Re: [PATCH] refs: drop "clear packed-refs while locked" assertion
On Fri, Dec 8, 2017 at 12:22 PM, Jeff King <peff@peff.net> wrote:
Show 5 quoted lines
> 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
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> -- >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.g8bd2eae3eaMichael Haggerty <mhagger@alum.mit.edu>