Hold a user media stream open while we get devices

As per comment.

src/media-utils.ts

@@ -14,10 +14,55 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
+import { logger } from "matrix-js-sdk/src/logger";
+
+/**
+ * Finds a media device with label matching 'deviceName'
+ * @param deviceName The label of the device to look for
+ * @param devices The list of devices to search
+ * @returns A matching media device or undefined if no matching device was found
+ */
 export async function findDeviceByName(
-  deviceName: string
+  deviceName: string,
+  devices: MediaDeviceInfo[]
 ): Promise<string | undefined> {
-  const devices = await navigator.mediaDevices.enumerateDevices();
   const deviceInfo = devices.find((d) => d.label === deviceName);
   return deviceInfo?.deviceId;
 }
+
+/**
+ * Gets the available audio input/output and video input devices
+ * from the browser: a wrapper around mediaDevices.enumerateDevices()
+ * that requests a stream and holds it while calling enumerateDevices().
+ * This is because some browsers (Firefox) only return device labels when
+ * the app has an active user media stream. In Chrome, this will get a
+ * stream from the default camera which can mean, for example, that the
+ * light for the FaceTime camera turns on briefly even if you selected
+ * another camera. Once the Permissions API
+ * (https://developer.mozilla.org/en-US/docs/Web/API/Permissions_API)
+ * is ready for primetime, this should allow us to avoid this.
+ *
+ * @return The available media devices
+ */
+export async function getDevices(): Promise<MediaDeviceInfo[]> {
+  let stream;
+  try {
+    stream = await navigator.mediaDevices.getUserMedia({
+      audio: true,
+      video: true,
+    });
+  } catch (e) {
+    logger.info("Couldn't get media stream for enumerateDevices: failing");
+    throw e;
+  }
+
+  try {
+    return await navigator.mediaDevices.enumerateDevices();
+  } catch (error) {
+    logger.warn("Unable to refresh WebRTC Devices: ", error);
+  } finally {
+    for (const track of stream.getTracks()) {
+      track.stop();
+    }
+  }
+}

src/room/GroupCallView.tsx

@@ -32,7 +32,7 @@ import { useRoomAvatar } from "./useRoomAvatar";
 import { useSentryGroupCallHandler } from "./useSentryGroupCallHandler";
 import { useLocationNavigation } from "../useLocationNavigation";
 import { useMediaHandler } from "../settings/useMediaHandler";
-import { findDeviceByName } from "../media-utils";
+import { findDeviceByName, getDevices } from "../media-utils";
 
 declare global {
   interface Window {
@@ -96,11 +96,16 @@ export function GroupCallView({
     if (widget && preload) {
       // In preload mode, wait for a join action before entering
       const onJoin = async (ev: CustomEvent<IWidgetApiRequest>) => {
+        // Get the available devices so we can match the selected device
+        // to its ID. This involves getting a media stream (see docs on
+        // the function) so we only do it once and re-use the result.
+        const devices = await getDevices();
+
         const { audioInput, videoInput } = ev.detail
           .data as unknown as JoinCallData;
 
         if (audioInput !== null) {
-          const deviceId = await findDeviceByName(audioInput);
+          const deviceId = await findDeviceByName(audioInput, devices);
           if (!deviceId) {
             logger.warn("Unknown audio input: " + audioInput);
           } else {
@@ -112,7 +117,7 @@ export function GroupCallView({
         }
 
         if (videoInput !== null) {
-          const deviceId = await findDeviceByName(videoInput);
+          const deviceId = await findDeviceByName(videoInput, devices);
           if (!deviceId) {
             logger.warn("Unknown video input: " + videoInput);
           } else {