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refs/files: add a comment about refs_reflog_exists() call

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Add a comment about why it is that we need to check for the the
existence of a reflog we're deleting after we've successfully acquired
the lock in files_reflog_expire(). As noted in [1] the lock protocol
for reflogs is somewhat intuitive.

This early exit code the comment applies to dates all the way back to
4264dc15e1 (git reflog expire, 2006-12-19).

1. https://lore.kernel.org/git/54DCDA42.2060800@alum.mit.edu/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Ævar Arnfjörð Bjarmason 2021-08-23 13:36:09 +02:00 committed by Junio C Hamano
parent 6f45ec88d2
commit 7aa7829f75
1 changed files with 13 additions and 0 deletions
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13
refs/files-backend.c
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@ -3061,6 +3061,19 @@ static int files_reflog_expire(struct ref_store *ref_store,
strbuf_release(&err);
return -1;
}
/*
* When refs are deleted, their reflog is deleted before the
* ref itself is deleted. This is because there is no separate
* lock for reflog; instead we take a lock on the ref with
* lock_ref_oid_basic().
*
* If a race happens and the reflog doesn't exist after we've
* acquired the lock that's OK. We've got nothing more to do;
* We were asked to delete the reflog, but someone else
* deleted it! The caller doesn't care that we deleted it,
* just that it is deleted. So we can return successfully.
*/
if (!refs_reflog_exists(ref_store, refname)) {
unlock_ref(lock);
return 0;