fix: don't let a slow browser link time out the relayed speed test

internal/server/handlers_panel_stat.go

@@ -27,6 +27,9 @@ const (
 	speedDownCap = 256 << 20
 	speedDownFor = 6 * time.Second
 	speedUpCap   = 64 << 20
+	// speedRelayUpFor is how long the agent's upload is passed on to the
+	// browser: the agent's own window with slack.
+	speedRelayUpFor = 4 * time.Second
 )
 
 // statCache holds the last sample per agent and serializes its refresh.
@@ -364,7 +367,14 @@ func (s *Server) handleSpeedRelayUp(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	_, _ = io.Copy(rl.upW, http.MaxBytesReader(w, r.Body, speedUpCap))
+	body := http.MaxBytesReader(w, r.Body, speedUpCap)
+	// A proxy in front may hand over the whole upload at once: pass on only
+	// what the browser takes in the window, or the agent would wait for the
+	// rest to trickle through a slower link.
+	t := time.AfterFunc(speedRelayUpFor, func() { rl.upW.Close() })
+	_, _ = io.Copy(rl.upW, body)
+	t.Stop()
 	rl.upW.Close()
+	_, _ = io.Copy(io.Discard, body)
 	w.WriteHeader(http.StatusNoContent)
 }