feat(trace): throttle the screencast

Author: pavelfeldman

packages/playwright-core/src/server/chromium/crPage.ts

@@ -857,7 +857,9 @@ class FrameSession {
   }
 
   _onScreencastFrame(payload: Protocol.Page.screencastFramePayload) {
-    this._client.send('Page.screencastFrameAck', { sessionId: payload.sessionId }).catch(() => {});
+    this._page.throttleScreencastFrameAck(() => {
+      this._client.send('Page.screencastFrameAck', { sessionId: payload.sessionId }).catch(() => {});
+    });
     const buffer = Buffer.from(payload.data, 'base64');
     this._page.emit(Page.Events.ScreencastFrame, {
       buffer,

packages/playwright-core/src/server/firefox/ffPage.ts

@@ -500,7 +500,10 @@ export class FFPage implements PageDelegate {
   private _onScreencastFrame(event: Protocol.Page.screencastFramePayload) {
     if (!this._screencastId)
       return;
-    this._session.send('Page.screencastFrameAck', { screencastId: this._screencastId }).catch(e => debugLogger.log('error', e));
+    const screencastId = this._screencastId;
+    this._page.throttleScreencastFrameAck(() => {
+      this._session.send('Page.screencastFrameAck', { screencastId }).catch(e => debugLogger.log('error', e));
+    });
 
     const buffer = Buffer.from(event.data, 'base64');
     this._page.emit(Page.Events.ScreencastFrame, {

packages/playwright-core/src/server/page.ts

@@ -144,6 +144,7 @@ export class Page extends SdkObject {
   _pageIsError: Error | undefined;
   _video: Artifact | null = null;
   _opener: Page | undefined;
+  private _frameThrottler = new FrameThrottler(10, 200);
 
   constructor(delegate: PageDelegate, browserContext: BrowserContext) {
     super(browserContext, 'page');
@@ -209,6 +210,7 @@ export class Page extends SdkObject {
 
   _didClose() {
     this._frameManager.dispose();
+    this._frameThrottler.setEnabled(false);
     assert(this._closedState !== 'closed', 'Page closed twice');
     this._closedState = 'closed';
     this.emit(Page.Events.Close);
@@ -217,12 +219,14 @@ export class Page extends SdkObject {
 
   _didCrash() {
     this._frameManager.dispose();
+    this._frameThrottler.setEnabled(false);
     this.emit(Page.Events.Crash);
     this._crashedPromise.resolve(new Error('Page crashed'));
   }
 
   _didDisconnect() {
     this._frameManager.dispose();
+    this._frameThrottler.setEnabled(false);
     assert(!this._disconnected, 'Page disconnected twice');
     this._disconnected = true;
     this._disconnectedPromise.resolve(new Error('Page closed'));
@@ -495,6 +499,16 @@ export class Page extends SdkObject {
 
   setScreencastOptions(options: { width: number, height: number, quality: number } | null) {
     this._delegate.setScreencastOptions(options).catch(e => debugLogger.log('error', e));
+    this._frameThrottler.setEnabled(!!options);
+  }
+
+  throttleScreencastFrameAck(ack: () => void) {
+    // Don't ack immediately, tracing has smart throttling logic that is implemented here.
+    this._frameThrottler.ack(ack);
+  }
+
+  temporarlyDisableTracingScreencastThrottling() {
+    this._frameThrottler.recharge();
   }
 
   firePageError(error: Error) {
@@ -631,3 +645,57 @@ function addPageBinding(bindingName: string, needsHandle: boolean) {
   };
   (globalThis as any)[bindingName].__installed = true;
 }
+
+class FrameThrottler {
+  private _acks: (() => void)[] = [];
+  private _interval: number;
+  private _nonThrottledFrames: number;
+  private _budget: number;
+  private _intervalId: NodeJS.Timeout | undefined;
+
+  constructor(nonThrottledFrames: number, interval: number) {
+    this._nonThrottledFrames = nonThrottledFrames;
+    this._budget = nonThrottledFrames;
+    this._interval = interval;
+  }
+
+  setEnabled(enabled: boolean) {
+    if (enabled) {
+      if (this._intervalId)
+        clearInterval(this._intervalId);
+      this._intervalId = setInterval(() => this._tick(), this._interval);
+    } else if (this._intervalId) {
+      clearInterval(this._intervalId);
+      this._intervalId = undefined;
+    }
+  }
+
+  recharge() {
+    // Send all acks, reset budget.
+    for (const ack of this._acks)
+      ack();
+    this._acks = [];
+    this._budget = this._nonThrottledFrames;
+  }
+
+  ack(ack: () => void) {
+    // Either not engaged or video is also recording, don't throttle.
+    if (!this._intervalId) {
+      ack();
+      return;
+    }
+
+    // Do we have enough budget to respond w/o throttling?
+    if (--this._budget > 0) {
+      ack();
+      return;
+    }
+
+    // Schedule.
+    this._acks.push(ack);
+  }
+
+  private _tick() {
+    this._acks.shift()?.();
+  }
+}

packages/playwright-core/src/server/trace/recorder/tracing.ts

@@ -254,6 +254,7 @@ export class Tracing implements InstrumentationListener, SnapshotterDelegate, Ha
   }
 
   async onBeforeCall(sdkObject: SdkObject, metadata: CallMetadata) {
+    sdkObject.attribution.page?.temporarlyDisableTracingScreencastThrottling();
     // Set afterSnapshot name for all the actions that operate selectors.
     // Elements resolved from selectors will be marked on the snapshot.
     metadata.afterSnapshot = `after@${metadata.id}`;
@@ -263,12 +264,14 @@ export class Tracing implements InstrumentationListener, SnapshotterDelegate, Ha
   }
 
   async onBeforeInputAction(sdkObject: SdkObject, metadata: CallMetadata, element: ElementHandle) {
+    sdkObject.attribution.page?.temporarlyDisableTracingScreencastThrottling();
     const actionSnapshot = this._captureSnapshot('action', sdkObject, metadata, element);
     this._pendingCalls.get(metadata.id)!.actionSnapshot = actionSnapshot;
     await actionSnapshot;
   }
 
   async onAfterCall(sdkObject: SdkObject, metadata: CallMetadata) {
+    sdkObject.attribution.page?.temporarlyDisableTracingScreencastThrottling();
     const pendingCall = this._pendingCalls.get(metadata.id);
     if (!pendingCall || pendingCall.afterSnapshot)
       return;

packages/playwright-core/src/server/webkit/wkPage.ts

@@ -873,7 +873,10 @@ export class WKPage implements PageDelegate {
   }
 
   private _onScreencastFrame(event: Protocol.Screencast.screencastFramePayload) {
-    this._pageProxySession.send('Screencast.screencastFrameAck', { generation: this._screencastGeneration }).catch(e => debugLogger.log('error', e));
+    const generation = this._screencastGeneration;
+    this._page.throttleScreencastFrameAck(() => {
+      this._pageProxySession.send('Screencast.screencastFrameAck', { generation }).catch(e => debugLogger.log('error', e));
+    });
     const buffer = Buffer.from(event.data, 'base64');
     this._page.emit(Page.Events.ScreencastFrame, {
       buffer,