refs/files-backend: fix memory leak in lock_ref_for_update

After the previous patch, none of the functions we call hold on to
`referent.buf`, so we can safely release the string buffer before
returning.

Reviewed-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Martin Ågren

refs/files-backend.c

@@ -2756,7 +2756,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 	struct strbuf referent = STRBUF_INIT;
 	int mustexist = (update->flags & REF_HAVE_OLD) &&
 		!is_null_oid(&update->old_oid);
-	int ret;
+	int ret = 0;
 	struct ref_lock *lock;
 
 	files_assert_main_repository(refs, "lock_ref_for_update");
@@ -2768,7 +2768,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 		ret = split_head_update(update, transaction, head_ref,
 					affected_refnames, err);
 		if (ret)
-			return ret;
+			goto out;
 	}
 
 	ret = lock_raw_ref(refs, update->refname, mustexist,
@@ -2782,7 +2782,7 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 		strbuf_addf(err, "cannot lock ref '%s': %s",
 			    original_update_refname(update), reason);
 		free(reason);
-		return ret;
+		goto out;
 	}
 
 	update->backend_data = lock;
@@ -2801,10 +2801,12 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 					strbuf_addf(err, "cannot lock ref '%s': "
 						    "error reading reference",
 						    original_update_refname(update));
-					return -1;
+					ret = -1;
+					goto out;
 				}
 			} else if (check_old_oid(update, &lock->old_oid, err)) {
-				return TRANSACTION_GENERIC_ERROR;
+				ret = TRANSACTION_GENERIC_ERROR;
+				goto out;
 			}
 		} else {
 			/*
@@ -2818,13 +2820,15 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 						  referent.buf, transaction,
 						  affected_refnames, err);
 			if (ret)
-				return ret;
+				goto out;
 		}
 	} else {
 		struct ref_update *parent_update;
 
-		if (check_old_oid(update, &lock->old_oid, err))
-			return TRANSACTION_GENERIC_ERROR;
+		if (check_old_oid(update, &lock->old_oid, err)) {
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
+		}
 
 		/*
 		 * If this update is happening indirectly because of a
@@ -2861,7 +2865,8 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 				    "cannot update ref '%s': %s",
 				    update->refname, write_err);
 			free(write_err);
-			return TRANSACTION_GENERIC_ERROR;
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
 		} else {
 			update->flags |= REF_NEEDS_COMMIT;
 		}
@@ -2875,10 +2880,14 @@ static int lock_ref_for_update(struct files_ref_store *refs,
 		if (close_ref(lock)) {
 			strbuf_addf(err, "couldn't close '%s.lock'",
 				    update->refname);
-			return TRANSACTION_GENERIC_ERROR;
+			ret = TRANSACTION_GENERIC_ERROR;
+			goto out;
 		}
 	}
-	return 0;
+
+out:
+	strbuf_release(&referent);
+	return ret;
 }
 
 /*