components/timers/rtc_alarm.h

   1 // Copyright 2014 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #ifndef COMPONENTS_TIMER_RTC_ALARM_H_
   6 #define COMPONENTS_TIMER_RTC_ALARM_H_
   7
   8 #include "base/macros.h"
   9 #include "base/memory/ref_counted.h"
  10 #include "base/memory/scoped_ptr.h"
  11 #include "base/memory/weak_ptr.h"
  12 #include "base/message_loop/message_loop.h"
  13 #include "base/time/time.h"
  14 #include "components/timers/alarm_timer.h"
  15
  16 namespace base {
  17 class MessageLoopProxy;
  18 }
  19
  20 namespace timers {
  21 // This class manages a Real Time Clock (RTC) alarm, a feature that is available
  22 // from linux version 3.11 onwards.  It creates a file descriptor for the RTC
  23 // alarm timer and then watches that file descriptor to see when it can be read
  24 // without blocking, indicating that the timer has fired.
  25 //
  26 // A major problem for this class is that watching file descriptors is only
  27 // available on a MessageLoopForIO but there is no guarantee the timer is going
  28 // to be created on one.  To get around this, the timer has a dedicated thread
  29 // with a MessageLoopForIO that posts tasks back to the thread that started the
  30 // timer.
  31 //
  32 // This class is designed to work together with the AlarmTimer class and is
  33 // tested through the AlarmTimer unit tests.  DO NOT TRY TO USE THIS CLASS
  34 // DIRECTLY.  Or use it anyway, I'm a comment not a cop.
  35 class RtcAlarm : public AlarmTimer::Delegate,
  36                  public base::MessageLoopForIO::Watcher {
  37  public:
  38   RtcAlarm();
  39
  40   // AlarmTimer::Delegate overrides.
  41   bool Init(base::WeakPtr<AlarmTimer> timer) override;
  42   void Stop() override;
  43   void Reset(base::TimeDelta delay) override;
  44
  45   // base::MessageLoopForIO::Watcher overrides.
  46   void OnFileCanReadWithoutBlocking(int fd) override;
  47   void OnFileCanWriteWithoutBlocking(int fd) override;
  48
  49  protected:
  50   // Needs to be protected because AlarmTimer::Delegate is a refcounted class.
  51   ~RtcAlarm() override;
  52
  53  private:
  54   // Actually performs the system calls to set up the timer.  This must be
  55   // called on a MessageLoopForIO.
  56   void ResetImpl(base::TimeDelta delay, int event_id);
  57
  58   // Callback that is run when the timer fires.  Must be run on
  59   // |origin_message_loop_|.
  60   void OnTimerFired(int event_id);
  61
  62   // File descriptor associated with the alarm timer.
  63   int alarm_fd_;
  64
  65   // Message loop which initially started the timer.
  66   scoped_refptr<base::MessageLoopProxy> origin_message_loop_;
  67
  68   // The parent timer that should be informed when the timer fires.  We may end
  69   // up outliving the parent so we need to ensure the reference is valid before
  70   // we try to call it.
  71   base::WeakPtr<AlarmTimer> parent_;
  72
  73   // Manages watching file descriptors.
  74   scoped_ptr<base::MessageLoopForIO::FileDescriptorWatcher> fd_watcher_;
  75
  76   // These two variables are used for coordinating between the thread that
  77   // started the timer and the IO thread being used to watch the timer file
  78   // descriptor.  When Reset() is called, the original thread increments
  79   // |origin_event_id_| and binds its value to ResetImpl(), which gets posted to
  80   // the IO thread.  When the IO thread runs, it saves this value in
  81   // |io_event_id_|.  Later, when the timer fires, the IO thread binds the value
  82   // of |io_event_id_| to OnTimerFired() and posts it to the original thread.
  83   // When the original thread runs OnTimerFired(), it calls
  84   // parent_->OnTimerFired() only if |origin_event_id_| matches the event id
  85   // that was passed in to it.  This is used to get around a race condition
  86   // where the user resets the timer on the original thread, while the event is
  87   // being fired on the IO thread at the same time.
  88   int origin_event_id_;
  89   int io_event_id_;
  90
  91   DISALLOW_COPY_AND_ASSIGN(RtcAlarm);
  92 };
  93
  94 }  // namespace timers
  95 #endif  // COMPONENTS_TIMER_RTC_ALARM_H_