Show an error screen when the SFU is at capacity

locales/en/app.json

@@ -82,6 +82,8 @@
     "e2ee_unsupported_description": "Your web browser does not support encrypted calls. Supported browsers include Chrome, Safari, and Firefox 117+.",
     "generic": "Something went wrong",
     "generic_description": "Submitting debug logs will help us track down the problem.",
+    "insufficient_capacity": "Insufficient capacity",
+    "insufficient_capacity_description": "The server has reached its maximum capacity and you cannot join the call at this time. Try again later, or contact your server admin if the problem persists.",
     "open_elsewhere": "Opened in another tab",
     "open_elsewhere_description": "{{brand}} has been opened in another tab. If that doesn't sound right, try reloading the page."
   },

src/RichError.tsx

@@ -7,7 +7,10 @@ Please see LICENSE in the repository root for full details.
 
 import { type FC, type ReactNode } from "react";
 import { useTranslation } from "react-i18next";
-import { PopOutIcon } from "@vector-im/compound-design-tokens/assets/web/icons";
+import {
+  HostIcon,
+  PopOutIcon,
+} from "@vector-im/compound-design-tokens/assets/web/icons";
 
 import { ErrorView } from "./ErrorView";
 
@@ -46,3 +49,19 @@ export class OpenElsewhereError extends RichError {
     super("App opened in another tab", <OpenElsewhere />);
   }
 }
+
+const InsufficientCapacity: FC = () => {
+  const { t } = useTranslation();
+
+  return (
+    <ErrorView Icon={HostIcon} title={t("error.insufficient_capacity")}>
+      <p>{t("error.insufficient_capacity_description")}</p>
+    </ErrorView>
+  );
+};
+
+export class InsufficientCapacityError extends RichError {
+  public constructor() {
+    super("Insufficient server capacity", <InsufficientCapacity />);
+  }
+}

src/livekit/useECConnectionState.test.tsx

@@ -0,0 +1,72 @@
+/*
+Copyright 2025 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial
+Please see LICENSE in the repository root for full details.
+*/
+
+import { type FC, useCallback, useState } from "react";
+import { test } from "vitest";
+import {
+  ConnectionError,
+  ConnectionErrorReason,
+  type Room,
+} from "livekit-client";
+import userEvent from "@testing-library/user-event";
+import { render, screen } from "@testing-library/react";
+import { ErrorBoundary } from "@sentry/react";
+import { MemoryRouter } from "react-router-dom";
+
+import { ErrorPage } from "../FullScreenView";
+import { useECConnectionState } from "./useECConnectionState";
+import { type SFUConfig } from "./openIDSFU";
+
+test.each<[string, ConnectionError]>([
+  [
+    "LiveKit",
+    new ConnectionError("", ConnectionErrorReason.InternalError, 503),
+  ],
+  [
+    "LiveKit Cloud",
+    new ConnectionError("", ConnectionErrorReason.NotAllowed, 429),
+  ],
+])(
+  "useECConnectionState throws error when %s hits track limit",
+  async (_server, error) => {
+    const mockRoom = {
+      on: () => {},
+      off: () => {},
+      once: () => {},
+      connect: () => {
+        throw error;
+      },
+      localParticipant: {
+        getTrackPublication: () => {},
+        createTracks: () => [],
+      },
+    } as unknown as Room;
+
+    const TestComponent: FC = () => {
+      const [sfuConfig, setSfuConfig] = useState<SFUConfig | undefined>(
+        undefined,
+      );
+      const connect = useCallback(
+        () => setSfuConfig({ url: "URL", jwt: "JWT token" }),
+        [],
+      );
+      useECConnectionState({}, false, mockRoom, sfuConfig);
+      return <button onClick={connect}>Connect</button>;
+    };
+
+    const user = userEvent.setup();
+    render(
+      <MemoryRouter>
+        <ErrorBoundary fallback={ErrorPage}>
+          <TestComponent />
+        </ErrorBoundary>
+      </MemoryRouter>,
+    );
+    await user.click(screen.getByRole("button", { name: "Connect" }));
+    screen.getByText("error.insufficient_capacity");
+  },
+);

src/livekit/useECConnectionState.ts

@@ -7,6 +7,7 @@ Please see LICENSE in the repository root for full details.
 
 import {
   type AudioCaptureOptions,
+  ConnectionError,
   ConnectionState,
   type LocalTrack,
   type Room,
@@ -19,6 +20,7 @@ import * as Sentry from "@sentry/react";
 
 import { type SFUConfig, sfuConfigEquals } from "./openIDSFU";
 import { PosthogAnalytics } from "../analytics/PosthogAnalytics";
+import { InsufficientCapacityError, RichError } from "../RichError";
 
 declare global {
   interface Window {
@@ -106,7 +108,8 @@ async function doConnect(
     await connectAndPublish(livekitRoom, sfuConfig, preCreatedAudioTrack, []);
   } catch (e) {
     preCreatedAudioTrack?.stop();
-    logger.warn("Stopped precreated audio tracks.", e);
+    logger.debug("Stopped precreated audio tracks.");
+    throw e;
   }
 }
 
@@ -129,12 +132,22 @@ async function connectAndPublish(
   tracker.cacheConnectStart();
   livekitRoom.once(RoomEvent.SignalConnected, tracker.cacheWsConnect);
 
-  await livekitRoom!.connect(sfuConfig!.url, sfuConfig!.jwt, {
-    // Due to stability issues on Firefox we are testing the effect of different
-    // timeouts, and allow these values to be set through the console
-    peerConnectionTimeout: window.peerConnectionTimeout ?? 45000,
-    websocketTimeout: window.websocketTimeout ?? 45000,
-  });
+  try {
+    await livekitRoom!.connect(sfuConfig!.url, sfuConfig!.jwt, {
+      // Due to stability issues on Firefox we are testing the effect of different
+      // timeouts, and allow these values to be set through the console
+      peerConnectionTimeout: window.peerConnectionTimeout ?? 45000,
+      websocketTimeout: window.websocketTimeout ?? 45000,
+    });
+  } catch (e) {
+    // LiveKit uses 503 to indicate that the server has hit its track limits
+    // or equivalently, 429 in LiveKit Cloud
+    // For reference, the 503 response is generated at: https://github.com/livekit/livekit/blob/fcb05e97c5a31812ecf0ca6f7efa57c485cea9fb/pkg/service/rtcservice.go#L171
+
+    if (e instanceof ConnectionError && (e.status === 503 || e.status === 429))
+      throw new InsufficientCapacityError();
+    throw e;
+  }
 
   // remove listener in case the connect promise rejects before `SignalConnected` is emitted.
   livekitRoom.off(RoomEvent.SignalConnected, tracker.cacheWsConnect);
@@ -175,6 +188,8 @@ export function useECConnectionState(
 
   const [isSwitchingFocus, setSwitchingFocus] = useState(false);
   const [isInDoConnect, setIsInDoConnect] = useState(false);
+  const [error, setError] = useState<RichError | null>(null);
+  if (error !== null) throw error;
 
   const onConnStateChanged = useCallback((state: ConnectionState) => {
     if (state == ConnectionState.Connected) setSwitchingFocus(false);
@@ -256,7 +271,9 @@ export function useECConnectionState(
         initialAudioOptions,
       )
         .catch((e) => {
-          logger.error("Failed to connect to SFU", e);
+          if (e instanceof RichError)
+            setError(e); // Bubble up any error screens to React
+          else logger.error("Failed to connect to SFU", e);
         })
         .finally(() => setIsInDoConnect(false));
     }