fix(room): a screen share publishes only while joined

js/room/src/local.test.ts

@@ -54,23 +54,30 @@ function setup() {
 	return { local, bandwidth, screen: sources.at(-1) as FakeSource, broadcasts: { camera, screen: screenBroadcast } };
 }
 
-// Replays LeavingStopsPublishing in `verify/Room.tla`: the screen broadcast is enabled by a live
-// share alone, so a participant who never joined, or who left, still publishes their screen.
-test("residual: a screen share publishes while not joined and after leaving", async () => {
+// Replays LeavingStopsPublishing in `verify/Room.tla`: a participant who has not joined, or has
+// left, neither publishes nor encodes their screen, even with a live share.
+test("a screen share publishes only while joined", async () => {
 	const { local, screen, broadcasts } = setup();
+	// The screen's hd, sd, and audio encoders are constructed last.
+	const screenEncoders = () => encoders.slice(-3).map((e) => (e.enabled as Signal<boolean>).peek());
 	try {
 		local.screenEnabled.set(true);
 		screen.out.state.set({ kind: "live", media: { video: {} } });
 		await flush();
 		expect(broadcasts.camera.enabled.peek()).toBe(false);
-		expect(broadcasts.screen.enabled.peek()).toBe(true);
+		expect(broadcasts.screen.enabled.peek()).toBe(false);
+		expect(screenEncoders()).toEqual([false, false, false]);
 
 		local.enabled.set(true);
 		await flush();
+		expect(broadcasts.screen.enabled.peek()).toBe(true);
+		expect(screenEncoders()).toEqual([true, true, true]);
+
 		local.enabled.set(false);
 		await flush();
 		expect(broadcasts.camera.enabled.peek()).toBe(false);
-		expect(broadcasts.screen.enabled.peek()).toBe(true);
+		expect(broadcasts.screen.enabled.peek()).toBe(false);
+		expect(screenEncoders()).toEqual([false, false, false]);
 	} finally {
 		local.close();
 	}

js/room/src/local.ts

@@ -33,7 +33,8 @@ export interface LocalProps {
  *
  * Enable {@link enabled} to join (announce `{identity}/camera.hang`). Camera and
  * microphone capture are separate knobs; the screenshare broadcast is
- * announced only while a share is live.
+ * announced only while joined and a share is live, so nothing is published
+ * before joining or after leaving.
  */
 export class Local {
 	/** Participant identity. */
@@ -96,6 +97,9 @@ export class Local {
 	#screenAudioSource = new Signal<Publish.Audio.Source | undefined>(undefined);
 	#screenLive = new Signal(false);
 
+	// The screen broadcast and its encoders run only while joined and sharing.
+	#screenPublished = new Signal(false);
+
 	#signals = new Effect();
 
 	#control(value: GetterInit<boolean> | undefined): Signal<boolean> {
@@ -189,9 +193,13 @@ export class Local {
 		});
 		this.#signals.cleanup(() => this.camera.close());
 
+		this.#signals.run((effect) => {
+			this.#screenPublished.set(effect.get(this.enabled) && effect.get(this.#screenLive));
+		});
+
 		this.screen = new Publish.Broadcast({
 			origin,
-			enabled: this.#screenLive,
+			enabled: this.#screenPublished,
 			name: screenName,
 			display: this.screenCapture.out.display,
 		});
@@ -236,7 +244,7 @@ export class Local {
 		const screenHd = new Publish.Video.Encoder("video/hd", {
 			broadcast: this.screen,
 			capture: this.screenCapture,
-			enabled: this.#screenLive,
+			enabled: this.#screenPublished,
 			bandwidth,
 			config: { maxPixels: 1920 * 1080 },
 		});
@@ -245,7 +253,7 @@ export class Local {
 		const screenSd = new Publish.Video.Encoder("video/sd", {
 			broadcast: this.screen,
 			capture: this.screenCapture,
-			enabled: this.#screenLive,
+			enabled: this.#screenPublished,
 			bandwidth,
 			config: { maxPixels: 960 * 540 },
 		});
@@ -260,7 +268,7 @@ export class Local {
 		const screenAudio = new Publish.Audio.Encoder("audio", {
 			broadcast: this.screen,
 			capture: screenAudioCapture,
-			enabled: this.#screenLive,
+			enabled: this.#screenPublished,
 			bandwidth,
 		});
 		this.#signals.cleanup(() => screenAudio.close());

js/room/verify/README.md

@@ -2,13 +2,13 @@
 
 `Room.tla` is a TLA+ model of you and Bob in a room, written to be drawn as a statechart with
 `/statechart`. Your side is `src/local.ts`: joining announces your camera broadcast, and a live
-screen share announces your screen broadcast. Your view of Bob is `src/room.ts`: each of his
+screen share announces your screen broadcast while joined. Your view of Bob is `src/room.ts`: each of his
 announcements adds or removes one of his broadcasts, and your connection dropping removes him
 until the reconnect announces him again.
 
-| Rule | Holds when | Today |
-| --- | --- | --- |
-| `LeavingStopsPublishing` | Someone who has not joined the room, or has left it, publishes nothing to it. | Breaks: the screen broadcast is enabled by a live share alone, so a share started before joining, or kept after leaving, stays published, and the room shows its owner. `residual: a screen share publishes while not joined and after leaving` in `src/local.test.ts` replays it. |
+| Rule | Holds when |
+| --- | --- |
+| `LeavingStopsPublishing` | Someone who has not joined the room, or has left it, publishes nothing to it. |
 
 The chart also shows every participant leaving your screen while your connection is down and
 coming back once it reconnects.
@@ -19,12 +19,11 @@ coming back once it reconnects.
 just js tla room/verify/Room.tla
 ```
 
-TLC exits non-zero while the rule above breaks. To draw the chart, run `/statechart` with
+To draw the chart, run `/statechart` with
 `Room.tla`, `Room.cfg`, and `Room.view.ts`. `Room.cfg` plays only Bob's camera; add `"screen"` to
 `Kinds` to play his share too.
 
 ## Left out
 
 Chat, typing, and user metadata, more than one remote participant, a participant restarting under
-the same identity, identity and prefix changes, and anything an app does on top of `Local`, such
-as turning sharing off when you leave.
+the same identity, identity and prefix changes, and anything an app does on top of `Local`.

js/room/verify/Room.tla

@@ -2,8 +2,7 @@
 \* You and another participant, Bob, in a room, as you see it.
 \*
 \* Your side is js/room/src/local.ts: `enabled` joins by announcing `{you}/camera.hang`, and the
-\* screen broadcast `{you}/screen.hang` is announced while a share is live (`#screenLive`),
-\* whether or not you joined. Your view of Bob is js/room/src/room.ts: each announcement under
+\* screen broadcast `{you}/screen.hang` is announced while joined and a share is live. Your view of Bob is js/room/src/room.ts: each announcement under
 \* the room adds or removes one of Bob's broadcasts, and Bob is on your screen while one is.
 \* Announcements reach you one at a time. When your connection drops, the entries it fed retract
 \* at once (reload.ts), and the reconnect announces them again.
@@ -24,7 +23,7 @@ Init ==
     /\ seen = {}
 
 \* What you publish into the room.
-Yours == (IF joined THEN {"camera"} ELSE {}) \cup (IF sharing THEN {"screen"} ELSE {})
+Yours == (IF joined THEN {"camera"} ELSE {}) \cup (IF joined /\ sharing THEN {"screen"} ELSE {})
 
 \* You.
 Join == ~joined /\ joined' = TRUE /\ UNCHANGED <<sharing, conn, bob, seen>>