core(fr): colocate PerformanceObserver installation with wait logic

lighthouse-core/gather/driver.js

@@ -899,15 +899,6 @@ class Driver {
     `);
   }
 
-  /**
-   * Install a performance observer that watches longtask timestamps for waitForCPUIdle.
-   * @return {Promise<void>}
-   */
-  async registerPerformanceObserver() {
-    const scriptStr = `(${pageFunctions.registerPerformanceObserverInPageString})()`;
-    await this.evaluateScriptOnNewDocument(scriptStr);
-  }
-
   /**
    * Use a RequestIdleCallback shim for tests run with simulated throttling, so that the deadline can be used without
    * a penalty

lighthouse-core/gather/driver/wait-for-condition.js

@@ -5,10 +5,11 @@
  */
 'use strict';
 
+/* global window */
+
 const log = require('lighthouse-logger');
 const LHError = require('../../lib/lh-error.js');
 const ExecutionContext = require('./execution-context.js');
-const pageFunctions = require('../../lib/page-functions.js');
 
 /** @typedef {import('./network-monitor.js')} NetworkMonitor */
 /** @typedef {import('./network-monitor.js').NetworkMonitorEvent} NetworkMonitorEvent */
@@ -221,7 +222,7 @@ function waitForCPUIdle(session, waitForCPUQuiet) {
   async function checkForQuiet(executionContext, resolve) {
     if (canceled) return;
     const timeSinceLongTask =
-      await executionContext.evaluate(pageFunctions.checkTimeSinceLastLongTask, {args: []});
+      await executionContext.evaluate(checkTimeSinceLastLongTaskInPage, {args: []});
     if (canceled) return;
 
     if (typeof timeSinceLongTask === 'number') {
@@ -241,9 +242,12 @@ function waitForCPUIdle(session, waitForCPUQuiet) {
     throw new Error('waitForCPUIdle.cancel() called before it was defined');
   };
 
+  const executionContext = new ExecutionContext(session);
   /** @type {Promise<void>} */
   const promise = new Promise((resolve, reject) => {
-    checkForQuiet(new ExecutionContext(session), resolve).catch(reject);
+    executionContext.evaluate(registerPerformanceObserverInPage, {args: []})
+      .then(() => checkForQuiet(executionContext, resolve))
+      .catch(reject);
     cancel = () => {
       if (canceled) return;
       canceled = true;
@@ -258,6 +262,69 @@ function waitForCPUIdle(session, waitForCPUQuiet) {
   };
 }
 
+/* c8 ignore start */
+
+/**
+ * This function is executed in the page itself when the document is first loaded.
+ *
+ * Used by _waitForCPUIdle and executed in the context of the page, updates the ____lastLongTask
+ * property on window to the end time of the last long task.
+ */
+function registerPerformanceObserverInPage() {
+  // Do not re-register if we've already run this script.
+  if (window.____lastLongTask === undefined) return;
+
+  window.____lastLongTask = window.__perfNow();
+  const observer = new window.PerformanceObserver(entryList => {
+    const entries = entryList.getEntries();
+    for (const entry of entries) {
+      if (entry.entryType === 'longtask') {
+        const taskEnd = entry.startTime + entry.duration;
+        window.____lastLongTask = Math.max(window.____lastLongTask || 0, taskEnd);
+      }
+    }
+  });
+
+  observer.observe({entryTypes: ['longtask'], buffered: true});
+}
+
+/**
+ * This function is executed in the page itself.
+ *
+ * Used by _waitForCPUIdle and executed in the context of the page, returns time since last long task.
+ * @return {Promise<number>}
+ */
+function checkTimeSinceLastLongTaskInPage() {
+  // This function attempts to return the time since the last long task occurred.
+  // `PerformanceObserver`s don't always immediately fire though, so we check twice with some time in
+  // between to make sure nothing has happened very recently.
+
+  // Chrome 88 introduced heavy throttling of timers which means our `setTimeout` will be executed
+  // at some point farish (several hundred ms) into the future and the time at which it executes isn't
+  // a reliable indicator of long task existence, instead we check if any information has changed.
+  // See https://developer.chrome.com/blog/timer-throttling-in-chrome-88/
+  return new window.__nativePromise(resolve => {
+    const firstAttemptTs = window.__perfNow();
+    const firstAttemptLastLongTaskTs = window.____lastLongTask || 0;
+
+    setTimeout(() => {
+      // We can't be sure a long task hasn't occurred since our first attempt, but if the `____lastLongTask`
+      // value is the same (i.e. the perf observer didn't have any new information), we can be pretty
+      // confident that the long task info was accurate *at the time of our first attempt*.
+      const secondAttemptLastLongTaskTs = window.____lastLongTask || 0;
+      const timeSinceLongTask =
+        firstAttemptLastLongTaskTs === secondAttemptLastLongTaskTs
+          ? // The time of the last long task hasn't changed, the information from our first attempt is accurate.
+            firstAttemptTs - firstAttemptLastLongTaskTs
+          : // The time of the last long task *did* change, we can't really trust the information we have.
+            0;
+      resolve(timeSinceLongTask);
+    }, 150);
+  });
+}
+
+/* c8 ignore stop */
+
 /**
  * Return a promise that resolves `pauseAfterLoadMs` after the load event
  * fires and a method to cancel internal listeners and timeout.
@@ -294,7 +361,6 @@ function waitForLoadEvent(session, pauseAfterLoadMs) {
   };
 }
 
-
 /**
  * Returns whether the page appears to be hung.
  * @param {LH.Gatherer.FRProtocolSession} session

lighthouse-core/gather/gather-runner.js

@@ -133,7 +133,6 @@ class GatherRunner {
     await driver.enableRuntimeEvents();
     await driver.enableAsyncStacks();
     await driver.cacheNatives();
-    await driver.registerPerformanceObserver();
     await driver.dismissJavaScriptDialogs();
     await driver.registerRequestIdleCallbackWrap(options.settings);
     if (resetStorage) {

lighthouse-core/lib/page-functions.js

@@ -47,57 +47,6 @@ function wrapRuntimeEvalErrorInBrowser(err) {
   };
 }
 
-/**
- * Used by _waitForCPUIdle and executed in the context of the page, updates the ____lastLongTask
- * property on window to the end time of the last long task.
- */
-function registerPerformanceObserverInPage() {
-  window.____lastLongTask = window.__perfNow();
-  const observer = new window.PerformanceObserver(entryList => {
-    const entries = entryList.getEntries();
-    for (const entry of entries) {
-      if (entry.entryType === 'longtask') {
-        const taskEnd = entry.startTime + entry.duration;
-        window.____lastLongTask = Math.max(window.____lastLongTask || 0, taskEnd);
-      }
-    }
-  });
-
-  observer.observe({entryTypes: ['longtask']});
-}
-
-/**
- * Used by _waitForCPUIdle and executed in the context of the page, returns time since last long task.
- * @return {Promise<number>}
- */
-function checkTimeSinceLastLongTask() {
-  // This function attempts to return the time since the last long task occurred.
-  // `PerformanceObserver`s don't always immediately fire though, so we check twice with some time in
-  // between to make sure nothing has happened very recently.
-
-  // Chrome 88 introduced heavy throttling of timers which means our `setTimeout` will be executed
-  // at some point farish (several hundred ms) into the future and the time at which it executes isn't
-  // a reliable indicator of long task existence, instead we check if any information has changed.
-  // See https://developer.chrome.com/blog/timer-throttling-in-chrome-88/
-  return new window.__nativePromise(resolve => {
-    const firstAttemptTs = window.__perfNow();
-    const firstAttemptLastLongTaskTs = window.____lastLongTask || 0;
-
-    setTimeout(() => {
-      // We can't be sure a long task hasn't occurred since our first attempt, but if the `____lastLongTask`
-      // value is the same (i.e. the perf observer didn't have any new information), we can be pretty
-      // confident that the long task info was accurate *at the time of our first attempt*.
-      const secondAttemptLastLongTaskTs = window.____lastLongTask || 0;
-      const timeSinceLongTask = firstAttemptLastLongTaskTs === secondAttemptLastLongTaskTs ?
-        // The time of the last long task hasn't changed, the information from our first attempt is accurate.
-        firstAttemptTs - firstAttemptLastLongTaskTs :
-        // The time of the last long task *did* change, we can't really trust the information we have.
-        0;
-      resolve(timeSinceLongTask);
-    }, 150);
-  });
-}
-
 /**
  * @template {string} T
  * @param {T} selector Optional simple CSS selector to filter nodes on.
@@ -577,8 +526,6 @@ const getNodeDetailsString = `function getNodeDetails(element) {
 module.exports = {
   wrapRuntimeEvalErrorInBrowserString: wrapRuntimeEvalErrorInBrowser.toString(),
   wrapRuntimeEvalErrorInBrowser,
-  registerPerformanceObserverInPageString: registerPerformanceObserverInPage.toString(),
-  checkTimeSinceLastLongTask,
   getElementsInDocument,
   getElementsInDocumentString: getElementsInDocument.toString(),
   getOuterHTMLSnippetString: getOuterHTMLSnippet.toString(),

lighthouse-core/test/gather/fake-driver.js

@@ -84,9 +84,6 @@ function makeFakeDriver({protocolGetVersionResponse}) {
     getScrollPosition() {
       return Promise.resolve(scrollPosition);
     },
-    registerPerformanceObserver() {
-      return Promise.resolve();
-    },
     beginTrace() {
       return Promise.resolve();
     },

lighthouse-core/test/gather/gather-runner-test.js

@@ -341,7 +341,6 @@ describe('GatherRunner', function() {
       enableAsyncStacks: asyncFunc,
       cacheNatives: asyncFunc,
       gotoURL: asyncFunc,
-      registerPerformanceObserver: asyncFunc,
       registerRequestIdleCallbackWrap: asyncFunc,
       cleanBrowserCaches: createCheck('calledCleanBrowserCaches'),
       clearDataForOrigin: createCheck('calledClearStorage'),