fix: multivalue tags

http/data.go

@@ -48,9 +48,32 @@ func (d *data) Check(path string) bool {
 }
 
 func handle(fn handleFunc, prefix string, store *storage.Storage, server *settings.Server) http.Handler {
-	handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+    // trackingWriter wraps the ResponseWriter to detect if headers/body were already written.
+    type trackingWriter struct {
+        http.ResponseWriter
+        wroteHeader bool
+        wroteBody   bool
+    }
+    func (tw *trackingWriter) WriteHeader(code int) {
+        if !tw.wroteHeader {
+            tw.wroteHeader = true
+            tw.ResponseWriter.WriteHeader(code)
+        }
+    }
+    func (tw *trackingWriter) Write(b []byte) (int, error) {
+        // Any Write implies headers committed (status 200 if none set)
+        if !tw.wroteHeader {
+            tw.wroteHeader = true
+        }
+        tw.wroteBody = true
+        return tw.ResponseWriter.Write(b)
+    }
+
+    handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+        // Wrap writer to prevent superfluous WriteHeader calls
+        tw := &trackingWriter{ResponseWriter: w}
 		for k, v := range globalHeaders {
-			w.Header().Set(k, v)
+			tw.Header().Set(k, v)
 		}
 
 		settings, err := store.Settings.Get()
@@ -59,7 +82,7 @@ func handle(fn handleFunc, prefix string, store *storage.Storage, server *settin
 			return
 		}
 
-		status, err := fn(w, r, &data{
+		status, err := fn(tw, r, &data{
 			Runner:   &runner.Runner{Enabled: server.EnableExec, Settings: settings},
 			store:    store,
 			settings: settings,
@@ -76,7 +99,10 @@ func handle(fn handleFunc, prefix string, store *storage.Storage, server *settin
 			if status == http.StatusBadRequest && err != nil {
 				txt += " (" + err.Error() + ")"
 			}
-			http.Error(w, strconv.Itoa(status)+" "+txt, status)
+			// Only write an error response if nothing was written yet.
+			if !tw.wroteHeader && !tw.wroteBody {
+				http.Error(tw, strconv.Itoa(status)+" "+txt, status)
+			}
 			return
 		}
 	})