[11/21] maintenance: take a lock on the objects directory

Commit Message

Linus Arver via GitGitGadget July 7, 2020, 2:21 p.m. UTC

From: Derrick Stolee <[e-mail]>

Performing maintenance on a Git repository involves writing data to the
.git directory, which is not safe to do with multiple writers attempting
the same operation. Ensure that only one 'git maintenance' process is
running at a time by holding a file-based lock. Simply the presence of
the .git/maintenance.lock file will prevent future maintenance. This
lock is never committed, since it does not represent meaningful data.
Instead, it is only a placeholder.

If the lock file already exists, then fail silently. This will become
very important later when we implement the 'fetch' task, as this is our
stop-gap from creating a recursive process loop between 'git fetch' and
'git maintenance run'.

Signed-off-by: Derrick Stolee <[e-mail]>
---
 builtin/gc.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

Patch

diff --git a/builtin/gc.c b/builtin/gc.c
index 8f2143862c..e3c634fc3b 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -849,6 +849,24 @@  static int maintenance_run(struct repository *r)
 {
 	int i;
 	int result = 0;
+	struct lock_file lk;
+	char *lock_path = xstrfmt("%s/maintenance", r->objects->odb->path);
+
+	if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {
+		/*
+		 * Another maintenance command is running.
+		 *
+		 * If --auto was provided, then it is likely due to a
+		 * recursive process stack. Do not report an error in
+		 * that case.
+		 */
+		if (!opts.auto_flag && !opts.quiet)
+			error(_("lock file '%s' exists, skipping maintenance"),
+			      lock_path);
+		free(lock_path);
+		return 0;
+	}
+	free(lock_path);
 
 	if (opts.tasks_selected)
 		QSORT(tasks, num_tasks, compare_tasks_by_selection);
@@ -863,6 +881,7 @@  static int maintenance_run(struct repository *r)
 		result = tasks[i]->fn(r);
 	}
 
+	rollback_lock_file(&lk);
 	return result;
 }