the
references being updated, a ".keep" lockfile is also generated by
git-index-pack(1) via the `--keep` option. Consequently, the ".keep"
lockfile is written into the ODB transaction alongside the packfile and
later migrated together to the primary ODB during transaction commit.

In a subsequent commit, writing the lockfile is deferred until
transaction commit and thus cannot rely on git-index-pack(1) to write
it. In preparation for this, update `odb_transaction_files_write_pack()`
to explicitly generate the same ".keep" lockfile itself.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
---
 object-file.c | 108 ++++++++++++++++++++++++++++++++++++++++----------
 1 file changed, 86 insertions(+), 22 deletions(-)

diff --git a/object-file.c b/object-file.c
index 210984f825..07a61c610a 100644
--- a/object-file.c
+++ b/object-file.c
@@ -1256,6 +1256,47 @@ int read_loose_object(struct repository *repo,
 	return ret;
 }
 
+static int write_pack_lockfile(struct odb_source *source, const char *pack_name,
+			       const char *keep_msg)
+{
+	struct strbuf lock_path = STRBUF_INIT;
+	struct strbuf contents = STRBUF_INIT;
+	int ret = -1;
+	int fd;
+
+	strbuf_addf(&lock_path, "%s/pack/pack-%s.keep",
+		    source->path, pack_name);
+	strbuf_addf(&contents, "%s\n", keep_msg);
+
+	fd = safe_create_file_with_leading_directories(source->odb->repo,
+						       lock_path.buf);
+	if (fd < 0) {
+		error_errno(_("unable to create '%s'"), lock_path.buf);
+		goto out;
+	}
+
+	if (write_in_full(fd, contents.buf, contents.len) < 0) {
+		error_errno(_("unable to write '%s'"), lock_path.buf);
+		close(fd);
+		unlink(lock_path.buf);
+		goto out;
+	}
+
+	if (close(fd) < 0) {
+		error_errno(_("unable to close '%s'"), lock_path.buf);
+		unlink(lock_path.buf);
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	strbuf_release(&lock_path);
+	strbuf_release(&contents);
+
+	return ret;
+}
+
 static int odb_transaction_files_commit(struct odb_transaction *base)
 {
 	struct odb_transaction_files *transaction =
@@ -1348,9 +1389,13 @@ static int odb_transaction_files_write_pack(struct odb_transaction *base,
 		container_of(base, struct odb_transaction_files, base);
 	struct repository *repo = base->source->odb->repo;
 	struct child_process child = CHILD_PROCESS_INIT;
+	struct strbuf final_path = STRBUF_INIT;
+	struct strbuf pack_name = STRBUF_INIT;
+	struct strbuf keep_msg = STRBUF_INIT;
 	struct pack_header hdr;
 	const char *hdr_err;
 	int err_fd = opts->err_fd;
+	int ret = -1;
 	int status;
 
 	hdr_err = parse_pack_header(&hdr, pack_fd);
@@ -1390,17 +1435,12 @@ static int odb_transaction_files_write_pack(struct odb_transaction *base,
 		}
 	} else {
 		char hostname[HOST_NAME_MAX + 1];
-		char *lockfile;
 
 		strvec_pushl(&child.args, "index-pack", "--stdin", NULL);
 		push_header_arg(&child.args, &hdr);
 
 		if (xgethostname(hostname, sizeof(hostname)))
 			xsnprintf(hostname, sizeof(hostname), "localhost");
-		strvec_pushf(&child.args,
-			     "--keep=receive-pack %"PRIuMAX" on %s",
-			     (uintmax_t)getpid(),
-			     hostname);
 
 		if (!opts->quiet && err_fd)
 			strvec_push(&child.args, "--show-resolving-progress");
@@ -1424,34 +1464,58 @@ static int odb_transaction_files_write_pack(struct odb_transaction *base,
 			return -1;
 		}
 
-		/*
-		 * The lockfile filepath is expected to be the final location of
-		 * the ".keep" file after being migrated to the main ODB source.
-		 * This ensures the lockfile can be found and removed later
-		 * after the ODB transaction has been committed.
-		 */
-		lockfile = index_pack_lockfile(base->source, child.out, NULL);
-		if (lockfile) {
-			ALLOC_GROW(transaction->pack_lockfiles,
-				   transaction->pack_lockfiles_nr + 1,
-				   transaction->pack_lockfiles_alloc);
-			transaction->pack_lockfiles[transaction->pack_lockfiles_nr++] =
-				register_tempfile(lockfile);
-			free(lockfile);
-		}
+		ret = read_index_pack_report(repo, child.out, &pack_name, NULL);
+
 		close(child.out);
 
 		status = finish_command(&child);
 		if (status) {
 			strbuf_addstr(err_msg, "index-pack abnormal exit");
-			return -1;
+			goto out;
+		}
+
+		if (ret) {
+			strbuf_addstr(err_msg, "index-pack failed to report pack name");
+			goto out;
 		}
 
+		strbuf_addf(&keep_msg, "receive-pack %"PRIuMAX" on %s",
+			    (uintmax_t)getpid(), hostname);
+
+		if (write_pack_lockfile(transaction->quarantine, pack_name.buf,
+					keep_msg.buf)) {
+			strbuf_addstr(err_msg, "lockfile write failed");
+			goto out;
+		}
+
+		ALLOC_GROW(transaction->pack_lockfiles,
+			   transaction->pack_lockfiles_nr + 1,
+			   transaction->pack_lockfiles_alloc);
+
+		/*
+		 * The lockfile filepath is expected to be the final location of
+		 * the ".keep" file after being migrated to the main ODB source.
+		 * This ensures the lockfile can be found and removed later
+		 * after the ODB transaction has been committed.
+		 */
+		strbuf_addf(&final_path, "%s/pack/pack-%s.keep",
+			    base->source->path, pack_name.buf);
+		transaction->pack_lockfiles[transaction->pack_lockfiles_nr++] =
+			register_tempfile(final_path.buf);
+
 		odb_source_prepare(transaction->quarantine,
 				   ODB_PREPARE_FLUSH_CACHES);
+
 	}
 
-	return 0;
+	ret = 0;
+
+out:
+	strbuf_release(&final_path);
+	strbuf_release(&pack_name);
+	strbuf_release(&keep_msg);
+
+	return ret;
 }
 
 static int odb_transaction_files_finalize(struct odb_transaction *base)
-- 
2.56.0.173.gf57c9a7388