Re: [PATCH v5 07/11] maintenance: take a lock on the objects directory

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Thu, Sep 17 2020, Derrick Stolee via GitGitGadget wrote:

> 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>
> Signed-off-by: Junio C Hamano <gitster@pobox.com>
> ---
>  builtin/gc.c | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/builtin/gc.c b/builtin/gc.c
> index 00fff59bdb..7ba9c6f7c9 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;

There's now two different lock strategies in builtin/gc.c, the existing
one introduced in 64a99eb476 ("gc: reject if another gc is running,
unless --force is given", 2013-08-08) where we write the hostname to the
gc.pid file, and then discard the lockfile depending on a heuristic of
whether or not it's the same etc., and this one.

With this as an entry point we'll entirely do away with the old one
since we don't use the "gc --auto" entry point.

All of that may or may not be desirable, but I think a description in
the docs & tests for how these lock files should interact would be
helpful. E.g. writing a different hostname in the gc lockfile and
setting the time on it with with "test-tool chmtime" will cause it to
plow ahead, but doing the same for "git maintenance" will stop it in its
tracks no matter the time or content.