CORE: prevent unbound growth of suspendedTimeouts and possible NaN va…

…lues (#12059)

* prevent unbound growth of suspendedTimeouts and possible NaN values in timeOutOfFocus

* Add tests

* Reintroduce outOfFocusStart default to 0

---------

Co-authored-by: Demetrio Girardi <dgirardi@prebid.org>

src/utils/focusTimeout.js

@@ -1,16 +1,27 @@
-let outOfFocusStart;
+let outOfFocusStart = null; // enforce null otherwise it could be undefined and the callback wouldn't execute
 let timeOutOfFocus = 0;
 let suspendedTimeouts = [];
 
-document.addEventListener('visibilitychange', () => {
+function trackTimeOutOfFocus() {
   if (document.hidden) {
     outOfFocusStart = Date.now()
   } else {
-    timeOutOfFocus += Date.now() - outOfFocusStart
-    suspendedTimeouts.forEach(({ callback, startTime, setTimerId }) => setTimerId(setFocusTimeout(callback, timeOutOfFocus - startTime)()))
+    timeOutOfFocus += Date.now() - (outOfFocusStart ?? 0); // when the page is loaded in hidden state outOfFocusStart is undefined, which results in timeoutOffset being NaN
     outOfFocusStart = null;
+    suspendedTimeouts.forEach(({ callback, startTime, setTimerId }) => setTimerId(setFocusTimeout(callback, timeOutOfFocus - startTime)()));
+    suspendedTimeouts = [];
   }
-});
+}
+
+document.addEventListener('visibilitychange', trackTimeOutOfFocus);
+
+export function reset() {
+  outOfFocusStart = null;
+  timeOutOfFocus = 0;
+  suspendedTimeouts = [];
+  document.removeEventListener('visibilitychange', trackTimeOutOfFocus);
+  document.addEventListener('visibilitychange', trackTimeOutOfFocus);
+}
 
 /**
  * Wraps native setTimeout function in order to count time only when page is focused
@@ -19,7 +30,7 @@ document.addEventListener('visibilitychange', () => {
  * @param {number} [milliseconds] - Minimum duration (in milliseconds) that the callback will be executed after
  * @returns {function(*): (number)} - Getter function for current timer id
  */
-export default function setFocusTimeout(callback, milliseconds) {
+export function setFocusTimeout(callback, milliseconds) {
   const startTime = timeOutOfFocus;
   let timerId = setTimeout(() => {
     if (timeOutOfFocus === startTime && outOfFocusStart == null) {

src/utils/ttlCollection.js

@@ -1,6 +1,6 @@
 import {GreedyPromise} from './promise.js';
 import {binarySearch, logError, timestamp} from '../utils.js';
-import setFocusTimeout from './focusTimeout.js';
+import {setFocusTimeout} from './focusTimeout.js';
 
 /**
  * Create a set-like collection that automatically forgets items after a certain time.

test/spec/unit/utils/focusTimeout_spec.js

@@ -1,4 +1,4 @@
-import setFocusTimeout from '../../../../src/utils/focusTimeout';
+import {setFocusTimeout, reset} from '../../../../src/utils/focusTimeout';
 
 export const setDocumentHidden = (hidden) => {
   Object.defineProperty(document, 'hidden', {
@@ -9,33 +9,41 @@ export const setDocumentHidden = (hidden) => {
 };
 
 describe('focusTimeout', () => {
-  let clock;
+  let clock, callback;
 
   beforeEach(() => {
+    reset()
     clock = sinon.useFakeTimers();
+    callback = sinon.spy();
   });
 
   afterEach(() => {
     clock.restore();
   })
 
   it('should invoke callback when page is visible', () => {
-    let callback = sinon.stub();
     setFocusTimeout(callback, 2000);
     clock.tick(2000);
     expect(callback.called).to.be.true;
   });
 
+  it('should not choke if page starts hidden', () => {
+    setDocumentHidden(true);
+    reset();
+    setDocumentHidden(false);
+    setFocusTimeout(callback, 1000);
+    clock.tick(1000);
+    sinon.assert.called(callback);
+  })
+
   it('should not invoke callback if page was hidden', () => {
-    let callback = sinon.stub();
     setFocusTimeout(callback, 2000);
     setDocumentHidden(true);
     clock.tick(3000);
     expect(callback.called).to.be.false;
   });
 
   it('should defer callback execution when page is hidden', () => {
-    let callback = sinon.stub();
     setFocusTimeout(callback, 4000);
     clock.tick(2000);
     setDocumentHidden(true);
@@ -46,8 +54,27 @@ describe('focusTimeout', () => {
     expect(callback.called).to.be.true;
   });
 
+  it('should not execute deferred callbacks again', () => {
+    setDocumentHidden(true);
+    setFocusTimeout(callback, 1000);
+    clock.tick(2000);
+    [false, true, false].forEach(setDocumentHidden);
+    clock.tick(2000);
+    sinon.assert.calledOnce(callback);
+  });
+
+  it('should run callbacks that expire while page is hidden', () => {
+    setFocusTimeout(callback, 1000);
+    clock.tick(500);
+    setDocumentHidden(true);
+    clock.tick(1000);
+    setDocumentHidden(false);
+    sinon.assert.notCalled(callback);
+    clock.tick(1000);
+    sinon.assert.called(callback);
+  })
+
   it('should return updated timerId after page was showed again', () => {
-    let callback = sinon.stub();
     const getCurrentTimerId = setFocusTimeout(callback, 4000);
     const oldTimerId = getCurrentTimerId();
     clock.tick(2000);