maintenance: take a lock on the objects directory

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 no maintenance tasks are attempted. This will become very important later when we implement the 'prefetch' task, as this is our stop-gap from creating a recursive process loop between 'git fetch' and 'git maintenance run --auto'. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Derrick Stolee committed Sep 17, 2020 at 18:11 UTC d7514f6ed57d20bcc9dcfb43016b95dba82ba790

1 file changed +20

builtin/gc.c

+20

@@ -798,6 +798,25 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts)

798 {

799 int i, found_selected = 0;

800 int result = 0;

801 + struct lock_file lk;

802 + struct repository *r = the_repository;

803 + char *lock_path = xstrfmt("%s/maintenance", r->objects->odb->path);

804 +

805 + if (hold_lock_file_for_update(&lk, lock_path, LOCK_NO_DEREF) < 0) {

806 + /*

807 + * Another maintenance command is running.

808 + *

809 + * If --auto was provided, then it is likely due to a

810 + * recursive process stack. Do not report an error in

811 + * that case.

812 + */

813 + if (!opts->auto_flag && !opts->quiet)

814 + warning(_("lock file '%s' exists, skipping maintenance"),

815 + lock_path);

816 + free(lock_path);

817 + return 0;

818 + }

819 + free(lock_path);

820

821 for (i = 0; !found_selected && i < TASK__COUNT; i++)

822 found_selected = tasks[i].selected_order >= 0;

@@ -818,6 +837,7 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts)

837 }

838 }

839

840 + rollback_lock_file(&lk);

841 return result;

842 }

843