13f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen// Copyright (c) 2011 The Chromium Authors. All rights reserved. 2c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// Use of this source code is governed by a BSD-style license that can be 3c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// found in the LICENSE file. 4c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 53f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen#include "base/synchronization/waitable_event_watcher.h" 6c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 7c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott#include "base/message_loop.h" 83f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen#include "base/synchronization/lock.h" 93f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen#include "base/synchronization/waitable_event.h" 10c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 11c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottnamespace base { 12c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 13c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 14c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// WaitableEventWatcher (async waits). 15c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// 16c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// The basic design is that we add an AsyncWaiter to the wait-list of the event. 17c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// That AsyncWaiter has a pointer to MessageLoop, and a Task to be posted to it. 18c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// The MessageLoop ends up running the task, which calls the delegate. 19c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// 20c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// Since the wait can be canceled, we have a thread-safe Flag object which is 21c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// set when the wait has been canceled. At each stage in the above, we check the 22c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// flag before going onto the next stage. Since the wait may only be canceled in 23c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// the MessageLoop which runs the Task, we are assured that the delegate cannot 24c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// be called after canceling... 25c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 26c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 27c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// A thread-safe, reference-counted, write-once flag. 28c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 29c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottclass Flag : public RefCountedThreadSafe<Flag> { 30c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott public: 31c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott Flag() { flag_ = false; } 32c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 33c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott void Set() { 34c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AutoLock locked(lock_); 35c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott flag_ = true; 36c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 37c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 38c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott bool value() const { 39c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AutoLock locked(lock_); 40c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return flag_; 41c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 42c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 43c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott private: 44c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott mutable Lock lock_; 45c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott bool flag_; 46c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott}; 47c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 48c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 49c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// This is an asynchronous waiter which posts a task to a MessageLoop when 50c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// fired. An AsyncWaiter may only be in a single wait-list. 51c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 52c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottclass AsyncWaiter : public WaitableEvent::Waiter { 53c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott public: 54c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AsyncWaiter(MessageLoop* message_loop, Task* task, Flag* flag) 55c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott : message_loop_(message_loop), 56c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cb_task_(task), 57c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott flag_(flag) { } 58c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 59c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott bool Fire(WaitableEvent* event) { 60c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (flag_->value()) { 61c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // If the callback has been canceled, we don't enqueue the task, we just 62c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // delete it instead. 63c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delete cb_task_; 64c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } else { 65c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_->PostTask(FROM_HERE, cb_task_); 66c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 67c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 68c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We are removed from the wait-list by the WaitableEvent itself. It only 69c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // remains to delete ourselves. 70c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delete this; 71c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 72c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We can always return true because an AsyncWaiter is never in two 73c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // different wait-lists at the same time. 74c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return true; 75c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 76c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 77c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // See StopWatching for discussion 78c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott bool Compare(void* tag) { 79c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return tag == flag_.get(); 80c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 81c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 82c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott private: 83c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott MessageLoop *const message_loop_; 84c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott Task *const cb_task_; 85c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott scoped_refptr<Flag> flag_; 86c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott}; 87c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 88c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 89c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// For async waits we need to make a callback in a MessageLoop thread. We do 90c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// this by posting this task, which calls the delegate and keeps track of when 91c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// the event is canceled. 92c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 93c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottclass AsyncCallbackTask : public Task { 94c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott public: 95c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AsyncCallbackTask(Flag* flag, WaitableEventWatcher::Delegate* delegate, 96c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott WaitableEvent* event) 97c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott : flag_(flag), 98c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delegate_(delegate), 99c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott event_(event) { 100c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 101c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 102c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott void Run() { 103c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // Runs in MessageLoop thread. 104c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (!flag_->value()) { 105c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // This is to let the WaitableEventWatcher know that the event has occured 106c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // because it needs to be able to return NULL from GetWatchedObject 107c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott flag_->Set(); 108c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delegate_->OnWaitableEventSignaled(event_); 109c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 110c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 111c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We are deleted by the MessageLoop 112c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 113c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 114c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott private: 115c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott scoped_refptr<Flag> flag_; 116c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott WaitableEventWatcher::Delegate *const delegate_; 117c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott WaitableEvent *const event_; 118c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott}; 119c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 120c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick ScottWaitableEventWatcher::WaitableEventWatcher() 1213f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen : message_loop_(NULL), 122c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_(NULL), 123c407dc5cd9bdc5668497f21b26b09d988ab439deBen Murdoch waiter_(NULL), 124c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott callback_task_(NULL), 1253f50c38dc070f4bb515c1b64450dae14f316474eKristian Monsen event_(NULL), 126c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delegate_(NULL) { 127c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 128c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 129c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick ScottWaitableEventWatcher::~WaitableEventWatcher() { 130c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott StopWatching(); 131c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 132c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 133c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 134c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// The Handle is how the user cancels a wait. After deleting the Handle we 135c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// insure that the delegate cannot be called. 136c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 137c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottbool WaitableEventWatcher::StartWatching 138c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott (WaitableEvent* event, WaitableEventWatcher::Delegate* delegate) { 139c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott MessageLoop *const current_ml = MessageLoop::current(); 140c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott DCHECK(current_ml) << "Cannot create WaitableEventWatcher without a " 141c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott "current MessageLoop"; 142c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 143c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // A user may call StartWatching from within the callback function. In this 144c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // case, we won't know that we have finished watching, expect that the Flag 145c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // will have been set in AsyncCallbackTask::Run() 146c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (cancel_flag_.get() && cancel_flag_->value()) { 147c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (message_loop_) { 148c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_->RemoveDestructionObserver(this); 149c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_ = NULL; 150c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 151c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 152c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = NULL; 153c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 154c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 155c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott DCHECK(!cancel_flag_.get()) << "StartWatching called while still watching"; 156c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 157c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = new Flag; 158c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott callback_task_ = new AsyncCallbackTask(cancel_flag_, delegate, event); 159c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott WaitableEvent::WaitableEventKernel* kernel = event->kernel_.get(); 160c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 161c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AutoLock locked(kernel->lock_); 162c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 163c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delegate_ = delegate; 164c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott event_ = event; 165c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 166c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (kernel->signaled_) { 167c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (!kernel->manual_reset_) 168c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott kernel->signaled_ = false; 169c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 170c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // No hairpinning - we can't call the delegate directly here. We have to 171c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // enqueue a task on the MessageLoop as normal. 172c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott current_ml->PostTask(FROM_HERE, callback_task_); 173c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return true; 174c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 175c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 176c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_ = current_ml; 177c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott current_ml->AddDestructionObserver(this); 178c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 179c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott kernel_ = kernel; 180c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott waiter_ = new AsyncWaiter(current_ml, callback_task_, cancel_flag_); 181c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott event->Enqueue(waiter_); 182c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 183c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return true; 184c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 185c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 186c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottvoid WaitableEventWatcher::StopWatching() { 187c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delegate_ = NULL; 188c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 189c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (message_loop_) { 190c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_->RemoveDestructionObserver(this); 191c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott message_loop_ = NULL; 192c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 193c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 194c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (!cancel_flag_.get()) // if not currently watching... 195c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return; 196c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 197c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (cancel_flag_->value()) { 198c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // In this case, the event has fired, but we haven't figured that out yet. 199c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // The WaitableEvent may have been deleted too. 200c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = NULL; 201c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return; 202c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 203c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 204c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (!kernel_.get()) { 205c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We have no kernel. This means that we never enqueued a Waiter on an 206c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // event because the event was already signaled when StartWatching was 207c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // called. 208c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // 209c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // In this case, a task was enqueued on the MessageLoop and will run. 210c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We set the flag in case the task hasn't yet run. The flag will stop the 211c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // delegate getting called. If the task has run then we have the last 212c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // reference to the flag and it will be deleted immedately after. 213c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_->Set(); 214c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = NULL; 215c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return; 216c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 217c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 218c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott AutoLock locked(kernel_->lock_); 219c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We have a lock on the kernel. No one else can signal the event while we 220c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // have it. 221c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 222c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // We have a possible ABA issue here. If Dequeue was to compare only the 223c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // pointer values then it's possible that the AsyncWaiter could have been 224c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // fired, freed and the memory reused for a different Waiter which was 225c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // enqueued in the same wait-list. We would think that that waiter was our 226c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // AsyncWaiter and remove it. 227c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // 228c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // To stop this, Dequeue also takes a tag argument which is passed to the 229c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // virtual Compare function before the two are considered a match. So we need 230c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // a tag which is good for the lifetime of this handle: the Flag. Since we 231c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // have a reference to the Flag, its memory cannot be reused while this object 232c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // still exists. So if we find a waiter with the correct pointer value, and 233c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // which shares a Flag pointer, we have a real match. 234c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (kernel_->Dequeue(waiter_, cancel_flag_.get())) { 235c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // Case 2: the waiter hasn't been signaled yet; it was still on the wait 236c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // list. We've removed it, thus we can delete it and the task (which cannot 237c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // have been enqueued with the MessageLoop because the waiter was never 238c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // signaled) 239c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delete waiter_; 240c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott delete callback_task_; 241c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = NULL; 242c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return; 243c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott } 244c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 245c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // Case 3: the waiter isn't on the wait-list, thus it was signaled. It may 246c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // not have run yet, so we set the flag to tell it not to bother enqueuing the 247c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // task on the MessageLoop, but to delete it instead. The Waiter deletes 248c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // itself once run. 249c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_->Set(); 250c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott cancel_flag_ = NULL; 251c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 252c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // If the waiter has already run then the task has been enqueued. If the Task 253c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // hasn't yet run, the flag will stop the delegate from getting called. (This 254c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // is thread safe because one may only delete a Handle from the MessageLoop 255c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // thread.) 256c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // 257c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // If the delegate has already been called then we have nothing to do. The 258c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott // task has been deleted by the MessageLoop. 259c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 260c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 261c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick ScottWaitableEvent* WaitableEventWatcher::GetWatchedEvent() { 262c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (!cancel_flag_.get()) 263c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return NULL; 264c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 265c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott if (cancel_flag_->value()) 266c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return NULL; 267c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 268c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott return event_; 269c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 270c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 271c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 272c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// This is called when the MessageLoop which the callback will be run it is 273c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// deleted. We need to cancel the callback as if we had been deleted, but we 274c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// will still be deleted at some point in the future. 275c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott// ----------------------------------------------------------------------------- 276c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scottvoid WaitableEventWatcher::WillDestroyCurrentMessageLoop() { 277c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott StopWatching(); 278c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} 279c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott 280c7f5f8508d98d5952d42ed7648c2a8f30a4da156Patrick Scott} // namespace base 281