Fix the drift in repeating timers.

Source/platform/Timer.cpp

 /*
  * Copyright (C) 2006, 2008 Apple Inc. All rights reserved.
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 87 matching lines @@
         m_nextFireTime = newTime;
         if (m_cancellableTimerTask)
             m_cancellableTimerTask->cancel();
         m_cancellableTimerTask = new CancellableTimerTask(this);
         if (newTime != m_unalignedNextFireTime) {
             // If the timer is being aligned, use postTimerTaskAt() to schedule it
             // so that the relative order of aligned timers is preserved.
             // TODO(skyostil): Move timer alignment into the scheduler.
             m_webScheduler->postTimerTaskAt(m_location, m_cancellableTimerTask, m_nextFireTime);
         } else {
-            // Round the delay up to the nearest millisecond to be consistant with the
-            // previous behavior of BlinkPlatformImpl::setSharedTimerFireInterval.
-            long long delayMs = static_cast<long long>(ceil((newTime - now) * 1000.0));
-            if (delayMs < 0)
-                delayMs = 0;
-            m_webScheduler->postTimerTask(m_location, m_cancellableTimerTask, delayMs);
+            m_webScheduler->postTimerTask(m_location, m_cancellableTimerTask, newTime - now);
         }
     }
 }
 
 NO_LAZY_SWEEP_SANITIZE_ADDRESS
 void TimerBase::runInternal()
 {
     if (!canFire())
         return;
 
     TRACE_EVENT0("blink", "TimerBase::run");
     ASSERT_WITH_MESSAGE(m_thread == currentThread(), "Timer posted by %s %s was run on a different thread", m_location.functionName(), m_location.fileName());
     TRACE_EVENT_SET_SAMPLING_STATE("blink", "BlinkInternal");
 
     m_nextFireTime = 0;
-    // Note: repeating timers drift, but this is preserving the functionality of the old timer heap.
-    // See crbug.com/328700.
-    if (m_repeatInterval)
-        setNextFireTime(monotonicallyIncreasingTime(), m_repeatInterval);
+    if (m_repeatInterval) {
+        double now = monotonicallyIncreasingTime();
+        ASSERT(now >= m_unalignedNextFireTime);
+        // This computation should be drift free, and it will cope if we miss a beat,
+        // which can easily happen if the thread is busy.
+        double intervalToNextFireTime = m_repeatInterval - fmod(now - m_unalignedNextFireTime, m_repeatInterval);

[Sami, 2015/08/03 12:57:21]

If (now - m_unalignedNextFireTime) % m_repeatInterval == 0, should the delay
equal zero too?

[alex clarke (OOO till 29th), 2015/08/03 13:37:26]

Interesting point.  I think for this API there's not really a strong preference
for or against that, so I'm going to default to simpler code with less special
cases.

+        setNextFireTime(now, intervalToNextFireTime);
+    }
     fired();
     TRACE_EVENT_SET_SAMPLING_STATE("blink", "Sleeping");
 }
 
 void TimerBase::didChangeAlignmentInterval(double now)
 {
     setNextFireTime(now, m_unalignedNextFireTime - now);
 }
 
 double TimerBase::nextUnalignedFireInterval() const
 {
     ASSERT(isActive());
     return std::max(m_unalignedNextFireTime - monotonicallyIncreasingTime(), 0.0);
 }
 
 bool TimerBase::Comparator::operator()(const TimerBase* a, const TimerBase* b) const
 {
     return a->m_unalignedNextFireTime < b->m_unalignedNextFireTime;
 }
 
 } // namespace blink