[PATCH] files-backend: unlock packed store only if locked

In commit 42c7f7ff9685 ("commit_packed_refs(): remove call to
`packed_refs_unlock()`", 2017-06-23), a call to packed_refs_unlock() was
added to files_initial_transaction_commit() in order to compensate for
removing that call from commit_packed_refs(). However, that call was
added in the cleanup section, which is run even if the packed_ref_store
was never locked (which happens if an error occurs earlier in the
function).

Create a new cleanup goto target which runs packed_refs_unlock(), and
ensure that only goto statements after a successful invocation of
packed_refs_lock() jump there.

Signed-off-by: Jonathan Tan <redacted>
---
I noticed this when one of our servers sent duplicate refs in its ref
advertisement (noticed through GIT_TRACE_PACKET). With this change (and
before the aforementioned commit 42c7f7ff9685), the error message is
"fatal: multiple updates for ref '<ref>' not allowed", which gives a
bigger clue to the problem. Currently, it is "fatal: BUG:
packed_refs_unlock() called when not locked".

(I couldn't replicate this problem in C Git.)
---
 refs/files-backend.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index f75d960e1..89bc5584a 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c

@@ -2931,13 +2931,14 @@ static int files_initial_transaction_commit(struct ref_store *ref_store,
 
 	if (initial_ref_transaction_commit(packed_transaction, err)) {
 		ret = TRANSACTION_GENERIC_ERROR;
-		goto cleanup;
+		goto locked_cleanup;
 	}
 
+locked_cleanup:
+	packed_refs_unlock(refs->packed_ref_store);
 cleanup:
 	if (packed_transaction)
 		ref_transaction_free(packed_transaction);
-	packed_refs_unlock(refs->packed_ref_store);
 	transaction->state = REF_TRANSACTION_CLOSED;
 	string_list_clear(&affected_refnames, 0);
 	return ret;

-- 
2.16.0.rc1.238.g530d649a79-goog

On Tue, Feb 06, 2018 at 12:36:15PM -0800, Jonathan Tan wrote:

In commit 42c7f7ff9685 ("commit_packed_refs(): remove call to
`packed_refs_unlock()`", 2017-06-23), a call to packed_refs_unlock() was
added to files_initial_transaction_commit() in order to compensate for
removing that call from commit_packed_refs(). However, that call was
added in the cleanup section, which is run even if the packed_ref_store
was never locked (which happens if an error occurs earlier in the
function).

Create a new cleanup goto target which runs packed_refs_unlock(), and
ensure that only goto statements after a successful invocation of
packed_refs_lock() jump there.

The explanation and patch look good to me.

But this all seemed strangely familiar... I think this is the same bug
as:

  https://public-inbox.org/git/20180118143841.1a4c674d@novascotia/

which is queued as mr/packed-ref-store-fix. It's listed as "will merge
to next" in the "what's cooking" from Jan 31st.

I actually like this double-label a bit more than what is queued on
mr/packed-ref-store-fix, though I am OK with either solution.

-Peff

On Wed, 7 Feb 2018 09:42:51 -0500
Jeff King [off-list ref] wrote:

But this all seemed strangely familiar... I think this is the same bug
as:

  https://public-inbox.org/git/20180118143841.1a4c674d@novascotia/

which is queued as mr/packed-ref-store-fix. It's listed as "will merge
to next" in the "what's cooking" from Jan 31st.

Ah...thanks, I didn't notice that.

I actually like this double-label a bit more than what is queued on
mr/packed-ref-store-fix, though I am OK with either solution.

Same here.