Attempting to resolve a race condition with PowerMonitor

base/power_monitor/power_monitor.cc

 // Copyright 2013 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 "base/power_monitor/power_monitor.h"
+
+#include "base/lazy_instance.h"
 #include "base/power_monitor/power_monitor_source.h"
+#include "base/synchronization/lock.h"
 
 namespace base {
 
+LazyInstance<Lock>::Leaky g_power_monitor_lock = LAZY_INSTANCE_INITIALIZER;
 static PowerMonitor* g_power_monitor = NULL;
 
+void PowerMonitor::Initialize(scoped_ptr<PowerMonitorSource> source) {
+  AutoLock(g_power_monitor_lock.Get());
+  DCHECK(!g_power_monitor);
+  g_power_monitor = new PowerMonitor(source.Pass());
+}
+
+void PowerMonitor::Shutdown() {
+  AutoLock(g_power_monitor_lock.Get());
+  if (g_power_monitor) {
+    delete g_power_monitor;
+    g_power_monitor = NULL;
+  }
+}
+
+bool PowerMonitor::IsInitialized() {
+  AutoLock(g_power_monitor_lock.Get());
+  return g_power_monitor != NULL;
+}
+
+bool PowerMonitor::AddObserver(PowerObserver* observer) {
+  AutoLock(g_power_monitor_lock.Get());
+  if (!g_power_monitor)
+    return false;

[Ken Russell (switch to Gerrit), 2014/02/28 21:30:09]

Maybe this, RemoveObserver and IsOnBatteryPower should DCHECK in debug builds if
the g_power_monitor is NULL to avoid surprises.

+  g_power_monitor->AddObserverImpl(observer);
+  return true;
+}
+
+bool PowerMonitor::RemoveObserver(PowerObserver* observer) {
+  AutoLock(g_power_monitor_lock.Get());
+  if (!g_power_monitor)
+    return false;
+  g_power_monitor->RemoveObserverImpl(observer);
+  return true;
+}
+
+bool PowerMonitor::IsOnBatteryPower() {
+  AutoLock(g_power_monitor_lock.Get());
+  if (!g_power_monitor)
+    return false;
+  return g_power_monitor->IsOnBatteryPowerImpl();
+}
+
 PowerMonitor::PowerMonitor(scoped_ptr<PowerMonitorSource> source)
     : observers_(new ObserverListThreadSafe<PowerObserver>()),
       source_(source.Pass()) {
-  DCHECK(!g_power_monitor);
-  g_power_monitor = this;
 }
 
 PowerMonitor::~PowerMonitor() {
-  DCHECK_EQ(this, g_power_monitor);
-  g_power_monitor = NULL;
 }
 
-// static
-PowerMonitor* PowerMonitor::Get() {
+Lock* PowerMonitor::GetSingletonLock() { return &g_power_monitor_lock.Get(); }
+
+PowerMonitor* PowerMonitor::GetSingleton() {
+  g_power_monitor_lock.Get().AssertAcquired();
   return g_power_monitor;
 }
 
-void PowerMonitor::AddObserver(PowerObserver* obs) {
+void PowerMonitor::AddObserverImpl(PowerObserver* obs) {
+  g_power_monitor_lock.Get().AssertAcquired();
   observers_->AddObserver(obs);
 }
 
-void PowerMonitor::RemoveObserver(PowerObserver* obs) {
+void PowerMonitor::RemoveObserverImpl(PowerObserver* obs) {
+  g_power_monitor_lock.Get().AssertAcquired();
   observers_->RemoveObserver(obs);
 }
 
-PowerMonitorSource* PowerMonitor::Source() {
-  return source_.get();
+bool PowerMonitor::IsOnBatteryPowerImpl() {
+  g_power_monitor_lock.Get().AssertAcquired();
+  return source_->IsOnBatteryPower();
 }
 
-bool PowerMonitor::IsOnBatteryPower() {
-  return source_->IsOnBatteryPower();
-}
+PowerMonitorSource* PowerMonitor::Source() { return source_.get(); }
 
 void PowerMonitor::NotifyPowerStateChange(bool battery_in_use) {
   DVLOG(1) << "PowerStateChange: " << (battery_in_use ? "On" : "Off")
            << " battery";
+  AutoLock(g_power_monitor_lock.Get());
   observers_->Notify(&PowerObserver::OnPowerStateChange, battery_in_use);
 }
 
 void PowerMonitor::NotifySuspend() {
   DVLOG(1) << "Power Suspending";
+  AutoLock(g_power_monitor_lock.Get());
   observers_->Notify(&PowerObserver::OnSuspend);
 }
 
 void PowerMonitor::NotifyResume() {
   DVLOG(1) << "Power Resuming";
+  AutoLock(g_power_monitor_lock.Get());
   observers_->Notify(&PowerObserver::OnResume);
 }
 
 }  // namespace base