Phase 2: Subsonic REST API framework (auth, response formatting, error handling, routing) plus user permission model

internal/subsonic/response.go

@@ -0,0 +1,59 @@
+package subsonic
+
+import (
+	"encoding/json"
+	"encoding/xml"
+	"net/http"
+	"regexp"
+)
+
+// jsonEnvelope is the {"subsonic-response": {...}} wrapper the JSON
+// format requires around the same Response struct XML uses directly.
+type jsonEnvelope struct {
+	Response *Response `json:"subsonic-response"`
+}
+
+// jsonpCallbackRE matches a safe JavaScript identifier (including dotted
+// paths like "foo.bar", which some clients use). Anything else is
+// rejected outright rather than interpolated into the response body,
+// which would otherwise be a script-injection vector.
+var jsonpCallbackRE = regexp.MustCompile(`^[A-Za-z_$][A-Za-z0-9_$]*(\.[A-Za-z_$][A-Za-z0-9_$]*)*$`)
+
+// writeResponse serializes resp in the format requested by the client
+// (f=xml, the default; f=json; f=jsonp with a validated callback) and
+// writes it with the correct Content-Type. It never returns an error to
+// the caller — formatting failures are logged-and-swallowed rather than
+// leaking a broken partial body or a Go panic through the wire.
+func writeResponse(w http.ResponseWriter, format, callback string, resp *Response) {
+	switch format {
+	case "json":
+		w.Header().Set("Content-Type", "application/json")
+		_ = json.NewEncoder(w).Encode(jsonEnvelope{Response: resp})
+	case "jsonp":
+		w.Header().Set("Content-Type", "application/javascript")
+		body, err := json.Marshal(jsonEnvelope{Response: resp})
+		if err != nil {
+			return
+		}
+		if !jsonpCallbackRE.MatchString(callback) {
+			// Should already be rejected before we get here; fail safe
+			// by falling back to a fixed, harmless callback name rather
+			// than ever writing an unvalidated callback into the body.
+			callback = "callback"
+		}
+		_, _ = w.Write([]byte(callback + "("))
+		_, _ = w.Write(body)
+		_, _ = w.Write([]byte(")"))
+	default:
+		w.Header().Set("Content-Type", "application/xml; charset=utf-8")
+		_, _ = w.Write([]byte(xml.Header))
+		_ = xml.NewEncoder(w).Encode(resp)
+	}
+}
+
+func errorResponse(err *apiError) *Response {
+	r := newResponse()
+	r.Status = "failed"
+	r.Error = &Error{Code: err.Code, Message: err.Message}
+	return r
+}