refs: specify error for regular refs with old_target

When a reference update tries to update a symref, but the ref in
question is actually a regular ref, we raise an error. However the error
raised in this situation is:

  verifying symref target: '<ref>': reference is missing but expected <old-target>

which is very generic and doesn't indicate the mismatch of types. Let's
make this error more specific:

  cannot lock ref '<ref>': expected symref with target '<old-target>': but is a regular ref

so that users have a clearer understanding.

Signed-off-by: Karthik Nayak <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: KarthikNayak

refs/files-backend.c

@@ -2490,14 +2490,16 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 
 		/*
 		 * Even if the ref is a regular ref, if `old_target` is set, we
-		 * check the referent value. Ideally `old_target` should only
-		 * be set for symrefs, but we're strict about its usage.
+		 * fail with an error.
 		 */
 		if (update->old_target) {
-			if (ref_update_check_old_target(referent.buf, update, err)) {
-				ret = TRANSACTION_GENERIC_ERROR;
-				goto out;
-			}
+			strbuf_addf(err, _("cannot lock ref '%s': "
+					   "expected symref with target '%s': "
+					   "but is a regular ref"),
+				    ref_update_original_update_refname(update),
+				    update->old_target);
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
 		} else if  (check_old_oid(update, &lock->old_oid, err)) {
 			ret = TRANSACTION_GENERIC_ERROR;
 			goto out;

refs/reftable-backend.c

@@ -931,6 +931,16 @@ static int reftable_be_transaction_prepare(struct ref_store *ref_store,
 		 * backend returns, which keeps our tests happy.
 		 */
 		if (u->old_target) {
+			if (!(u->type & REF_ISSYMREF)) {
+				strbuf_addf(err, _("cannot lock ref '%s': "
+					   "expected symref with target '%s': "
+					   "but is a regular ref"),
+					    ref_update_original_update_refname(u),
+					    u->old_target);
+				ret = -1;
+				goto done;
+			}
+
 			if (ref_update_check_old_target(referent.buf, u, err)) {
 				ret = -1;
 				goto done;