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create_symref: write reflog while holding lock 

We generally hold a lock on the matching ref while writing
to its reflog; this prevents two simultaneous writers from
clobbering each other's reflog lines (it does not even have
to be two symref updates; because we don't hold the lock, we
could race with somebody writing to the pointed-to ref via
HEAD, for example).

We can fix this by writing the reflog before we commit the
lockfile. This runs the risk of writing the reflog but
failing the final rename(), but at least we now err on the
same side as the rest of the ref code.

Noticed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Jeff King 2015-12-29 00:57:25 -05:00 committed by Junio C Hamano
parent 370e5ad65e
commit 396da8f7a0
1 changed files with 2 additions and 1 deletions
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3
refs/files-backend.c
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@ -2850,12 +2850,13 @@ static int create_symref_locked(struct ref_lock *lock, const char *refname,
return error("unable to fdopen %s: %s",
lock->lk->tempfile.filename.buf, strerror(errno));
update_symref_reflog(lock, refname, target, logmsg);
/* no error check; commit_ref will check ferror */
fprintf(lock->lk->tempfile.fp, "ref: %s\n", target);
if (commit_ref(lock) < 0)
return error("unable to write symref for %s: %s", refname,
strerror(errno));
update_symref_reflog(lock, refname, target, logmsg);
return 0;
}