Rewriting WebRTC output level tests to be more reliable under load.

content/test/data/media/webrtc_test_audio.js

Index: content/test/data/media/webrtc_test_audio.js
diff --git a/content/test/data/media/webrtc_test_audio.js b/content/test/data/media/webrtc_test_audio.js
index 1e69211d961dd1c7c69e5ab6fe5a3fc6e189eae1..836862d069f1deedf9d1744f26e1eff94dec7505 100644
--- a/content/test/data/media/webrtc_test_audio.js
+++ b/content/test/data/media/webrtc_test_audio.js
@@ -15,8 +15,7 @@ var MAX_AUDIO_OUTPUT_ENERGY = 32768;
 function ensureAudioPlaying(peerConnection, beLenient) {
   addExpectedEvent();
 
-  // Gather 50 samples per second for 2 seconds.
-  gatherAudioLevelSamples(peerConnection, 100, 50, function(samples) {
+  gatherAudioLevelSamples(peerConnection, 3 * 1000, function(samples) {
     identifyFakeDeviceSignal_(samples, beLenient);
     eventOccured();
   });
@@ -27,7 +26,7 @@ function ensureAudioPlaying(peerConnection, beLenient) {
 function ensureSilence(peerConnection) {
   addExpectedEvent();
   setTimeout(function() {
-    gatherAudioLevelSamples(peerConnection, 100, 50, function(samples) {
+    gatherAudioLevelSamples(peerConnection, 1 * 1000, function(samples) {
       identifySilence_(samples);
       eventOccured();
     });
@@ -48,32 +47,38 @@ function workAroundSeveralReportsIssue(audioOutputLevels) {
   return Math.max(audioOutputLevels[0], audioOutputLevels[1]);
 }
 
-// Gathers |numSamples| samples at |frequency| number of times per second and
-// calls back |callback| with an array with numbers in the [0, 32768] range.
-function gatherAudioLevelSamples(peerConnection, numSamples, frequency,
-                                 callback) {
-  console.log('Gathering ' + numSamples + ' audio samples...');
+// Gathers samples from WebRTC stats as fast as possible for |durationMs|
+// milliseconds and calls back |callback| with an array with numbers in the
+// [0, 32768] range. There are no guarantees for how often we will be able to
+// collect values, but this function deliberately avoids setTimeout calls in
+// order be as insensitive as possible to starvation (particularly when this
+// code runs in parallel with other tests on a heavily loaded bot).
+function gatherAudioLevelSamples(peerConnection, durationMs, callback) {
+  console.log('Gathering audio samples for ' + durationMs + ' milliseconds...');
   var audioLevelSamples = []
 
   // If this times out and never found any audio output levels, the call
   // probably doesn't have an audio stream.
-  var gatherSamples = setInterval(function() {
-    peerConnection.getStats(function(response) {
-      audioOutputLevels = getAudioLevelFromStats_(response);
-      if (audioOutputLevels.length == 0) {
-        // The call probably isn't up yet.
-        return;
-      }
-      var outputLevel = workAroundSeveralReportsIssue(audioOutputLevels);
-      audioLevelSamples.push(outputLevel);
-
-      if (audioLevelSamples.length == numSamples) {
-        console.log('Gathered all samples.');
-        clearInterval(gatherSamples);
-        callback(audioLevelSamples);
-      }
-    });
-  }, 1000 / frequency);
+  var startTime = new Date();
+  var gotStats = function(response) {
+    audioOutputLevels = getAudioLevelFromStats_(response);
+    if (audioOutputLevels.length == 0) {
+      // The call probably isn't up yet.
+      peerConnection.getStats(gotStats);
+      return;
+    }
+    var outputLevel = workAroundSeveralReportsIssue(audioOutputLevels);
+    audioLevelSamples.push(outputLevel);
+
+    var elapsed = new Date() - startTime;
+    if (elapsed > durationMs) {
+      console.log('Gathered all samples.');
+      callback(audioLevelSamples);
+      return;
+    }
+    peerConnection.getStats(gotStats);
+  }
+  peerConnection.getStats(gotStats);
 }
 
 /**
@@ -96,7 +101,7 @@ function identifyFakeDeviceSignal_(samples, beLenient) {
   var currentlyOverThreshold = false;
 
   // Detect when we have been been over the threshold and is going back again
-  // (i.e. count peaks). We should see about one peak per second.
+  // (i.e. count peaks). We should see about two peaks per second.
   for (var i = 0; i < samples.length; ++i) {
     if (currentlyOverThreshold && samples[i] < threshold)
       numPeaks++;