refs: fix interleaving hook calls with reference-transaction hook

In order to not repeatedly search for the reference-transaction hook in
case it's getting called multiple times, we use a caching mechanism to
only call `find_hook()` once. What was missed though is that the return
value of `find_hook()` actually comes from a static strbuf, which means
it will get overwritten when calling `find_hook()` again. As a result,
we may call the wrong hook with parameters of the reference-transaction
hook.

This scenario was spotted in the wild when executing a git-push(1) with
multiple references, where there are interleaving calls to both the
update and the reference-transaction hook. While initial calls to the
reference-transaction hook work as expected, it will stop working after
the next invocation of the update hook. The result is that we now start
calling the update hook with parameters and stdin of the
reference-transaction hook.

This commit fixes the issue by storing a copy of `find_hook()`'s return
value in the cache.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.c

@@ -2001,7 +2001,7 @@ static int run_transaction_hook(struct ref_transaction *transaction,
 	if (hook == &hook_not_found)
 		return ret;
 	if (!hook)
-		hook = find_hook("reference-transaction");
+		hook = xstrdup_or_null(find_hook("reference-transaction"));
 	if (!hook) {
 		hook = &hook_not_found;
 		return ret;

t/t1416-ref-transaction-hooks.sh

@@ -106,4 +106,30 @@ test_expect_success 'hook gets all queued updates in aborted state' '
 	test_cmp expect actual
 '
 
+test_expect_success 'interleaving hook calls succeed' '
+	test_when_finished "rm -r target-repo.git" &&
+
+	git init --bare target-repo.git &&
+
+	write_script target-repo.git/hooks/reference-transaction <<-\EOF &&
+		echo $0 "$@" >>actual
+	EOF
+
+	write_script target-repo.git/hooks/update <<-\EOF &&
+		echo $0 "$@" >>actual
+	EOF
+
+	cat >expect <<-EOF &&
+		hooks/update refs/tags/PRE 0000000000000000000000000000000000000000 63ac8e7bcdb882293465435909f54a96de17d4f7
+		hooks/reference-transaction prepared
+		hooks/reference-transaction committed
+		hooks/update refs/tags/POST 0000000000000000000000000000000000000000 99d53161c3a0a903b6561b9f6c0c665b3a476401
+		hooks/reference-transaction prepared
+		hooks/reference-transaction committed
+	EOF
+
+	git push ./target-repo.git PRE POST &&
+	test_cmp expect target-repo.git/actual
+'
+
 test_done