refs: create and use `ref_update_expects_existing_old_ref()`

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_expects_existing_old_ref()` and expose it internally via
'refs-internal.h'.

Helped-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/refs.c b/refs.c
index fa5471d219..00e87f8ee3 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_expects_existing_old_ref(struct ref_update *update)
+{
+	return (update->flags & REF_HAVE_OLD) &&
+		(!is_null_oid(&update->old_oid) || update->old_target);
+}