Revert of C++ readability review (patchset #10 id:180001 of...

Revert of C++ readability review (patchset #10 id:180001 of https://codereview.chromium.org/706993003/)

Reason for revert:
It looks like this broke tests the following tests on Linux Chromium OS:
AlarmTimerTest.OneShotTimerConcurrentResetAndTimerFired
AlarmTimerTest.RepeatingTimerConcurrentResetAndTimerFired

Example output:
AlarmTimerTest.OneShotTimerConcurrentResetAndTimerFired (run #1):
[ RUN      ] AlarmTimerTest.OneShotTimerConcurrentResetAndTimerFired
[11334:11334:0406/201742:404219967:INFO:alarm_timer_chromeos.cc(166)] CLOCK_REALTIME_ALARM not supported on this system: Invalid argument
[11334:11334:0406/201742:404230154:FATAL:timer.cc(116)] Check failed: !user_task_.is_null().

AlarmTimerTest.RepeatingTimerConcurrentResetAndTimerFired (run #1):
[ RUN      ] AlarmTimerTest.RepeatingTimerConcurrentResetAndTimerFired
[11382:11382:0406/201748:411022458:INFO:alarm_timer_chromeos.cc(166)] CLOCK_REALTIME_ALARM not supported on this system: Invalid argument
../../components/timers/alarm_timer_unittest.cc:469: Failure
Value of: did_run
  Actual: true
Expected: false

Example build:
http://build.chromium.org/p/chromium.chromiumos/builders/Linux%20ChromiumOS%20Tests%20%281%29/builds/996

Original issue's description:
> C++ readability review
>
> BUG=b/18275087
>
> Committed: https://crrev.com/05e4d854dca737cfb43cd967cd72d024f6b1625b
> Cr-Commit-Position: refs/heads/master@{#324004}

TBR=derat@chromium.org,gromer@google.com,zea@chromium.org,chirantan@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=b/18275087

Review URL: https://codereview.chromium.org/1058693003

Cr-Commit-Position: refs/heads/master@{#324009}

components/gcm_driver/gcm_driver_desktop.cc

@@ -25,7 +25,7 @@
 #include "net/url_request/url_request_context_getter.h"
 
 #if defined(OS_CHROMEOS)
-#include "components/timers/alarm_timer_chromeos.h"
+#include "components/timers/alarm_timer.h"
 #endif
 
 namespace gcm {
@@ -344,7 +344,7 @@ void GCMDriverDesktop::IOWorker::WakeFromSuspendForHeartbeat(bool wake) {
 
   scoped_ptr<base::Timer> timer;
   if (wake)
-    timer.reset(new timers::SimpleAlarmTimer());
+    timer.reset(new timers::AlarmTimer(true, false));
   else
     timer.reset(new base::Timer(true, false));
 

components/timers.gypi

@@ -15,8 +15,10 @@
         '../base/base.gyp:base',
       ],
       'sources': [
-        'timers/alarm_timer_chromeos.cc',
-        'timers/alarm_timer_chromeos.h',
+        'timers/alarm_timer.cc',
+        'timers/alarm_timer.h',
+        'timers/rtc_alarm.cc',
+        'timers/rtc_alarm.h',
       ],
     },
   ],

components/timers/BUILD.gn

@@ -4,8 +4,10 @@
 
 static_library("timers") {
   sources = [
-    "alarm_timer_chromeos.cc",
-    "alarm_timer_chromeos.h",
+    "alarm_timer.cc",
+    "alarm_timer.h",
+    "rtc_alarm.cc",
+    "rtc_alarm.h",
   ]
 
   deps = [

components/timers/alarm_timer.cc

+// 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.
+
+#include "components/timers/alarm_timer.h"
+
+#include "base/bind.h"
+#include "base/bind_helpers.h"
+#include "base/files/file_util.h"
+#include "base/logging.h"
+#include "base/pending_task.h"
+#include "components/timers/rtc_alarm.h"
+
+namespace timers {
+
+AlarmTimer::AlarmTimer(bool retain_user_task, bool is_repeating)
+    : base::Timer(retain_user_task, is_repeating),
+      delegate_(new RtcAlarm()),
+      can_wake_from_suspend_(false),
+      origin_message_loop_(NULL),
+      weak_factory_(this) {
+  can_wake_from_suspend_ = delegate_->Init(weak_factory_.GetWeakPtr());
+}
+
+AlarmTimer::AlarmTimer(const tracked_objects::Location& posted_from,
+                       base::TimeDelta delay,
+                       const base::Closure& user_task,
+                       bool is_repeating)
+    : base::Timer(posted_from, delay, user_task, is_repeating),
+      delegate_(new RtcAlarm()),
+      can_wake_from_suspend_(false),
+      origin_message_loop_(NULL),
+      weak_factory_(this) {
+  can_wake_from_suspend_ = delegate_->Init(weak_factory_.GetWeakPtr());
+}
+
+AlarmTimer::~AlarmTimer() {
+  Stop();
+}
+
+void AlarmTimer::OnTimerFired() {
+  if (!base::Timer::IsRunning())
+    return;
+
+  DCHECK(pending_task_.get());
+
+  // Take ownership of the pending user task, which is going to be cleared by
+  // the Stop() or Reset() functions below.
+  scoped_ptr<base::PendingTask> pending_user_task(pending_task_.Pass());
+
+  // Re-schedule or stop the timer as requested.
+  if (base::Timer::is_repeating())
+    Reset();
+  else
+    Stop();
+
+  // Now run the user task.
+  base::MessageLoop::current()->task_annotator()->RunTask(
+      "AlarmTimer::Reset", "AlarmTimer::OnTimerFired", *pending_user_task);
+}
+
+void AlarmTimer::Stop() {
+  if (!can_wake_from_suspend_) {
+    base::Timer::Stop();
+    return;
+  }
+
+  // Clear the running flag, stop the delegate, and delete the pending task.
+  base::Timer::set_is_running(false);
+  delegate_->Stop();
+  pending_task_.reset();
+
+  // Stop is called when the AlarmTimer is destroyed so we need to remove
+  // ourselves as a MessageLoop::DestructionObserver to prevent a segfault
+  // later.
+  if (origin_message_loop_) {
+    origin_message_loop_->RemoveDestructionObserver(this);
+    origin_message_loop_ = NULL;
+  }
+
+  if (!base::Timer::retain_user_task())
+    base::Timer::set_user_task(base::Closure());
+}
+
+void AlarmTimer::Reset() {
+  if (!can_wake_from_suspend_) {
+    base::Timer::Reset();
+    return;
+  }
+
+  DCHECK(!base::Timer::user_task().is_null());
+  DCHECK(!origin_message_loop_ ||
+         origin_message_loop_->task_runner()->RunsTasksOnCurrentThread());
+
+  // Make sure that the timer will stop if the underlying message loop is
+  // destroyed.
+  if (!origin_message_loop_) {
+    origin_message_loop_ = base::MessageLoop::current();
+    origin_message_loop_->AddDestructionObserver(this);
+  }
+
+  // Set up the pending task.
+  if (base::Timer::GetCurrentDelay() > base::TimeDelta::FromMicroseconds(0)) {
+    base::Timer::set_desired_run_time(
+        base::TimeTicks::Now() + base::Timer::GetCurrentDelay());
+    pending_task_.reset(new base::PendingTask(base::Timer::posted_from(),
+                                              base::Timer::user_task(),
+                                              base::Timer::desired_run_time(),
+                                              true  /* nestable */));
+  } else {
+    base::Timer::set_desired_run_time(base::TimeTicks());
+    pending_task_.reset(new base::PendingTask(base::Timer::posted_from(),
+                                              base::Timer::user_task()));
+  }
+  base::MessageLoop::current()->task_annotator()->DidQueueTask(
+      "AlarmTimer::Reset", *pending_task_);
+
+  // Now start up the timer.
+  delegate_->Reset(base::Timer::GetCurrentDelay());
+  base::Timer::set_is_running(true);
+}
+
+void AlarmTimer::WillDestroyCurrentMessageLoop() {
+  Stop();
+}
+
+}  // namespace timers

components/timers/alarm_timer_chromeos.h → components/timers/alarm_timer.h

@@ -2,18 +2,18 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef COMPONENTS_TIMERS_ALARM_TIMER_CHROMEOS_H_
-#define COMPONENTS_TIMERS_ALARM_TIMER_CHROMEOS_H_
+#ifndef COMPONENTS_TIMER_ALARM_TIMER_H_
+#define COMPONENTS_TIMER_ALARM_TIMER_H_
 
 #include "base/callback.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/weak_ptr.h"
+#include "base/message_loop/message_loop.h"
 #include "base/time/time.h"
 #include "base/timer/timer.h"
 
 namespace base {
-class MessageLoop;
 struct PendingTask;
 }
 
@@ -24,45 +24,61 @@ namespace timers {
 // Currently, this feature is only available on Chrome OS systems running linux
 // version 3.11 or higher.  On all other platforms, the AlarmTimer behaves
 // exactly the same way as a regular Timer.
-class AlarmTimer : public base::Timer {
+class AlarmTimer : public base::Timer,
+                   public base::MessageLoop::DestructionObserver {
  public:
-  ~AlarmTimer() override;
-
-  bool can_wake_from_suspend() const { return can_wake_from_suspend_; }
+  // The delegate is responsible for managing the system level details for
+  // actually setting up and monitoring a timer that is capable of waking the
+  // system from suspend.  This class is reference counted because it may need
+  // to outlive the timer in order to clean up after itself.
+  class Delegate : public base::RefCountedThreadSafe<Delegate> {
+   public:
+    // Initializes the timer.  Should return true iff the system has timers that
+    // can wake it up from suspend.  Will only be called once.
+    virtual bool Init(base::WeakPtr<AlarmTimer> timer) = 0;
 
-  // Sets a hook that will be called when the timer fires and a task has been
-  // queued on |origin_message_loop_|.  Used by tests to wait until a task is
-  // pending in the MessageLoop.
-  void SetTimerFiredCallbackForTest(base::Closure test_callback);
+    // Stops the currently running timer.  It should be safe to call this more
+    // than once.
+    virtual void Stop() = 0;
 
-  // Timer overrides.
-  void Stop() override;
-  void Reset() override;
+    // Resets the timer to fire after |delay| has passed.  Cancels any
+    // pre-existing delay.
+    virtual void Reset(base::TimeDelta delay) = 0;
 
    protected:
-  // The constructors for this class are protected because consumers should
-  // instantiate one of the specialized sub-classes defined below instead.
+    virtual ~Delegate() {}
+
+   private:
+    friend class base::RefCountedThreadSafe<Delegate>;
+  };
+
   AlarmTimer(bool retain_user_task, bool is_repeating);
+
   AlarmTimer(const tracked_objects::Location& posted_from,
              base::TimeDelta delay,
              const base::Closure& user_task,
              bool is_repeating);
 
- private:
-  // Common initialization that must be performed by both constructors.  This
-  // really should live in a delegated constructor but the way base::Timer's
-  // constructors are written makes it really hard to do so.
-  void Init();
+  ~AlarmTimer() override;
 
-  // Will be called by the delegate to indicate that the timer has fired and
+  bool can_wake_from_suspend() const { return can_wake_from_suspend_; }
+
+  // Must be called by the delegate to indicate that the timer has fired and
   // that the user task should be run.
   void OnTimerFired();
 
-  // Called when |origin_message_loop_| will be destroyed.
-  void WillDestroyCurrentMessageLoop();
+  // Timer overrides.
+  void Stop() override;
+  void Reset() override;
+
+  // MessageLoop::DestructionObserver overrides.
+  void WillDestroyCurrentMessageLoop() override;
+
+ private:
+  // Initializes the timer with the appropriate delegate.
+  void Init();
 
   // Delegate that will manage actually setting the timer.
-  class Delegate;
   scoped_refptr<Delegate> delegate_;
 
   // Keeps track of the user task we want to run.  A new one is constructed
@@ -79,68 +95,11 @@ class AlarmTimer : public base::Timer {
   // destruction of that message loop.
   base::MessageLoop* origin_message_loop_;
 
-  // Observes |origin_message_loop_| and informs this class if it will be
-  // destroyed.
-  class MessageLoopObserver;
-  scoped_ptr<MessageLoopObserver> message_loop_observer_;
-
   base::WeakPtrFactory<AlarmTimer> weak_factory_;
 
   DISALLOW_COPY_AND_ASSIGN(AlarmTimer);
 };
 
-// As its name suggests, a OneShotAlarmTimer runs a given task once.  It does
-// not remember the task that was given to it after it has fired and does not
-// repeat.  Useful for fire-and-forget tasks.
-class OneShotAlarmTimer : public AlarmTimer {
- public:
-  // Constructs a basic OneShotAlarmTimer.  An AlarmTimer constructed this way
-  // requires that Start() is called before Reset() is called.
-  OneShotAlarmTimer();
-  ~OneShotAlarmTimer() override;
-};
-
-// A RepeatingAlarmTimer takes a task and delay and repeatedly runs the task
-// using the specified delay as an interval between the runs until it is
-// explicitly stopped.  It remembers both the task and the delay it was given
-// after it fires.
-class RepeatingAlarmTimer : public AlarmTimer {
- public:
-  // Constructs a basic RepeatingAlarmTimer.  An AlarmTimer constructed this way
-  // requires that Start() is called before Reset() is called.
-  RepeatingAlarmTimer();
-
-  // Constructs a RepeatingAlarmTimer with pre-populated parameters but does not
-  // start it.  Useful if |user_task| or |delay| are not going to change.
-  // Reset() can be called immediately after constructing an AlarmTimer in this
-  // way.
-  RepeatingAlarmTimer(const tracked_objects::Location& posted_from,
-                      base::TimeDelta delay,
-                      const base::Closure& user_task);
-
-  ~RepeatingAlarmTimer() override;
-};
-
-// A SimpleAlarmTimer only fires once but remembers the task that it was given
-// even after it has fired.  Useful if you want to run the same task multiple
-// times but not at a regular interval.
-class SimpleAlarmTimer : public AlarmTimer {
- public:
-  // Constructs a basic SimpleAlarmTimer.  An AlarmTimer constructed this way
-  // requires that Start() is called before Reset() is called.
-  SimpleAlarmTimer();
-
-  // Constructs a SimpleAlarmTimer with pre-populated parameters but does not
-  // start it.  Useful if |user_task| or |delay| are not going to change.
-  // Reset() can be called immediately after constructing an AlarmTimer in this
-  // way.
-  SimpleAlarmTimer(const tracked_objects::Location& posted_from,
-                   base::TimeDelta delay,
-                   const base::Closure& user_task);
-
-  ~SimpleAlarmTimer() override;
-};
-
 }  // namespace timers
 
-#endif  // COMPONENTS_TIMERS_ALARM_TIMER_CHROMEOS_H_
+#endif  // COMPONENTS_TIMER_ALARM_TIMER_H_

components/timers/rtc_alarm.cc

+// 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.
+
+#include "components/timers/rtc_alarm.h"
+
+#include <sys/timerfd.h>
+
+#include "base/bind.h"
+#include "base/files/file_util.h"
+#include "base/lazy_instance.h"
+#include "base/logging.h"
+#include "base/macros.h"
+#include "base/message_loop/message_loop_proxy.h"
+#include "base/threading/thread.h"
+
+namespace timers {
+
+namespace {
+// Helper class to ensure that the IO thread we will use for watching file
+// descriptors is started only once.
+class IOThreadStartHelper {
+ public:
+  IOThreadStartHelper() : thread_(new base::Thread("RTC Alarm IO Thread")) {
+    CHECK(thread_->StartWithOptions(
+        base::Thread::Options(base::MessageLoop::TYPE_IO, 0)));
+  }
+  ~IOThreadStartHelper() {}
+
+  base::Thread& operator*() const { return *thread_.get(); }
+
+  base::Thread* operator->() const { return thread_.get(); }
+
+ private:
+  scoped_ptr<base::Thread> thread_;
+};
+
+base::LazyInstance<IOThreadStartHelper> g_io_thread = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+RtcAlarm::RtcAlarm()
+    : alarm_fd_(timerfd_create(CLOCK_REALTIME_ALARM, 0)),
+      origin_event_id_(0),
+      io_event_id_(0) {
+}
+
+RtcAlarm::~RtcAlarm() {
+  if (alarm_fd_ != -1)
+    close(alarm_fd_);
+}
+
+bool RtcAlarm::Init(base::WeakPtr<AlarmTimer> parent) {
+  parent_ = parent;
+
+  return alarm_fd_ != -1;
+}
+
+void RtcAlarm::Stop() {
+  // Make sure that we stop the RTC from a MessageLoopForIO.
+  if (!base::MessageLoopForIO::IsCurrent()) {
+    g_io_thread.Get()->task_runner()->PostTask(
+        FROM_HERE,
+        base::Bind(&RtcAlarm::Stop, scoped_refptr<RtcAlarm>(this)));
+    return;
+  }
+
+  // Stop watching for events.
+  fd_watcher_.reset();
+
+  // Now clear the timer.
+  DCHECK_NE(alarm_fd_, -1);
+  itimerspec blank_time = {};
+  timerfd_settime(alarm_fd_, 0, &blank_time, NULL);
+}
+
+void RtcAlarm::Reset(base::TimeDelta delay) {
+  // Get a proxy for the current message loop.  When the timer fires, we will
+  // post tasks to this proxy to let the parent timer know.
+  origin_message_loop_ = base::MessageLoopProxy::current();
+
+  // Increment the event id.  Used to invalidate any events that have been
+  // queued but not yet run since the last time Reset() was called.
+  origin_event_id_++;
+
+  // Calling timerfd_settime with a zero delay actually clears the timer so if
+  // the user has requested a zero delay timer, we need to handle it
+  // differently.  We queue the task here but we still go ahead and call
+  // timerfd_settime with the zero delay anyway to cancel any previous delay
+  // that might have been programmed.
+  if (delay <= base::TimeDelta::FromMicroseconds(0)) {
+    // The timerfd_settime documentation is vague on what happens when it is
+    // passed a negative delay.  We can sidestep the issue by ensuring that the
+    // delay is 0.
+    delay = base::TimeDelta::FromMicroseconds(0);
+    origin_message_loop_->PostTask(FROM_HERE,
+                                   base::Bind(&RtcAlarm::OnTimerFired,
+                                              scoped_refptr<RtcAlarm>(this),
+                                              origin_event_id_));
+  }
+
+  // Make sure that we are running on a MessageLoopForIO.
+  if (!base::MessageLoopForIO::IsCurrent()) {
+    g_io_thread.Get()->task_runner()->PostTask(
+        FROM_HERE,
+        base::Bind(&RtcAlarm::ResetImpl,
+                   scoped_refptr<RtcAlarm>(this),
+                   delay,
+                   origin_event_id_));
+    return;
+  }
+
+  ResetImpl(delay, origin_event_id_);
+}
+
+void RtcAlarm::OnFileCanReadWithoutBlocking(int fd) {
+  DCHECK_EQ(alarm_fd_, fd);
+
+  // Read from the fd to ack the event.
+  char val[sizeof(uint64_t)];
+  base::ReadFromFD(alarm_fd_, val, sizeof(uint64_t));
+
+  // Make sure that the parent timer is informed on the proper message loop.
+  if (origin_message_loop_->RunsTasksOnCurrentThread()) {
+    OnTimerFired(io_event_id_);
+    return;
+  }
+
+  origin_message_loop_->PostTask(FROM_HERE,
+                                 base::Bind(&RtcAlarm::OnTimerFired,
+                                            scoped_refptr<RtcAlarm>(this),
+                                            io_event_id_));
+}
+
+void RtcAlarm::OnFileCanWriteWithoutBlocking(int fd) {
+  NOTREACHED();
+}
+
+void RtcAlarm::ResetImpl(base::TimeDelta delay, int event_id) {
+  DCHECK(base::MessageLoopForIO::IsCurrent());
+  DCHECK_NE(alarm_fd_, -1);
+
+  // Store the event id in the IO thread variable.  When the timer fires, we
+  // will bind this value to the OnTimerFired callback to ensure that we do the
+  // right thing if the timer gets reset.
+  io_event_id_ = event_id;
+
+  // If we were already watching the fd, this will stop watching it.
+  fd_watcher_.reset(new base::MessageLoopForIO::FileDescriptorWatcher);
+
+  // Start watching the fd to see when the timer fires.
+  if (!base::MessageLoopForIO::current()->WatchFileDescriptor(
+          alarm_fd_,
+          false,
+          base::MessageLoopForIO::WATCH_READ,
+          fd_watcher_.get(),
+          this)) {
+    LOG(ERROR) << "Error while attempting to watch file descriptor for RTC "
+               << "alarm.  Timer will not fire.";
+  }
+
+  // Actually set the timer.  This will also clear the pre-existing timer, if
+  // any.
+  itimerspec alarm_time = {};
+  alarm_time.it_value.tv_sec = delay.InSeconds();
+  alarm_time.it_value.tv_nsec =
+      (delay.InMicroseconds() % base::Time::kMicrosecondsPerSecond) *
+      base::Time::kNanosecondsPerMicrosecond;
+  if (timerfd_settime(alarm_fd_, 0, &alarm_time, NULL) < 0)
+    PLOG(ERROR) << "Error while setting alarm time.  Timer will not fire";
+}
+
+void RtcAlarm::OnTimerFired(int event_id) {
+  DCHECK(origin_message_loop_->RunsTasksOnCurrentThread());
+
+  // Check to make sure that the timer was not reset in the time between when
+  // this task was queued to run and now.  If it was reset, then don't do
+  // anything.
+  if (event_id != origin_event_id_)
+    return;
+
+  if (parent_)
+    parent_->OnTimerFired();
+}
+
+}  // namespace timers

components/timers/rtc_alarm.h

+// 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.
+  ~RtcAlarm() override;
+
+ 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_