fix: end the browser speed test feed without a 410 or 502

internal/server/handlers_panel_stat.go

@@ -298,18 +298,23 @@ func (s *Server) handleSpeedRelayOpen(w http.ResponseWriter, r *http.Request) {
 }
 
 // POST /api/panel/agents/{id}/speed/down - a piece of the browser's upload,
-// passed on to the agent. 410 once the agent has stopped reading.
+// passed on to the agent. 204 asks for the next piece, 200 says the agent has
+// stopped reading.
 func (s *Server) handleSpeedRelayFeed(w http.ResponseWriter, r *http.Request) {
+	body := http.MaxBytesReader(w, r.Body, speedUpCap)
 	rl := s.relays.get(r.PathValue("id"))
-	if rl == nil {
-		http.Error(w, "gone", http.StatusGone)
-		return
+	if rl != nil {
+		if _, err := io.Copy(rl.downW, body); err == nil {
+			w.WriteHeader(http.StatusNoContent)
+			return
+		}
 	}
-	if _, err := io.Copy(rl.downW, http.MaxBytesReader(w, r.Body, speedUpCap)); err != nil {
-		http.Error(w, "gone", http.StatusGone)
-		return
-	}
-	w.WriteHeader(http.StatusNoContent)
+	// Take the rest of the piece before answering: an answer sent over an
+	// unread body makes Go drop the connection, which a proxy in front turns
+	// into a 502.
+	_, _ = io.Copy(io.Discard, body)
+	w.Header().Set("Cache-Control", "no-store")
+	_, _ = io.WriteString(w, "done")
 }
 
 // GET /api/panel/agents/{id}/speed/up - the agent's upload, for the browser to