Patrick Steinhardt [off-list ref] writes:

On Tue, May 14, 2024 at 02:44:06PM +0200, Karthik Nayak wrote:

quoted

From: Karthik Nayak <redacted>

The files and reftable backend, need to check if a ref must exist, so
that the required validation can be done. A ref must exist only when the
`old_oid` value of the update has been explicitly set and it is not the
`null_oid` value.

Since we also support symrefs now, we need to ensure that even when
`old_target` is set a ref must exist. While this was missed when we
added symref support in transactions, there are no active users of this
path. As we introduce the 'symref-verify' command in the upcoming
commits, it is important to fix this.

So let's export this to a function called `ref_update_ref_must_exist()`
and expose it internally via 'refs-internal.h'.

Signed-off-by: Karthik Nayak <redacted>
---
 refs.c                  | 6 ++++++
 refs/files-backend.c    | 3 +--
 refs/refs-internal.h    | 6 ++++++
 refs/reftable-backend.c | 2 +-
 4 files changed, 14 insertions(+), 3 deletions(-)

diff --git a/refs.c b/refs.c
index fa5471d219..59858fafdb 100644
--- a/refs.c
+++ b/refs.c

@@ -2863,3 +2863,9 @@ int ref_update_check_old_target(const char *referent, struct ref_update *update,
 			    referent, update->old_target);
 	return -1;
 }
+
+int ref_update_ref_must_exist(struct ref_update *update)
+{
+	return (update->flags & REF_HAVE_OLD) &&
+		(!is_null_oid(&update->old_oid) || update->old_target);
+}

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 3957bfa579..2df204f891 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c

@@ -2411,8 +2411,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 			       struct strbuf *err)
 {
 	struct strbuf referent = STRBUF_INIT;
-	int mustexist = (update->flags & REF_HAVE_OLD) &&
-		!is_null_oid(&update->old_oid);
+	int mustexist = ref_update_ref_must_exist(update);

Okay. So we didn't notice this was broken because even though we started
writing symrefs via transactions now, none of the calles ever assert
that the old ref exists?

Yup, that's correct. The `git-symbolic-ref(1)` command doesn't ever
check for the old value and it is the only user of transactional symrefs
at this point.

quoted

 	int ret = 0;
 	struct ref_lock *lock;

diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index 53a6c5d842..5da3029e6c 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h

@@ -765,4 +765,10 @@ int ref_update_has_null_new_value(struct ref_update *update);
 int ref_update_check_old_target(const char *referent, struct ref_update *update,
 				struct strbuf *err);

+/*
+ * Check if the ref must exist, this means that the old_oid or
+ * old_target is non NULL.
+ */
+int ref_update_ref_must_exist(struct ref_update *update);

Seeing `ref_update_ref_must_exist()` as a standalone function wouldn't
quite tell me what it really does. It sounds a bit like this would
already assert the ref exists at the time of calling it.

We could call this `ref_upate_expects_existing_old_ref()`, which might
clarify the intent a bit.

Patrick

Yeah, that's better, will change. Thanks.