refs: atomically record overwritten ref in update_symref

When updating a symref with update_symref it's currently not possible to
know for sure what was the previous value that was overwritten. Extend
refs_update_symref under a new function name, to record the value after
the ref has been locked if the caller of refs_update_symref_extended
requests it via a new variable in the function call. Make the return
value of the function notify the caller, if the previous value was
actually not a symbolic reference. Keep the original refs_update_symref
function with the same signature, but now as a wrapper around
refs_update_symref_extended.

Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ferdinandyb

refs.c

@@ -2115,6 +2115,13 @@ int peel_iterated_oid(struct repository *r, const struct object_id *base, struct
 
 int refs_update_symref(struct ref_store *refs, const char *ref,
 		       const char *target, const char *logmsg)
+{
+	return refs_update_symref_extended(refs, ref, target, logmsg, NULL);
+}
+
+int refs_update_symref_extended(struct ref_store *refs, const char *ref,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
@@ -2125,10 +2132,22 @@ int refs_update_symref(struct ref_store *refs, const char *ref,
 	    ref_transaction_update(transaction, ref, NULL, NULL,
 				   target, NULL, REF_NO_DEREF,
 				   logmsg, &err) ||
-	    ref_transaction_commit(transaction, &err)) {
+	    ref_transaction_prepare(transaction, &err)) {
 		ret = error("%s", err.buf);
+		goto cleanup;
 	}
+	if (referent && refs_read_symbolic_ref(refs, ref, referent) == NOT_A_SYMREF) {
+		struct object_id oid;
+		if (!refs_read_ref(refs, ref, &oid)) {
+			strbuf_addstr(referent, oid_to_hex(&oid));
+			ret = NOT_A_SYMREF;
+		}
+	}
+
+	if (ref_transaction_commit(transaction, &err))
+		ret = error("%s", err.buf);
 
+cleanup:
 	strbuf_release(&err);
 	if (transaction)
 		ref_transaction_free(transaction);
@@ -2948,4 +2967,3 @@ 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);
 }
-

refs.h

@@ -584,6 +584,10 @@ int refs_copy_existing_ref(struct ref_store *refs, const char *oldref,
 int refs_update_symref(struct ref_store *refs, const char *refname,
 		       const char *target, const char *logmsg);
 
+int refs_update_symref_extended(struct ref_store *refs, const char *refname,
+		       const char *target, const char *logmsg,
+		       struct strbuf *referent);
+
 enum action_on_err {
 	UPDATE_REFS_MSG_ON_ERR,
 	UPDATE_REFS_DIE_ON_ERR,