Re: [PATCH] refs: drop "clear packed-refs while locked" assertion

[Michael Haggerty <mhagger@alum.mit.edu>]

On Fri, Dec 8, 2017 at 12:22 PM, Jeff King <peff@peff.net> wrote:
> This patch fixes a regression in v2.14.0. It's actually fixed already in
> v2.15.0 because all of the packed-ref code there was rewritten. So
> there's no point in applying this on "master" or even "maint". But I
> figured it was worth sharing here in case somebody else runs across it,
> and in case we ever do a v2.14.4 release.

I forgot to respond to this. +1

Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>

Michael

> -- >8 --
> In clear_packed_ref_cache(), we assert that we're not
> currently holding the packed-refs lock. But in each of the
> three code paths that can hit this, the assertion is either
> a noop or actively does the wrong thing:
>
>  1. in rollback_packed_refs(), we will have just released
>     the lock before calling the function, and so the
>     assertion can never trigger.
>
>  2. get_packed_ref_cache() can reach this assertion via
>     validate_packed_ref_cache(). But it calls the validate
>     function only when it knows that we're not holding the
>     lock, so again, the assertion can never trigger.
>
>  3. lock_packed_refs() also calls validate_packed_ref_cache().
>     In this case we're _always_ holding the lock, which
>     means any time the validate function has to clear the
>     cache, we'll trigger this assertion and die.
>
>     This doesn't happen often in practice because the
>     validate function clears the cache only if we find that
>     somebody else has racily rewritten the packed-refs file
>     between the time we read it and the time we took the lock.
>
>     So most of the time we don't reach the assertion at all
>     (nobody has racily written the file so there's no need
>     to clear the cache). And when we do, it is not actually
>     indicative of a bug; clearing the cache while holding
>     the lock is the right thing to do here.
>
> This final case is relatively new, being triggerd by the
> extra validation added in fed6ebebf1 (lock_packed_refs():
> fix cache validity check, 2017-06-12).
>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
>  refs/files-backend.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index f21a954ce7..dd41e1d382 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -99,8 +99,6 @@ static void clear_packed_ref_cache(struct files_ref_store *refs)
>         if (refs->packed) {
>                 struct packed_ref_cache *packed_refs = refs->packed;
>
> -               if (is_lock_file_locked(&refs->packed_refs_lock))
> -                       die("BUG: packed-ref cache cleared while locked");
>                 refs->packed = NULL;
>                 release_packed_ref_cache(packed_refs);
>         }
> --
> 2.15.1.659.g8bd2eae3ea