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/refs-internal.h b/refs/refs-internal.h
index 53a6c5d842..ee298ec0d5 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_expects_existing_old_ref(struct ref_update *update);
+
 #endif /* REFS_REFS_INTERNAL_H */