[PATCH v4] gc: reject if another gc is running, unless --force is given

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

This may happen when `git gc --auto` is run automatically, then the
user, to avoid wait time, switches to a new terminal, keeps working
and `git gc --auto` is started again because the first gc instance has
not clean up the repository.

This patch tries to avoid multiple gc running, especially in --auto
mode. In the worst case, gc may be delayed 12 hours if a daemon reuses
the pid stored in gc.pid.

kill(pid, 0) support is added to MinGW port so it should work on
Windows too.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 this patch is getting cooler:

 - uname() is dropped in favor of gethostbyname(), which is supported
   by MinGW port.
 - host name is stored in gc.pid as junio suggested so...
 - now we can say "gc is already running on _this host_ with _this pid_..."

 Documentation/git-gc.txt |  6 ++++-
 builtin/gc.c             | 67 ++++++++++++++++++++++++++++++++++++++++++++++++
 compat/mingw.c           |  6 +++++
 3 files changed, 78 insertions(+), 1 deletion(-)

diff --git a/Documentation/git-gc.txt b/Documentation/git-gc.txt
index 2402ed6..e158a3b 100644
--- a/Documentation/git-gc.txt
+++ b/Documentation/git-gc.txt
@@ -9,7 +9,7 @@ git-gc - Cleanup unnecessary files and optimize the local repository
 SYNOPSIS
 --------
 [verse]
-'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune]
+'git gc' [--aggressive] [--auto] [--quiet] [--prune=<date> | --no-prune] [--force]
 
 DESCRIPTION
 -----------
@@ -72,6 +72,10 @@ automatic consolidation of packs.
 --quiet::
 	Suppress all progress reports.
 
+--force::
+	Force `git gc` to run even if there may be another `git gc`
+	instance running on this repository.
+
 Configuration
 -------------
 
diff --git a/builtin/gc.c b/builtin/gc.c
index 6be6c8d..99682f0 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -167,11 +167,69 @@ static int need_to_gc(void)
 	return 1;
 }
 
+/* return NULL on success, else hostname running the gc */
+static const char *lock_repo_for_gc(int force, pid_t* ret_pid)
+{
+	static struct lock_file lock;
+	static char locking_host[128];
+	char my_host[128];
+	struct strbuf sb = STRBUF_INIT;
+	struct stat st;
+	uintmax_t pid;
+	FILE *fp;
+	int fd, should_exit;
+
+	if (gethostname(my_host, sizeof(my_host)))
+		strcpy(my_host, "unknown");
+
+	fd = hold_lock_file_for_update(&lock, git_path("gc.pid"),
+				       LOCK_DIE_ON_ERROR);
+	if (!force) {
+		fp = fopen(git_path("gc.pid"), "r");
+		memset(locking_host, 0, sizeof(locking_host));
+		should_exit =
+			fp != NULL &&
+			!fstat(fileno(fp), &st) &&
+			/*
+			 * 12 hour limit is very generous as gc should
+			 * never take that long. On the other hand we
+			 * don't really need a strict limit here,
+			 * running gc --auto one day late is not a big
+			 * problem. --force can be used in manual gc
+			 * after the user verifies that no gc is
+			 * running.
+			 */
+			time(NULL) - st.st_mtime <= 12 * 3600 &&
+			fscanf(fp, "%"PRIuMAX" %127c", &pid, locking_host) == 2 &&
+			!strcmp(locking_host, my_host) &&
+			!kill(pid, 0);
+		if (fp != NULL)
+			fclose(fp);
+		if (should_exit) {
+			if (fd >= 0)
+				rollback_lock_file(&lock);
+			*ret_pid = pid;
+			return locking_host;
+		}
+	}
+
+	strbuf_addf(&sb, "%"PRIuMAX" %s",
+		    (uintmax_t) getpid(), my_host);
+	write_in_full(fd, sb.buf, sb.len);
+	strbuf_release(&sb);
+	commit_lock_file(&lock);
+
+	return NULL;
+}
+
 int cmd_gc(int argc, const char **argv, const char *prefix)
 {
 	int aggressive = 0;
 	int auto_gc = 0;
 	int quiet = 0;
+	int force = 0;
+	const char *name;
+	pid_t pid;
 
 	struct option builtin_gc_options[] = {
 		OPT__QUIET(&quiet, N_("suppress progress reporting")),
@@ -180,6 +238,7 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 			PARSE_OPT_OPTARG, NULL, (intptr_t)prune_expire },
 		OPT_BOOLEAN(0, "aggressive", &aggressive, N_("be more thorough (increased runtime)")),
 		OPT_BOOLEAN(0, "auto", &auto_gc, N_("enable auto-gc mode")),
+		OPT_BOOL(0, "force", &force, N_("force running gc even if there may be another gc running")),
 		OPT_END()
 	};
 
@@ -225,6 +284,14 @@ int cmd_gc(int argc, const char **argv, const char *prefix)
 	} else
 		add_repack_all_option();
 
+	name = lock_repo_for_gc(force, &pid);
+	if (name) {
+		if (auto_gc)
+			return 0; /* be quiet on --auto */
+		die(_("gc is already running on machine '%s' pid %"PRIuMAX" (use --force if not)"),
+		    name, (uintmax_t)pid);
+	}
+
 	if (pack_refs && run_command_v_opt(pack_refs_cmd.argv, RUN_GIT_CMD))
 		return error(FAILED_RUN, pack_refs_cmd.argv[0]);
 
diff --git a/compat/mingw.c b/compat/mingw.c
index bb92c43..22ee9ef 100644
--- a/compat/mingw.c
+++ b/compat/mingw.c
@@ -1086,6 +1086,12 @@ int mingw_kill(pid_t pid, int sig)
 		errno = err_win_to_posix(GetLastError());
 		CloseHandle(h);
 		return -1;
+	} else if (pid > 0 && sig == 0) {
+		HANDLE h = OpenProcess(PROCESS_QUERY_INFORMATION, FALSE, pid);
+		if (h) {
+			CloseHandle(h);
+			return 0;
+		}
 	}
 
 	errno = EINVAL;
-- 
1.8.2.83.gc99314b