Turn a camera back on rather than replace it for its processor

The SDK now builds a processor chosen while the camera was off when the
camera turns on, so the call view model no longer unpublishes the
camera and publishes a new one. TrackSupply.replace, the record of
which camera took which processor, and followProcessor's applied
callback go with it.

Answers robintown's review on #4282 ("what's wrong with attaching
processors to already published camera tracks?").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/state/CallViewModel/CallViewModel.test.ts

@@ -1697,7 +1697,9 @@ describe("a camera with a video processor", () => {
     ).toMatchObject({ processor });
   });
 
-  it("replaces a camera turned off before the processor was chosen", async () => {
+  // The SDK builds the processor on the camera's restart, before the room
+  // sees a frame, so the track is unmuted rather than published anew
+  it("turns on a camera turned off before the processor was chosen", async () => {
     const setHandler = vi.spyOn(MuteState.prototype, "setHandler");
     onTestFinished(() => setHandler.mockRestore());
     const videoProcessor$ = new BehaviorSubject<VideoProcessor | undefined>(
@@ -1711,31 +1713,6 @@ describe("a camera with a video processor", () => {
       },
       { videoProcessor$ },
     );
-    // The camera is off, so it is stopped and can't take the processor
-    videoProcessor$.next(processor);
-    await cameraHandler(setHandler)(true);
-    await flushPromises();
-
-    expect(participation!.unpublish).toHaveBeenCalledWith("camera");
-    expect(participation!.publish).toHaveBeenCalledWith(
-      expect.objectContaining({ source: "camera", processor }),
-    );
-  });
-
-  it("turns on a camera that took the processor while running", async () => {
-    const setHandler = vi.spyOn(MuteState.prototype, "setHandler");
-    onTestFinished(() => setHandler.mockRestore());
-    const videoProcessor$ = new BehaviorSubject<VideoProcessor | undefined>(
-      undefined,
-    );
-    let participation: RTCParticipation | undefined;
-    withCallViewModel(
-      { videoEnabled: new Map([[localParticipant, constant(true)]]) },
-      (_vm, _session, { rtcParticipation }) => {
-        participation = rtcParticipation;
-      },
-      { videoProcessor$ },
-    );
     videoProcessor$.next(processor);
     await cameraHandler(setHandler)(true);
     await flushPromises();

src/state/CallViewModel/CallViewModel.ts

@@ -14,7 +14,6 @@ import {
   type EncryptionSystem,
   generateItems,
   type LocalAudioMediaTrack,
-  type LocalMediaTrack,
   type LocalRTCMember,
   type LocalVideoMediaTrack,
   type RTCParticipation,
@@ -513,16 +512,6 @@ export function createCallViewModel$(
     "screenShare",
   ) as Behavior<LocalVideoMediaTrack | undefined>;
 
-  // Camera tracks the processor reached while they ran. A camera turned off is
-  // stopped, and a stopped track can't take one.
-  const tookProcessor = new WeakMap<LocalVideoMediaTrack, VideoProcessor>();
-  const missesProcessor = (track: LocalMediaTrack): boolean => {
-    const processor = videoProcessor$.value;
-    return (
-      processor !== undefined &&
-      tookProcessor.get(track as LocalVideoMediaTrack) !== processor
-    );
-  };
   const supply = (source: "microphone" | "camera"): TrackSupply => ({
     publish: async () =>
       rtcParticipation.publish(
@@ -533,16 +522,6 @@ export function createCallViewModel$(
           source === "camera" ? videoProcessor$.value : undefined,
         ),
       ),
-    // A camera turned off before the processor was chosen is replaced, so
-    // that the new one starts with it
-    replace:
-      source === "camera"
-        ? async (track): Promise<boolean> => {
-            if (!missesProcessor(track)) return false;
-            await rtcParticipation.unpublish("camera");
-            return true;
-          }
-        : undefined,
   });
   followMuteState(
     scope,
@@ -582,13 +561,7 @@ export function createCallViewModel$(
         });
       });
 
-  followProcessor(
-    scope,
-    camera$,
-    videoProcessor$,
-    logger,
-    (camera, processor) => tookProcessor.set(camera, processor),
-  );
+  followProcessor(scope, camera$, videoProcessor$, logger);
 
   const members$ = scope.behavior<(LocalRTCMember | RemoteRTCMember)[]>(
     combineLatest(

src/state/localMediaControls.ts

@@ -18,14 +18,9 @@ import { combineLatest } from "rxjs";
 import { type MediaDevice } from "./MediaDevices";
 import { type Handler } from "./MuteStates";
 
-/**
- * How a mute switch gets a track where there is none and, where given,
- * whether a track it has should be let go and published anew instead of
- * unmuted: resolves true once it is gone.
- */
+/** How a mute switch gets a track where there is none. */
 export interface TrackSupply {
   publish: () => Promise<unknown>;
-  replace?: (track: LocalMediaTrack) => Promise<boolean>;
 }
 
 /**
@@ -51,9 +46,7 @@ export function followMuteState(
   muteState.setHandler(async (enabled) => {
     desired = enabled;
     const track = track$.value;
-    const replaced =
-      enabled && track !== undefined && (await supply.replace?.(track));
-    if (track !== undefined && !replaced) {
+    if (track !== undefined) {
       desired = await track.setEnabled(enabled);
       return desired;
     }
@@ -107,15 +100,11 @@ export function followProcessor(
   camera$: Behavior<LocalVideoMediaTrack | undefined>,
   videoProcessor$: Behavior<VideoProcessor | undefined>,
   logger: Logger,
-  /** Told of each camera a processor reaches while it runs. */
-  applied?: (camera: LocalVideoMediaTrack, processor: VideoProcessor) => void,
 ): void {
   combineLatest([camera$, videoProcessor$])
     .pipe(scope.bind())
     .subscribe(([camera, processor]) => {
-      if (camera === undefined) return;
-      if (processor && !camera.muted$.value) applied?.(camera, processor);
-      camera.setProcessor(processor).catch((e) => {
+      camera?.setProcessor(processor).catch((e) => {
         logger.error("Failed to set the video processor", e);
       });
     });