feat：support $keep_only_declared and deduped $append for header token overrides

relay/common/override.go

@@ -847,24 +847,30 @@ func resolveHeaderOverrideValueByMapping(context map[string]interface{}, headerN
 		return "", false, fmt.Errorf("header value mapping cannot be empty")
 	}
 
-	sourceValue, exists := getHeaderValueFromContext(context, headerName)
-	if !exists {
-		return "", false, nil
+	appendTokens, err := parseHeaderAppendTokens(mapping)
+	if err != nil {
+		return "", false, err
 	}
-	sourceTokens := splitHeaderListValue(sourceValue)
-	if len(sourceTokens) == 0 {
-		return "", false, nil
+	keepOnlyDeclared := parseHeaderKeepOnlyDeclared(mapping)
+
+	sourceValue, exists := getHeaderValueFromContext(context, headerName)
+	sourceTokens := make([]string, 0)
+	if exists {
+		sourceTokens = splitHeaderListValue(sourceValue)
 	}
 
 	wildcardValue, hasWildcard := mapping["*"]
-	resultTokens := make([]string, 0, len(sourceTokens))
+	resultTokens := make([]string, 0, len(sourceTokens)+len(appendTokens))
 	for _, token := range sourceTokens {
 		replacementRaw, hasReplacement := mapping[token]
-		if !hasReplacement && hasWildcard {
+		if !hasReplacement && hasWildcard && !keepOnlyDeclared {
 			replacementRaw = wildcardValue
 			hasReplacement = true
 		}
 		if !hasReplacement {
+			if keepOnlyDeclared {
+				continue
+			}
 			resultTokens = append(resultTokens, token)
 			continue
 		}
@@ -875,6 +881,7 @@ func resolveHeaderOverrideValueByMapping(context map[string]interface{}, headerN
 		resultTokens = append(resultTokens, replacementTokens...)
 	}
 
+	resultTokens = append(resultTokens, appendTokens...)
 	resultTokens = lo.Uniq(resultTokens)
 	if len(resultTokens) == 0 {
 		return "", false, nil
@@ -882,6 +889,26 @@ func resolveHeaderOverrideValueByMapping(context map[string]interface{}, headerN
 	return strings.Join(resultTokens, ","), true, nil
 }
 
+func parseHeaderAppendTokens(mapping map[string]interface{}) ([]string, error) {
+	appendRaw, ok := mapping["$append"]
+	if !ok {
+		return nil, nil
+	}
+	return parseHeaderReplacementTokens(appendRaw)
+}
+
+func parseHeaderKeepOnlyDeclared(mapping map[string]interface{}) bool {
+	keepOnlyDeclaredRaw, ok := mapping["$keep_only_declared"]
+	if !ok {
+		return false
+	}
+	keepOnlyDeclared, ok := keepOnlyDeclaredRaw.(bool)
+	if !ok {
+		return false
+	}
+	return keepOnlyDeclared
+}
+
 func parseHeaderReplacementTokens(value interface{}) ([]string, error) {
 	switch raw := value.(type) {
 	case nil: