Re: [PATCH v2 7/7] reflog expire: don't assert the OID when locking refs

[Duy Nguyen <pclouds@gmail.com>]

On Thu, Mar 14, 2019 at 7:35 PM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
>
> During reflog expiry, the cmd_reflog_expire() function first iterates
> over all reflogs in logs/*, and then one-by-one acquires the lock for
> each one to expire its reflog by getting a *.lock file on the
> corresponding loose ref[1] (even if the actual ref is packed).
>
> This lock is needed, but what isn't needed is locking the loose ref as
> a function of the OID we found from that first iteration. By the time
> we get around to re-visiting the reference some of the OIDs may have
> changed.
>
> Thus the verify_lock() function called by the lock_ref_oid_basic()
> function being changed here would fail with e.g. "ref '%s' is at %s
> but expected %s" if the repository was being updated concurrent to the
> "reflog expire".
>
> By not passing the OID to it we'll try to lock the reference
> regardless of it last known OID. Locking as a function of the OID
> would make "reflog expire" exit with a non-zero exit status under such
> contention, which in turn meant that a "gc" command (which expires
> reflogs before forking to the background) would encounter a hard
> error.

Passing NULL oid has another side(?) effect, which I don't know if it
matters at all. Before, the mustexist flag in lock_ref_oid_basic() is
true. Now it's false. This affects refs_resolve_ref_unsafe() calls in
there. But that's where I'm stuck.

> diff --git a/refs/files-backend.c b/refs/files-backend.c
> index ef053f716c..4d4d226601 100644
> --- a/refs/files-backend.c
> +++ b/refs/files-backend.c
> @@ -3037,7 +3037,7 @@ static int files_reflog_expire(struct ref_store *ref_store,
>          * reference itself, plus we might need to update the
>          * reference if --updateref was specified:
>          */
> -       lock = lock_ref_oid_basic(refs, refname, oid,
> +       lock = lock_ref_oid_basic(refs, refname, NULL /* NOT oid! */,

Maybe mention this not oid in the comment block above. Or just drop
it. Reading this comment without the commit message does not really
help answer "why not oid?". Or perhaps /* don't verify oid */

>                                   NULL, NULL, REF_NO_DEREF,
>                                   &type, &err);
>         if (!lock) {
> --
-- 
Duy