[PATCH v4 07/11] maintenance: take a lock on the objects directory

From: Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Date: Sep 4, 2020, 13:09 UTC
Message-ID: <c57998a1c8844fc39cc95877a85790566e8f3331.1599224956.git.gitgitgadget@gmail.com>
In-Reply-To: <pull.695.v4.git.1599224956.gitgitgadget@gmail.com>

From: Derrick Stolee <dstolee@microsoft.com>

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>
---
 builtin/gc.c | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/builtin/gc.c b/builtin/gc.c
index e94f263c77..1cebb7282d 100644
--- a/builtin/gc.c
+++ b/builtin/gc.c
@@ -798,6 +798,25 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts)
 {
 	int i, found_selected = 0;
 	int result = 0;
+	struct lock_file lk;
+	struct repository *r = the_repository;
+	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)
+			warning(_("lock file '%s' exists, skipping maintenance"),
+				lock_path);
+		free(lock_path);
+		return 0;
+	}
+	free(lock_path);
 
 	for (i = 0; !found_selected && i < TASK__COUNT; i++)
 		found_selected = tasks[i].selected_order >= 0;
@@ -818,6 +837,7 @@ static int maintenance_run_tasks(struct maintenance_run_opts *opts)
 		}
 	}
 
+	rollback_lock_file(&lk);
 	return result;
 }

-- 
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