refs/reftable: don't fail empty transactions in repo without HEAD

Under normal circumstances, it shouldn't ever happen that a repository
has no HEAD reference. In fact, git-update-ref(1) would fail any request
to delete the HEAD reference, and a newly initialized repository always
pre-creates it, too.

We have however changed git-clone(1) to partially initialize the
refdb just up to the point where remote helpers can find the
repository. With that change, we are going to run into a situation
where repositories have no refs at all.

Now there is a very particular edge case in this situation: when
preparing an empty ref transacton, we end up returning whatever value
`read_ref_without_reload()` returned to the caller. Under normal
conditions this would be fine: "HEAD" should usually exist, and thus the
function would return `0`. But if "HEAD" doesn't exist, the function
returns a positive value which we end up returning to the caller.

Fix this bug by resetting the return code to `0` and add a test.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

refs/reftable-backend.c

@@ -821,6 +821,7 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 				      &head_referent, &head_type);
 	if (ret < 0)
 		goto done;
+	ret = 0;
 
 	for (i = 0; i < transaction->nr; i++) {
 		struct ref_update *u = transaction->updates[i];

t/t0610-reftable-basics.sh

@@ -328,6 +328,19 @@ test_expect_success 'ref transaction: writes are synced' '
 	EOF
 '
 
+test_expect_success 'ref transaction: empty transaction in empty repo' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_commit -C repo --no-tag A &&
+	COMMIT=$(git -C repo rev-parse HEAD) &&
+	git -C repo update-ref -d refs/heads/main &&
+	test-tool -C repo ref-store main delete-refs REF_NO_DEREF msg HEAD &&
+	git -C repo update-ref --stdin <<-EOF
+	prepare
+	commit
+	EOF
+'
+
 test_expect_success 'pack-refs: compacts tables' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&