Bug 749894: performance.now() mochitest3: Better spec compliance, stricter test, better slack. r=bz

dom/tests/mochitest/general/test_performance_now.html

@@ -15,22 +15,51 @@
     ok(window.performance.now() >= n, "The value of now() should monotonically increase.");
     SimpleTest.waitForExplicitFinish();
 
+    // The spec says performance.now() should have micro-second resolution, but allows 1ms if the platform doesn't support it.
+    // Our implementation does provide micro-second resolution, except for windows XP combined with some HW properties
+    // where we can't use QueryPerformanceCounters (see comments at mozilla-central/xpcom/ds/TimeStamp_windows.cpp).
+    // This XP-low-res case results in about 15ms resolutions, and can be identified when perf.now() returns only integers.
+    //
+    // Since setTimeout might return too early/late, our goal is that perf.now() changed within 2ms
+    // (or 25ms for XP-low-res), rather than specific number of setTimeout(N) invocations.
+    // See bug 749894 (intermittent failures of this test)
+    var platformPossiblyLowRes = navigator.oscpu.indexOf("Windows NT 5.1") == 0; // XP only
+    var allInts = (n % 1) == 0; // Indicator of limited HW resolution.
     var checks = 0;
+
     function checkAfterTimeout() {
       checks++;
       var d2 = Date.now();
-      // Timeouts aren't extremely reliable and especially on Windows we are prone to fallback to
-      // millisecond timers, in which case this test might occasionally fail. This is a workaround:
-      if (navigator.platform.indexOf("Win") == 0 &&
-          window.performance.now() == n && d == d2 &&
-          checks < 5) {
-        setTimeout(checkAfterTimeout, 20);
+      var n2 = window.performance.now();
+
+      allInts = allInts && (n2 % 1) == 0;
+      var lowResCounter = platformPossiblyLowRes && allInts;
+
+      if ( n2 == n && checks < 50 && // 50 is just a failsafe. Our real goals are 2ms or 25ms.
+           ( (d2 - d) < 2 // The spec allows 1ms resolution. We allow up to measured 2ms to ellapse.
+             ||
+             lowResCounter &&
+             (d2 - d) < 25
+           )
+         ) {
+        setTimeout(checkAfterTimeout, 1);
         return;
       }
-      ok(window.performance.now() > n, "After a timeout, the value of now() should be strictly greater than before.");
+
+      // Loose spec: 1ms resolution, or 15ms resolution for the XP-low-res case.
+      // We shouldn't test that dt is actually within 2/25ms since the iterations break if it isn't, and timeout could be late.
+      ok(n2 > n, "Loose - the value of now() should increase within 2ms (or 25ms if low-res counter) (delta now(): " + (n2 - n) + " ms).");
+
+      // Strict spec: if it's not the XP-low-res case, while the spec allows 1ms resolution, it prefers microseconds, which we provide.
+      // Since the fastest setTimeout return which I observed was ~500 microseconds, a microseconds counter should change in 1 iteretion.
+      ok(n2 > n && (lowResCounter || checks == 1),
+         "Strict - [if high-res counter] the value of now() should increase after one setTimeout (hi-res: " + (!lowResCounter) +
+                                                                                                  ", iters: " + checks +
+                                                                                                  ", dt: " + (d2 - d) +
+                                                                                                  ", now(): " + n2 + ").");
       SimpleTest.finish();
     };
-    setTimeout(checkAfterTimeout, 20);
+    setTimeout(checkAfterTimeout, 1);
   </script>
 </body>
 </html>