checker: add JoinRelative helper for service-relative owner names

checker/names.go

@@ -0,0 +1,37 @@
+// Copyright 2020-2026 The happyDomain Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package checker
+
+import "strings"
+
+// JoinRelative treats name as relative to origin, as happyDomain encodes
+// service-embedded record owners and subdomains. An empty or "@" name
+// resolves to the origin itself; an empty origin returns the trimmed name
+// unchanged. A name already suffixed by origin is returned as-is so that
+// absolute encodings round-trip safely. Trailing dots are stripped.
+func JoinRelative(name, origin string) string {
+	origin = strings.TrimSuffix(origin, ".")
+	name = strings.TrimSuffix(name, ".")
+	if origin == "" {
+		return name
+	}
+	if name == "" || name == "@" {
+		return origin
+	}
+	if name == origin || strings.HasSuffix(name, "."+origin) {
+		return name
+	}
+	return name + "." + origin
+}