components: Introduce AlarmTimer class and use it for GCM heartbeat

components/timers/rtc_alarm.h

Index: components/timers/rtc_alarm.h
diff --git a/components/timers/rtc_alarm.h b/components/timers/rtc_alarm.h
new file mode 100644
index 0000000000000000000000000000000000000000..bbde8c7a8f8bffe1fab5b8e6c90575cdd9b29507
--- /dev/null
+++ b/components/timers/rtc_alarm.h
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+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef COMPONENTS_TIMER_RTC_ALARM_H_
+#define COMPONENTS_TIMER_RTC_ALARM_H_
+
+#include "base/macros.h"
+#include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
+#include "base/memory/weak_ptr.h"
+#include "base/message_loop/message_loop.h"
+#include "base/time/time.h"
+#include "components/timers/alarm_timer.h"
+
+namespace base {
+class MessageLoopProxy;
+}
+
+namespace timers {
+// This class manages a Real Time Clock (RTC) alarm, a feature that is available
+// from linux version 3.11 onwards.  It creates a file descriptor for the RTC
+// alarm timer and then watches that file descriptor to see when it can be read
+// without blocking, indicating that the timer has fired.
+//
+// A major problem for this class is that watching file descriptors is only
+// available on a MessageLoopForIO but there is no guarantee the timer is going
+// to be created on one.  To get around this, the timer has a dedicated thread
+// with a MessageLoopForIO that posts tasks back to the thread that started the
+// timer.
+//
+// This class is designed to work together with the AlarmTimer class and is
+// tested through the AlarmTimer unit tests.  DO NOT TRY TO USE THIS CLASS
+// DIRECTLY.  Or use it anyway, I'm a comment not a cop.
+class RtcAlarm : public AlarmTimer::Delegate,
+                 public base::MessageLoopForIO::Watcher {
+ public:
+  RtcAlarm();
+
+  // AlarmTimer::Delegate overrides.
+  bool Init(base::WeakPtr<AlarmTimer> timer) override;
+  void Stop() override;
+  void Reset(base::TimeDelta delay) override;
+
+  // base::MessageLoopForIO::Watcher overrides.
+  void OnFileCanReadWithoutBlocking(int fd) override;
+  void OnFileCanWriteWithoutBlocking(int fd) override;
+
+ protected:
+  // Needs to be protected because AlarmTimer::Delegate is a refcounted class.
+  virtual ~RtcAlarm();
+
+ private:
+  // Actually performs the system calls to set up the timer.  This must be
+  // called on a MessageLoopForIO.
+  void ResetImpl(base::TimeDelta delay, int event_id);
+
+  // Callback that is run when the timer fires.  Must be run on
+  // |origin_message_loop_|.
+  void OnTimerFired(int event_id);
+
+  // File descriptor associated with the alarm timer.
+  int alarm_fd_;
+
+  // Message loop which initially started the timer.
+  scoped_refptr<base::MessageLoopProxy> origin_message_loop_;
+
+  // The parent timer that should be informed when the timer fires.  We may end
+  // up outliving the parent so we need to ensure the reference is valid before
+  // we try to call it.
+  base::WeakPtr<AlarmTimer> parent_;
+
+  // Manages watching file descriptors.
+  scoped_ptr<base::MessageLoopForIO::FileDescriptorWatcher> fd_watcher_;
+
+  // These two variables are used for coordinating between the thread that
+  // started the timer and the IO thread being used to watch the timer file
+  // descriptor.  When Reset() is called, the original thread increments
+  // |origin_event_id_| and binds its value to ResetImpl(), which gets posted to
+  // the IO thread.  When the IO thread runs, it saves this value in
+  // |io_event_id_|.  Later, when the timer fires, the IO thread binds the value
+  // of |io_event_id_| to OnTimerFired() and posts it to the original thread.
+  // When the original thread runs OnTimerFired(), it calls
+  // parent_->OnTimerFired() only if |origin_event_id_| matches the event id
+  // that was passed in to it.  This is used to get around a race condition
+  // where the user resets the timer on the original thread, while the event is
+  // being fired on the IO thread at the same time.
+  int origin_event_id_;
+  int io_event_id_;
+
+  DISALLOW_COPY_AND_ASSIGN(RtcAlarm);
+};
+
+}  // namespace timers
+#endif  // COMPONENTS_TIMER_RTC_ALARM_H_