make the transport discovery lk edgecase a EC exclusive workaround

Dont add it to the sdk.

package.json

@@ -107,7 +107,7 @@
     "livekit-client": "^2.18.1",
     "lodash-es": "^4.17.21",
     "loglevel": "^1.9.1",
-    "matrix-js-sdk": "github:matrix-org/matrix-js-sdk#9405287c22797e9d69acd366c47bf412fa4384c8",
+    "matrix-js-sdk": "github:matrix-org/matrix-js-sdk#3f05dc4d104a3dfc753c5e3549a18c74fab4b076",
     "matrix-widget-api": "^1.18.0",
     "node-stdlib-browser": "^1.3.1",
     "normalize.css": "^8.0.1",

pnpm-lock.yaml

@@ -191,8 +191,8 @@ importers:
         specifier: ^1.9.1
         version: 1.9.2
       matrix-js-sdk:
-        specifier: github:matrix-org/matrix-js-sdk#9405287c22797e9d69acd366c47bf412fa4384c8
-        version: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/9405287c22797e9d69acd366c47bf412fa4384c8
+        specifier: github:matrix-org/matrix-js-sdk#3f05dc4d104a3dfc753c5e3549a18c74fab4b076
+        version: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076
       matrix-widget-api:
         specifier: ^1.18.0
         version: 1.20.0
@@ -4673,8 +4673,8 @@ packages:
   matrix-events-sdk@0.0.1:
     resolution: {integrity: sha512-1QEOsXO+bhyCroIe2/A5OwaxHvBm7EsSQ46DEDn8RBIfQwN5HWBpFvyWWR4QY0KHPPnnJdI99wgRiAl7Ad5qaA==}
 
-  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/9405287c22797e9d69acd366c47bf412fa4384c8:
-    resolution: {gitHosted: true, integrity: sha512-pYk38ggTEdM0WWXLDiFoKmyRn3wo3o8m1ge0iCtxjBmgAW5qoQ2Ifd8oHVy+gu6PQe/CfLuKPXEXH8hff9C8GA==, tarball: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/9405287c22797e9d69acd366c47bf412fa4384c8}
+  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076:
+    resolution: {gitHosted: true, integrity: sha512-Ly8eS7fNNKDo9KmPvTkE49KvD8iMUmrVWJMNERNuxc8UksMLMG36NLz3fs8HytdwrB7AKPfyVo392VZsxILDIg==, tarball: https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076}
     version: 44.0.0
     engines: {node: '>=22.0.0'}
 
@@ -10227,7 +10227,7 @@ snapshots:
 
   matrix-events-sdk@0.0.1: {}
 
-  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/9405287c22797e9d69acd366c47bf412fa4384c8:
+  matrix-js-sdk@https://codeload.github.com/matrix-org/matrix-js-sdk/tar.gz/3f05dc4d104a3dfc753c5e3549a18c74fab4b076:
     dependencies:
       '@babel/runtime': 8.0.0
       '@matrix-org/matrix-sdk-crypto-wasm': 18.9.0

sdk/SdkArchitecture.md

@@ -101,8 +101,8 @@ LiveKit participants.
   │  create ───▶│                                       │                         │
   │             │ GET rtc/transports (session) ────────▶│                         │
   │             │◀── first transport of the backend's type                        │
-  │             │   (none: findRemotePublishTransport,  │                         │
-  │             │    a member's SFU we may publish on) ─▶│                         │
+  │             │   (none, LiveKit only: a member's SFU │                         │
+  │             │    that lets us publish, get_token) ─▶│                         │
   │             │ prepareLocalTransport (backend):      │                         │
   │             │   publisher token: CS API get_token,  │                         │
   │             │   or OpenID token then /sfu/get ─────▶│                         │
@@ -539,10 +539,6 @@ type MemberMediaFields = Pick<RTCMember, "tracks$" | "encryptionError$">;
 
 export interface MediaBackend {
   readonly transportType: string; // "livekit"
-  /** Another member's transport we may publish on, where the client found none. */
-  findRemotePublishTransport(
-    memberships: CallMembership[],
-  ): Promise<Transport | null>;
   /** Authenticate with the transport the client found; the probe for delegation runs here. */
   prepareLocalTransport(
     transport: Transport,
@@ -562,9 +558,10 @@ export interface MediaBackend {
 ```
 
 What stays on the client's side: discovery of the local transport from the
-homeserver's list (a homeserver endpoint, so not a backend's), asking the
-backend for another member's transport only where that list and the host's
-fallback are empty, the membership
+homeserver's list (a homeserver endpoint, so not a backend's) with, for the
+LiveKit backend only, a last resort of another member's SFU that lets us
+publish (`findRemotePublishLivekitTransport`, wired in by `RTCParticipation` rather
+than by the backend interface, since it is an MSC4195 matter), the membership
 state machine in `LocalMembership.ts` and its combination with
 `local.connectionState$` into `state$`, the choice of delayed leave timings,
 mapping a data packet's sender to a member, and the transport registry, which keys on the raw
@@ -574,8 +571,7 @@ transport serialised with sorted keys and derives `connectedTransports$` and eac
 The backend's context carries no Matrix client and no membership list. A backend
 that needs the client takes a `Pick` of exactly what it calls in its own options
 (`createLivekitBackend(scope, context, { client, mediaQuality })`),
-and learns of remote members only through `mediaFor$` and the one membership
-snapshot `findRemotePublishTransport` is handed, registering each
+and learns of remote members only through `mediaFor$`, registering each
 membership's transport for as long as the member's scope lives. Media keys reach
 it as a stream, `mediaKeys$`, replayed on subscribe; the js-sdk session never
 does. A host supplies its own backend through `MatrixRTCClientOptions.backend`;

sdk/src/RTCParticipation.test.ts

@@ -8,6 +8,7 @@ Please see LICENSE in the repository root for full details.
 import { type MatrixClient, type Room, SyncState } from "matrix-js-sdk";
 import { logger } from "matrix-js-sdk/lib/logger";
 import {
+  type CallMembership,
   MatrixRTCSessionEvent,
   MembershipManagerEvent,
   type Transport,
@@ -38,13 +39,26 @@ import { constant } from "./reactive/Behavior";
 import { ObservableScope } from "./reactive/ObservableScope";
 import { createRTCParticipation } from "./RTCParticipation";
 import { transportId } from "./matrixrtc/Transports";
-import { flushPromises, mockEmitter, MockRTCSession } from "./utils/test";
+import {
+  flushPromises,
+  mockEmitter,
+  mockRtcMembership,
+  MockRTCSession,
+} from "./utils/test";
 
 const transport: Transport = {
   type: "livekit",
   livekit_service_url: "https://lk.example.org",
 };
 const userId = "@alice:example.org";
+/** A token with the publish grant, as a homeserver issues it for its own SFU. */
+const publisherJWT = [
+  {},
+  { sub: "identity", video: { room: "room", canPublish: true } },
+  {},
+]
+  .map((d) => global.btoa(JSON.stringify(d)))
+  .join(".");
 const deviceId = "AAAA";
 const legacyMemberId = `${userId}:${deviceId}`;
 
@@ -84,7 +98,6 @@ interface FakeBackend {
   connectionState$: BehaviorSubject<MediaConnectionState | Error>;
   data$: Subject<DataPacket>;
   setPublishing: Mock<(publish: boolean) => void>;
-  findRemotePublishTransport: Mock<() => Promise<Transport | null>>;
   delegateDelayedLeave: Mock<(delayId: string) => Promise<void>>;
   mediaFor: Mock<(scope: ObservableScope, member: RTCMember) => MemberMedia>;
 }
@@ -92,7 +105,13 @@ interface FakeBackend {
 function setup(
   options: Partial<RTCParticipationOptions> = {},
   slotOptions: { matrixRTCMode?: MatrixRTCMode } = {},
-  homeserverTransports: Transport[] = [transport],
+  {
+    homeserverTransports = [transport],
+    memberships = [],
+  }: {
+    homeserverTransports?: Transport[];
+    memberships?: CallMembership[];
+  } = {},
 ): {
   scope: ObservableScope;
   participation: ReturnType<typeof createRTCParticipation>["participation"];
@@ -105,7 +124,7 @@ function setup(
 } {
   const scope = new ObservableScope();
   const room = { roomId: "!room:example.org" } as Room;
-  const session = new MockRTCSession(room);
+  const session = new MockRTCSession(room, memberships);
   const mockedSession = session.asMockedSession();
   const join = vi.spyOn(session, "join");
   const client = {
@@ -115,6 +134,7 @@ function setup(
     getDomain: () => "example.org",
     getSyncState: () => SyncState.Syncing,
     _unstable_getRTCTransports: vi.fn().mockResolvedValue(homeserverTransports),
+    _unstable_getLivekitToken: vi.fn().mockResolvedValue({ jwt: publisherJWT }),
   } as unknown as MatrixClient;
   const members$ = new BehaviorSubject<RTCMember[]>([]);
 
@@ -126,7 +146,6 @@ function setup(
     ),
     data$: new Subject<DataPacket>(),
     setPublishing: vi.fn(),
-    findRemotePublishTransport: vi.fn(async () => Promise.resolve(null)),
     delegateDelayedLeave: vi.fn(async () => Promise.resolve()),
     mediaFor: vi.fn(() => ({
       tracks$: constant(null),
@@ -139,7 +158,6 @@ function setup(
   ): MediaBackend => {
     const backend: MediaBackend = {
       transportType: "livekit",
-      findRemotePublishTransport: fake.findRemotePublishTransport,
       prepareLocalTransport: async () => fake.prepared.promise,
       delegateDelayedLeave: fake.delegateDelayedLeave,
       local: {
@@ -225,22 +243,27 @@ describe("createRTCParticipation", () => {
   });
 
   it("publishes on another member's SFU where the homeserver advertises none", async () => {
-    const bobsTransport: Transport = {
-      type: "livekit",
-      url: "wss://sfu.bob.com",
-      server_name: "bob.com",
-    };
-    const { session, join, fake } = setup({}, {}, []);
-    fake.findRemotePublishTransport.mockResolvedValue(bobsTransport);
+    const bobOnSfu = mockRtcMembership("@bob:example.org", "BOB", {
+      fociPreferred: [{ type: "livekit", url: "wss://sfu.example.org" }],
+    });
+    const { join, fake } = setup(
+      {},
+      {},
+      { homeserverTransports: [], memberships: [bobOnSfu] },
+    );
     fake.prepared.resolve({ canDelegateDelayedLeave: false });
     await flushPromises();
+    await flushPromises();
 
-    expect(fake.findRemotePublishTransport).toHaveBeenCalledWith(
-      session.memberships,
-    );
     expect(join).toHaveBeenCalledWith(
       expect.anything(),
-      [bobsTransport],
+      [
+        {
+          type: "livekit",
+          url: "wss://sfu.example.org",
+          server_name: "example.org",
+        },
+      ],
       expect.anything(),
     );
   });

sdk/src/RTCParticipation.ts

@@ -11,6 +11,7 @@ import {
   type MatrixRTCSession as JsSdkRTCSession,
   MatrixRTCSessionEvent,
   MembershipManagerEvent,
+  type Transport,
 } from "matrix-js-sdk/lib/matrixrtc";
 import { type IMembershipManager } from "matrix-js-sdk/lib/matrixrtc/IMembershipManager";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
@@ -50,6 +51,7 @@ import {
   type MediaKey,
 } from "./media-backend/api";
 import { createLivekitBackend } from "./media-backend/livekit/LivekitBackend";
+import { findRemotePublishLivekitTransport } from "./media-backend/livekit/RemotePublishTransport";
 import { createHomeserverConnected$ } from "./matrixrtc/HomeserverConnected";
 import { joinJsSdkSession } from "./matrixrtc/joinJsSdkSession";
 import {
@@ -130,14 +132,29 @@ export function createRTCParticipation(
     logger,
   });
 
+  // If the backend is a LiveKit: We check it for transports as well. (as a fallbacke)
+  // The livekit backend is a source of transports too.
+  // (MSC4195). LiveKit only, so it stays out of the backend interface
+  const transportsFromBackends = async (): Promise<Transport[]> =>
+    backend.transportType === "livekit"
+      ? findRemotePublishLivekitTransport({
+          client,
+          // A snapshot: if nobody with a usable SFU is here yet, the user has
+          // to come back later rather than wait in a half-working lobby
+          memberships: jsSdkSession.memberships,
+          ownMembershipIdentity,
+          roomId: room.roomId,
+          slotId: jsSdkSession.slotId,
+          logger,
+        })
+      : [];
+
   const preparedTransport$ = from(
     discoverLocalTransport(
       client,
-      backend,
-      // A snapshot: if nobody with a usable SFU is here yet, the user has to
-      // come back later rather than wait in a half-working lobby
-      jsSdkSession.memberships,
+      backend.transportType,
       options,
+      transportsFromBackends,
       logger,
     ).then(async (transport): Promise<PreparedTransport> => ({
       transport,

sdk/src/matrixrtc/LocalTransport.test.ts

@@ -10,7 +10,6 @@ import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
 import { describe, expect, it, vi } from "vitest";
 
 import { MatrixRTCTransportMissingError } from "../errors";
-import { mockRtcMembership } from "../utils/test";
 import { discoverLocalTransport } from "./LocalTransport";
 
 const homeserverTransport: Transport = {
@@ -26,7 +25,11 @@ const bobsTransport: Transport = {
   url: "wss://sfu.bob.com",
   server_name: "bob.com",
 };
-const bob = mockRtcMembership("@bob:bob.com", "BOB");
+const carolsTransport: Transport = {
+  type: "livekit",
+  url: "wss://sfu.carol.com",
+  server_name: "carol.com",
+};
 
 function client(
   advertised: Transport[],
@@ -37,13 +40,10 @@ function client(
   };
 }
 
-function backend(
-  remote: Transport | null = null,
-): Parameters<typeof discoverLocalTransport>[1] {
-  return {
-    transportType: "livekit",
-    findRemotePublishTransport: vi.fn().mockResolvedValue(remote),
-  };
+function fromBackends(
+  transports: Transport[] = [],
+): ReturnType<typeof vi.fn<() => Promise<Transport[]>>> {
+  return vi.fn(async () => Promise.resolve(transports));
 }
 
 describe("discoverLocalTransport", () => {
@@ -53,9 +53,9 @@ describe("discoverLocalTransport", () => {
     await expect(
       discoverLocalTransport(
         c,
-        backend(),
-        [],
+        "livekit",
         { transport: hostTransport },
+        fromBackends(),
         logger,
       ),
     ).resolves.toBe(hostTransport);
@@ -66,9 +66,9 @@ describe("discoverLocalTransport", () => {
     await expect(
       discoverLocalTransport(
         client([homeserverTransport]),
-        backend(),
-        [],
+        "livekit",
         { fallbackTransport: hostTransport },
+        fromBackends(),
         logger,
       ),
     ).resolves.toEqual(homeserverTransport);
@@ -78,44 +78,47 @@ describe("discoverLocalTransport", () => {
     await expect(
       discoverLocalTransport(
         client([{ type: "not_livekit" }]),
-        backend(),
-        [],
+        "livekit",
         { fallbackTransport: hostTransport },
+        fromBackends(),
         logger,
       ),
     ).resolves.toEqual(hostTransport);
   });
 
-  it("prefers a local transport over other members' SFUs", async () => {
-    const b = backend(bobsTransport);
+  it("prefers a local transport over the backends' without asking them", async () => {
+    const backends = fromBackends([bobsTransport]);
 
     await expect(
       discoverLocalTransport(
         client([homeserverTransport]),
-        b,
-        [bob],
+        "livekit",
         {},
+        backends,
         logger,
       ),
     ).resolves.toEqual(homeserverTransport);
-    expect(b.findRemotePublishTransport).not.toHaveBeenCalled();
+    expect(backends).not.toHaveBeenCalled();
   });
 
-  it("publishes on another member's SFU where there is no local transport", async () => {
-    const b = backend(bobsTransport);
-
+  it("takes the first transport the backends provide where there is no local one", async () => {
     await expect(
-      discoverLocalTransport(client([]), b, [bob], {}, logger),
+      discoverLocalTransport(
+        client([]),
+        "livekit",
+        {},
+        fromBackends([bobsTransport, carolsTransport]),
+        logger,
+      ),
     ).resolves.toEqual(bobsTransport);
-    expect(b.findRemotePublishTransport).toHaveBeenCalledWith([bob]);
   });
 
   it("fails naming the homeserver where there is no transport at all", async () => {
     const promise = discoverLocalTransport(
       client([]),
-      backend(),
-      [bob],
+      "livekit",
       {},
+      fromBackends(),
       logger,
     );
 

sdk/src/matrixrtc/LocalTransport.ts

@@ -7,42 +7,39 @@ Please see LICENSE in the repository root for full details.
 
 import { type MatrixClient } from "matrix-js-sdk";
 import { type Logger } from "matrix-js-sdk/lib/logger";
-import {
-  type CallMembership,
-  type Transport,
-} from "matrix-js-sdk/lib/matrixrtc";
+import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
 
 import { type RTCParticipationOptions } from "../api";
 import { MatrixRTCTransportMissingError } from "../errors";
-import { type MediaBackend } from "../media-backend/api";
 import { TransportAutoDiscovery } from "./TransportAutoDiscovery";
 
 /**
  * The transport the local member advertises: the one the host names, else the
  * homeserver's first of the type the backend serves, else the host's fallback,
- * else another member's that the backend can publish on.
+ * else the first the backends can provide, which are asked only then.
  *
  * @throws {MatrixRTCTransportMissingError} where none of these exists.
  */
 export async function discoverLocalTransport(
   client: Pick<MatrixClient, "getDomain" | "_unstable_getRTCTransports">,
-  backend: Pick<MediaBackend, "transportType" | "findRemotePublishTransport">,
-  memberships: CallMembership[],
+  transportType: string,
   {
     transport: override,
     fallbackTransport,
   }: Pick<RTCParticipationOptions, "transport" | "fallbackTransport">,
+  transportsFromBackends: () => Promise<Transport[]>,
   logger: Logger,
 ): Promise<Transport> {
   const transport =
     override ??
     (await new TransportAutoDiscovery({
       client,
-      transportType: backend.transportType,
+      transportType,
       fallbackTransport,
       logger,
     }).discoverPreferredTransport()) ??
-    (await backend.findRemotePublishTransport(memberships));
+    (await transportsFromBackends()).at(0) ??
+    null;
   if (transport === null)
     throw new MatrixRTCTransportMissingError(client.getDomain() ?? "");
   return transport;

sdk/src/media-backend/api.ts

@@ -13,10 +13,7 @@ Please see LICENSE in the repository root for full details.
  */
 
 import { type Logger } from "matrix-js-sdk/lib/logger";
-import {
-  type CallMembership,
-  type Transport,
-} from "matrix-js-sdk/lib/matrixrtc";
+import { type Transport } from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
 import { type Observable } from "rxjs";
 
@@ -119,17 +116,6 @@ export interface MediaBackend {
   /** The membership transport type this backend serves: "livekit". */
   readonly transportType: string;
 
-  /**
-   * A transport to publish on where neither the homeserver nor the host
-   * provides one of the backend's type: for LiveKit, the SFU of another
-   * member whose homeserver lets us publish on it (MSC4195). Resolves null
-   * where no member's transport will do. Called at most once per session,
-   * with the memberships at that moment, before `prepareLocalTransport`.
-   */
-  findRemotePublishTransport(
-    memberships: CallMembership[],
-  ): Promise<Transport | null>;
-
   /**
    * Makes `transport` usable as the one the local member publishes on. The
    * session found it; the backend does whatever has to succeed before it is

sdk/src/media-backend/livekit/LivekitBackend.test.ts

@@ -25,7 +25,6 @@ import { defaultSessionTimings } from "../../config";
 import {
   flushPromises,
   mockEmitter,
-  mockRtcMembership,
   ownMemberMock,
   testScope,
 } from "../../utils/test";
@@ -276,35 +275,6 @@ describe("createLivekitBackend", () => {
     expect(connections[0].connection.serverName).toBe("sample.com");
   });
 
-  it("finds another member's SFU to publish on through the homeserver", async () => {
-    client._unstable_getLivekitToken.mockResolvedValue({
-      jwt: [
-        {},
-        { sub: "identity", video: { room: "room", canPublish: true } },
-        {},
-      ]
-        .map((d) => global.btoa(JSON.stringify(d)))
-        .join("."),
-    });
-    const carl = mockRtcMembership("@carl:sample.com", "CARL", {
-      fociPreferred: [{ type: "livekit", url: "wss://sfu.sample.com" }],
-    });
-
-    await expect(backend().findRemotePublishTransport([carl])).resolves.toEqual(
-      {
-        type: "livekit",
-        url: "wss://sfu.sample.com",
-        server_name: "sample.com",
-      },
-    );
-    expect(client._unstable_getLivekitToken).toHaveBeenCalledWith(
-      expect.objectContaining({
-        url: "wss://sfu.sample.com",
-        server_name: "sample.com",
-      }),
-    );
-  });
-
   it("waits for the publisher connection to hold its token before the join", async () => {
     fetchMock.post(homeserverEndpoint, 404);
     fetchMock.post(`${exampleServiceUrl}/delegate_delayed_leave`, 404);

sdk/src/media-backend/livekit/LivekitBackend.ts

@@ -8,7 +8,6 @@ Please see LICENSE in the repository root for full details.
 import { type MatrixClient } from "matrix-js-sdk";
 import { type Logger } from "matrix-js-sdk/lib/logger";
 import {
-  type CallMembership,
   isUnstableLivekitTransport,
   type Transport,
 } from "matrix-js-sdk/lib/matrixrtc";
@@ -63,7 +62,6 @@ import { createDataChannel$ } from "./DataChannel";
 import { createKeyProvider } from "./KeyProvider";
 import { createLivekitMemberMedia } from "./LivekitMemberMedia";
 import { createLivekitLocalMedia } from "./LocalMedia";
-import { findRemotePublishTransport } from "./RemotePublishTransport";
 
 export interface LivekitBackendOptions {
   /**
@@ -150,18 +148,6 @@ export function createLivekitBackend(
 
   const ownServerName = serverNameOf(ownMembershipIdentity.userId);
 
-  const remotePublishTransport = async (
-    memberships: CallMembership[],
-  ): Promise<Transport | null> =>
-    findRemotePublishTransport({
-      client,
-      memberships,
-      ownMembershipIdentity,
-      roomId,
-      slotId,
-      logger,
-    });
-
   const prepareLocalTransport = async (
     transport: Transport,
   ): Promise<TransportCapabilities> => {
@@ -286,7 +272,6 @@ export function createLivekitBackend(
 
   return {
     transportType: "livekit",
-    findRemotePublishTransport: remotePublishTransport,
     prepareLocalTransport,
     delegateDelayedLeave: delegateLeave,
     local: localMedia_,

sdk/src/media-backend/livekit/RemotePublishTransport.test.ts

@@ -12,7 +12,7 @@ import { beforeEach, describe, expect, it, type Mocked, vi } from "vitest";
 
 import { mockRtcMembership, ownMemberMock } from "../../utils/test";
 import { type ClientGetTokenParts } from "./auth/csApi";
-import { findRemotePublishTransport } from "./RemotePublishTransport";
+import { findRemotePublishLivekitTransport } from "./RemotePublishTransport";
 
 /** A JWT whose payload states whether the issuing server lets us publish. */
 function jwtWithPublishGrant(canPublish: boolean): string {
@@ -27,6 +27,8 @@ function jwtWithPublishGrant(canPublish: boolean): string {
 
 const bobSfu = { type: "livekit", url: "wss://sfu.bob.com" };
 const carolSfu = { type: "livekit", url: "wss://sfu.carol.com" };
+const bobsTransport = { ...bobSfu, server_name: "bob.com" };
+const carolsTransport = { ...carolSfu, server_name: "carol.com" };
 
 // Bob is the oldest member, Carol joined after him
 const bob = mockRtcMembership("@bob:bob.com", "BOB", {
@@ -37,24 +39,41 @@ const carol = mockRtcMembership("@carol:carol.com", "CAROL", {
   fociPreferred: [carolSfu],
   membership: { created_ts: 200 },
 });
-const legacyDave = mockRtcMembership("@dave:dave.com", "DAVE", {
+// Bob's other device, and Dave who publishes on Bob's SFU remotely
+const bobsLaptop = mockRtcMembership("@bob:bob.com", "LAPTOP", {
+  fociPreferred: [bobSfu],
+  membership: { created_ts: 150 },
+});
+const dave = mockRtcMembership("@dave:dave.com", "DAVE", {
+  fociPreferred: [bobsTransport],
+  membership: { created_ts: 160 },
+});
+const legacyErin = mockRtcMembership("@erin:erin.com", "ERIN", {
   fociPreferred: [
-    { type: "livekit", livekit_service_url: "https://jwt.dave.com" },
+    { type: "livekit", livekit_service_url: "https://jwt.erin.com" },
   ],
   membership: { created_ts: 50 },
 });
+const ourselves = mockRtcMembership(
+  ownMemberMock.userId,
+  ownMemberMock.deviceId,
+  {
+    fociPreferred: [{ type: "livekit", url: "wss://sfu.example.org" }],
+    membership: { created_ts: 10 },
+  },
+);
 
 const csApiMissing = new MatrixError(
   { errcode: "M_UNRECOGNIZED", error: "" },
   404,
 );
 
-describe("findRemotePublishTransport", () => {
+describe("findRemotePublishLivekitTransport", () => {
   let client: Mocked<ClientGetTokenParts>;
   const find = async (
     memberships: CallMembership[],
-  ): ReturnType<typeof findRemotePublishTransport> =>
-    findRemotePublishTransport({
+  ): ReturnType<typeof findRemotePublishLivekitTransport> =>
+    findRemotePublishLivekitTransport({
       client,
       memberships,
       ownMembershipIdentity: ownMemberMock,
@@ -72,11 +91,7 @@ describe("findRemotePublishTransport", () => {
   });
 
   it("publishes on the oldest member's SFU with its server_name", async () => {
-    await expect(find([carol, bob])).resolves.toEqual({
-      type: "livekit",
-      url: "wss://sfu.bob.com",
-      server_name: "bob.com",
-    });
+    await expect(find([carol, bob])).resolves.toEqual(bobsTransport);
     expect(client._unstable_getLivekitToken).toHaveBeenCalledTimes(1);
     expect(client._unstable_getLivekitToken).toHaveBeenCalledWith({
       url: "wss://sfu.bob.com",
@@ -87,10 +102,22 @@ describe("findRemotePublishTransport", () => {
     });
   });
 
-  it("ignores members publishing through a legacy JWT service", async () => {
-    await expect(find([legacyDave, bob])).resolves.toMatchObject({
-      server_name: "bob.com",
+  it("asks about an SFU once however many members are on it", async () => {
+    client._unstable_getLivekitToken.mockResolvedValue({
+      jwt: jwtWithPublishGrant(false),
     });
+
+    await expect(find([dave, bobsLaptop, bob])).resolves.toBeNull();
+    expect(client._unstable_getLivekitToken).toHaveBeenCalledTimes(1);
+  });
+
+  it("skips our own membership", async () => {
+    await expect(find([ourselves, bob])).resolves.toEqual(bobsTransport);
+    expect(client._unstable_getLivekitToken).toHaveBeenCalledTimes(1);
+  });
+
+  it("ignores members publishing through a legacy JWT service", async () => {
+    await expect(find([legacyErin, bob])).resolves.toEqual(bobsTransport);
     expect(client._unstable_getLivekitToken).toHaveBeenCalledTimes(1);
   });
 
@@ -102,9 +129,7 @@ describe("findRemotePublishTransport", () => {
         }),
     );
 
-    await expect(find([bob, carol])).resolves.toMatchObject({
-      server_name: "carol.com",
-    });
+    await expect(find([bob, carol])).resolves.toEqual(carolsTransport);
     expect(client._unstable_getLivekitToken).toHaveBeenCalledTimes(2);
   });
 
@@ -121,9 +146,7 @@ describe("findRemotePublishTransport", () => {
           : Promise.resolve({ jwt: jwtWithPublishGrant(true) }),
     );
 
-    await expect(find([bob, carol])).resolves.toMatchObject({
-      server_name: "carol.com",
-    });
+    await expect(find([bob, carol])).resolves.toEqual(carolsTransport);
   });
 
   it("finds nothing once all candidates are exhausted", async () => {
@@ -143,7 +166,7 @@ describe("findRemotePublishTransport", () => {
   });
 
   it("finds nothing where there are no other members", async () => {
-    await expect(find([])).resolves.toBeNull();
+    await expect(find([ourselves])).resolves.toBeNull();
     expect(client._unstable_getLivekitToken).not.toHaveBeenCalled();
   });
 });

sdk/src/media-backend/livekit/RemotePublishTransport.ts

@@ -8,8 +8,7 @@ Please see LICENSE in the repository root for full details.
 import { type Logger } from "matrix-js-sdk/lib/logger";
 import {
   type CallMembership,
-  getRemoteLivekitPublishCandidates,
-  type LivekitPublishCandidate,
+  isUnstableLivekitTransport,
   type Transport,
 } from "matrix-js-sdk/lib/matrixrtc";
 import { type CallMembershipIdentityParts } from "matrix-js-sdk/lib/matrixrtc/EncryptionManager";
@@ -30,15 +29,15 @@ export interface RemotePublishTransportParams {
 
 /**
  * The SFU of another member that its homeserver lets us publish on, as the
- * transport to advertise, or null where no member's SFU will do. Candidates
- * are tried oldest membership first, and each one is checked by asking for a
+ * transport to advertise, or null where none does. Candidates are tried
+ * oldest membership first, each SFU once, and each is checked by asking for a
  * publisher token through our homeserver (MSC4195): a homeserver may restrict
  * remote publishers, which shows as a token without the publish grant.
  */
-export async function findRemotePublishTransport({
+export async function findRemotePublishLivekitTransport({
   client,
   memberships,
-  ownMembershipIdentity,
+  ownMembershipIdentity: own,
   roomId,
   slotId,
   logger,
@@ -46,45 +45,51 @@ export async function findRemotePublishTransport({
   logger.info(
     "No local transport available, looking for another member's SFU to publish on",
   );
-  const candidates = getRemoteLivekitPublishCandidates(
-    memberships,
-    ownMembershipIdentity,
-  );
-  for (const candidate of candidates) {
-    const outcome = await tryCandidate(candidate);
-    if (outcome === "usable") return candidate.transport;
-    if (outcome === "noCsApi") return null;
+  const tried = new Set<string>();
+  const byAge = [...memberships].sort((a, b) => a.createdTs() - b.createdTs());
+  for (const membership of byAge) {
+    if (
+      membership.userId === own.userId &&
+      membership.deviceId === own.deviceId
+    )
+      continue;
+    for (const transport of membership.transports) {
+      // Publishing remotely goes through the client-server API, which a JWT
+      // service cannot stand in for, so only a `url` qualifies
+      if (!isUnstableLivekitTransport(transport) || !("url" in transport))
+        continue;
+      const { url } = transport;
+      const serverName = membership.getTransportServerName(transport);
+      const key = JSON.stringify([url, serverName]);
+      if (tried.has(key)) continue;
+      tried.add(key);
+      try {
+        const { jwt } = await csApi.getSFUConfig({
+          client,
+          membership: own,
+          url,
+          serverName,
+          roomId,
+          slotId,
+        });
+        if (decodeSFUJWTPayload(jwt).video.canPublish) {
+          logger.info(`Publishing on remote SFU ${url} of ${serverName}`);
+          return { type: "livekit", url, server_name: serverName };
+        }
+        logger.info(
+          `${serverName} does not let us publish on ${url}, skipping`,
+        );
+      } catch (e) {
+        if (isCsApiUnavailable(e)) {
+          // Without the endpoint no further candidate can work
+          logger.warn(
+            `Cannot use the MSC4195 get_token endpoint (${e}), so no remote SFU can be used`,
+          );
+          return null;
+        }
+        logger.warn(`Failed to get a token for ${url} of ${serverName}:`, e);
+      }
+    }
   }
   return null;
-
-  /** "noCsApi" ends the search: without the endpoint no candidate can work. */
-  async function tryCandidate({
-    url,
-    serverName,
-  }: LivekitPublishCandidate): Promise<"usable" | "unusable" | "noCsApi"> {
-    try {
-      const { jwt } = await csApi.getSFUConfig({
-        client,
-        membership: ownMembershipIdentity,
-        url,
-        serverName,
-        roomId,
-        slotId,
-      });
-      if (decodeSFUJWTPayload(jwt).video.canPublish) {
-        logger.info(`Publishing on remote SFU ${url} of ${serverName}`);
-        return "usable";
-      }
-      logger.info(`${serverName} does not let us publish on ${url}, skipping`);
-    } catch (e) {
-      if (isCsApiUnavailable(e)) {
-        logger.warn(
-          `Cannot use the MSC4195 get_token endpoint (${e}), so no remote SFU can be used`,
-        );
-        return "noCsApi";
-      }
-      logger.warn(`Failed to get a token for ${url} of ${serverName}:`, e);
-    }
-    return "unusable";
-  }
 }