add key rotation debug information

src/settings/DeveloperSettingsTab.tsx

@@ -69,6 +69,23 @@ import { Slider } from "../Slider";
 import { useUrlParams } from "../UrlParams";
 import { type CallViewModel } from "../state/CallViewModel/CallViewModel.ts";
 import { getSFUConfigWithOpenID } from "../livekit/openIDSFU";
+import { useBehavior } from "../useBehavior";
+
+/**
+ * Shows whether the call is large enough that MatrixRTC has stopped rotating the media key.
+ */
+const KeyRotationStatus: FC<{ vm: CallViewModel }> = ({ vm }) => {
+  const suppressed = useBehavior(vm.keyRotationSuppressed$);
+  const participantCount = useBehavior(vm.participantCount$);
+  return (
+    <p>
+      Media key rotation:{" "}
+      {suppressed
+        ? `suppressed, participant limit reached (${participantCount} participants)`
+        : `active (${participantCount} participants)`}
+    </p>
+  );
+};
 
 interface Props {
   client: MatrixClient;
@@ -368,6 +385,7 @@ export const DeveloperSettingsTab: FC<Props> = ({
           id: client.getDeviceId() || "unknown",
         })}
       </p>
+      {vm && <KeyRotationStatus vm={vm} />}
       <Separator />
       <FieldRow>
         <InputField

src/state/CallViewModel/CallViewModel.ts

@@ -113,6 +113,7 @@ import {
   type LocalTransport,
 } from "./localMember/LocalTransport.ts";
 import {
+  createKeyRotationSuppressed$,
   createMemberships$,
   membershipsAndTransports$,
 } from "../SessionBehaviors.ts";
@@ -301,6 +302,12 @@ export interface CallViewModel {
    *    multiple devices.
    */
   participantCount$: Behavior<number>;
+  /**
+   * Whether the call has grown large enough that MatrixRTC has stopped rotating the media
+   * encryption key. While this is true the key in use is still shared with new joiners, but no new
+   * key is generated when someone joins or leaves.
+   */
+  keyRotationSuppressed$: Behavior<boolean>;
   allConnections$: Behavior<ConnectionManagerData>;
   /** Participants sorted by livekit room so they can be used in the audio rendering */
   livekitRoomItems$: Behavior<LivekitRoomItem[]>;
@@ -856,6 +863,11 @@ export function createCallViewModel$(
     matrixLivekitMembers$.pipe(map((ms) => ms.length)),
   );
 
+  const keyRotationSuppressed$ = createKeyRotationSuppressed$(
+    scope,
+    matrixRTCSession,
+  );
+
   const leaveSoundEffect$ = userMedia$.pipe(
     pairwise(),
     filter(
@@ -1782,6 +1794,7 @@ export function createCallViewModel$(
     ),
     allConnections$,
     participantCount$: participantCount$,
+    keyRotationSuppressed$: keyRotationSuppressed$,
     handsRaised$: handsRaised$,
     reactions$: reactions$,
     joinSoundEffect$: joinSoundEffect$,

src/state/SessionBehaviors.ts

@@ -88,3 +88,22 @@ export const createMemberships$ = (
     new Epoch(matrixRTCSession.memberships),
   );
 };
+
+/**
+ * Whether the session has grown large enough that MatrixRTC has stopped rotating the media
+ * encryption key. While this is true the key in use is still shared with new joiners, but no new
+ * key is generated when someone joins or leaves.
+ */
+export const createKeyRotationSuppressed$ = (
+  scope: ObservableScope,
+  matrixRTCSession: MatrixRTCSession,
+): Behavior<boolean> => {
+  return scope.behavior(
+    fromEvent(
+      matrixRTCSession,
+      MatrixRTCSessionEvent.KeyRotationSuppressedChanged,
+      (suppressed: boolean) => suppressed,
+    ),
+    matrixRTCSession.isKeyRotationSuppressed,
+  );
+};

src/utils/test.ts

@@ -490,6 +490,8 @@ export class MockRTCSession extends TypedEventEmitter<
     return this.joined;
   }
 
+  public isKeyRotationSuppressed = false;
+
   public withMemberships(
     rtcMembers$: Behavior<Partial<CallMembership>[]>,
   ): MockRTCSession {