WaitableEvent

base/waitable_event_watcher_posix.cc

+// Copyright (c) 2006-2008 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/waitable_event_watcher.h"
+
+#include "base/condition_variable.h"
+#include "base/lock.h"
+#include "base/message_loop.h"
+#include "base/waitable_event.h"
+
+namespace base {
+
+// -----------------------------------------------------------------------------
+// WaitableEventWatcher (async waits).
+//
+// The basic design is that we add an AsyncWaiter to the wait-list of the event.
+// That AsyncWaiter has a pointer to MessageLoop, and a Task to be posted to it.
+// The MessageLoop ends up running the task, which calls the delegate.
+//
+// Since the wait can be canceled, we have a thread-safe Flag object which is
+// set when the wait has been canceled. At each stage in the above, we check the
+// flag before going onto the next stage. Since the wait may only be canceled in
+// the MessageLoop which runs the Task, we are assured that the delegate cannot
+// be called after canceling...
+
+// -----------------------------------------------------------------------------
+// A thread-safe, reference-counted, write-once flag.
+// -----------------------------------------------------------------------------
+class Flag : public RefCountedThreadSafe<Flag> {
+ public:
+  Flag() { flag_ = false; }
+
+  void Set() {
+    AutoLock locked(lock_);
+    flag_ = true;
+  }
+
+  bool value() const {
+    AutoLock locked(lock_);
+    return flag_;
+  }
+
+ private:
+  mutable Lock lock_;
+  bool flag_;
+};
+
+// -----------------------------------------------------------------------------
+// This is an asynchronous waiter which posts a task to a MessageLoop when
+// fired. An AsyncWaiter may only be in a single wait-list.
+// -----------------------------------------------------------------------------
+class AsyncWaiter : public WaitableEvent::Waiter {
+ public:
+  AsyncWaiter(MessageLoop* message_loop, Task* task, Flag* flag)
+      : message_loop_(message_loop),
+        cb_task_(task),
+        flag_(flag) { }
+
+  bool Fire(WaitableEvent* event) {
+    if (flag_->value()) {
+      // If the callback has been canceled, we don't enqueue the task, we just
+      // delete it instead.
+      delete cb_task_;
+    } else {
+      message_loop_->PostTask(FROM_HERE, cb_task_);
+    }
+
+    // We are removed from the wait-list by the WaitableEvent itself. It only
+    // remains to delete ourselves.
+    delete this;
+
+    // We can always return true because an AsyncWaiter is never in two
+    // different wait-lists at the same time.
+    return true;
+  }
+
+  // See StopWatching for discussion
+  bool Compare(void* tag) {
+    return tag == flag_.get();
+  }
+
+  MessageLoop *const message_loop_;
+  Task *const cb_task_;
+  scoped_refptr<Flag> flag_;
+};
+
+// -----------------------------------------------------------------------------
+// For async waits we need to make a callback in a MessageLoop thread. We do
+// this by posting this task, which calls the delegate and keeps track of when
+// the event is canceled.
+// -----------------------------------------------------------------------------
+class AsyncCallbackTask : public Task {
+ public:
+  AsyncCallbackTask(Flag* flag, WaitableEventWatcher::Delegate* delegate,
+                    WaitableEvent* event)
+      : flag_(flag),
+        delegate_(delegate),
+        event_(event) {
+  }
+
+  void Run() {
+    // Runs in MessageLoop thread.
+    if (!flag_->value())
+      delegate_->OnWaitableEventSignaled(event_);
+
+    // This is to let the WaitableEventWatcher know that the event has occured
+    // because it needs to be able to return NULL from GetWatchedEvent
+    flag_->Set();
+
+    // We are deleted by the MessageLoop
+  }
+
+ private:
+  scoped_refptr<Flag> flag_;
+  WaitableEventWatcher::Delegate *const delegate_;
+  WaitableEvent *const event_;
+};
+
+WaitableEventWatcher::WaitableEventWatcher()
+    : event_(NULL),
+      message_loop_(NULL),
+      cancel_flag_(NULL),
+      callback_task_(NULL) {
+}
+
+WaitableEventWatcher::~WaitableEventWatcher() {
+  StopWatching();
+}
+
+// -----------------------------------------------------------------------------
+// The Handle is how the user cancels a wait. After deleting the Handle we
+// insure that the delegate cannot be called.
+// -----------------------------------------------------------------------------
+bool WaitableEventWatcher::StartWatching
+    (WaitableEvent* event, WaitableEventWatcher::Delegate* delegate) {
+  MessageLoop *const current_ml = MessageLoop::current();
+  DCHECK(current_ml) << "Cannot create WaitableEventWatcher without a "
+                        "current MessageLoop";
+
+  DCHECK(!cancel_flag_.get()) << "StartWatching called while still watching";
+
+  cancel_flag_ = new Flag;
+  callback_task_ = new AsyncCallbackTask(cancel_flag_, delegate, event);
+
+  AutoLock locked(event->lock_);
+
+  if (event->signaled_) {
+    if (!event->manual_reset_)
+      event->signaled_ = false;
+
+    // No hairpinning - we can't call the delegate directly here. We have to
+    // enqueue a task on the MessageLoop as normal.
+    current_ml->PostTask(FROM_HERE, callback_task_);
+    return true;
+  }
+
+  message_loop_ = current_ml;
+  current_ml->AddDestructionObserver(this);
+
+  event_ = event;
+  waiter_ = new AsyncWaiter(current_ml, callback_task_, cancel_flag_);
+  event->Enqueue(waiter_);
+
+  return true;
+}
+
+void WaitableEventWatcher::StopWatching() {
+  if (message_loop_) {
+    message_loop_->RemoveDestructionObserver(this);
+    message_loop_ = NULL;
+  }
+
+  if (!cancel_flag_.get())  // if not currently watching...
+    return;
+
+  if (!event_) {
+    // We have no WaitableEvent. This means that we never enqueued a Waiter on
+    // an event because the event was already signaled when StartWatching was
+    // called.
+    //
+    // In this case, a task was enqueued on the MessageLoop and will run.
+    // We set the flag in case the task hasn't yet run. The flag will stop the
+    // delegate getting called. If the task has run then we have the last
+    // reference to the flag and it will be deleted immedately after.
+    cancel_flag_->Set();
+    cancel_flag_ = NULL;
+    return;
+  }
+
+  AutoLock locked(event_->lock_);
+  // We have a lock on the WaitableEvent. No one else can signal the event while
+  // we have it.
+
+  // We have a possible ABA issue here. If Dequeue was to compare only the
+  // pointer values then it's possible that the AsyncWaiter could have been
+  // fired, freed and the memory reused for a different Waiter which was
+  // enqueued in the same wait-list. We would think that that waiter was our
+  // AsyncWaiter and remove it.
+  //
+  // To stop this, Dequeue also takes a tag argument which is passed to the
+  // virtual Compare function before the two are considered a match. So we need
+  // a tag which is good for the lifetime of this handle: the Flag. Since we
+  // have a reference to the Flag, its memory cannot be reused while this object
+  // still exists. So if we find a waiter with the correct pointer value, and
+  // which shares a Flag pointer, we have a real match.
+  if (event_->Dequeue(waiter_, cancel_flag_.get())) {
+    // Case 2: the waiter hasn't been signaled yet; it was still on the wait
+    // list. We've removed it, thus we can delete it and the task (which cannot
+    // have been enqueued with the MessageLoop because the waiter was never
+    // signaled)
+    delete waiter_;
+    delete callback_task_;
+    cancel_flag_ = NULL;
+    return;
+  }
+
+  // Case 3: the waiter isn't on the wait-list, thus it was signaled. It may
+  // not have run yet, so we set the flag to tell it not to bother enqueuing the
+  // task on the MessageLoop, but to delete it instead. The Waiter deletes
+  // itself once run.
+  cancel_flag_->Set();
+  cancel_flag_ = NULL;
+
+  // If the waiter has already run then the task has been enqueued. If the Task
+  // hasn't yet run, the flag will stop the delegate from getting called. (This
+  // is thread safe because one may only delete a Handle from the MessageLoop
+  // thread.)
+  //
+  // If the delegate has already been called then we have nothing to do. The
+  // task has been deleted by the MessageLoop.
+}
+
+WaitableEvent* WaitableEventWatcher::GetWatchedEvent() {
+  if (!cancel_flag_.get())
+    return NULL;
+
+  if (cancel_flag_->value())
+    return NULL;
+
+  return event_;
+}
+
+// -----------------------------------------------------------------------------
+// This is called when the MessageLoop which the callback will be run it is
+// deleted. We need to cancel the callback as if we had been deleted, but we
+// will still be deleted at some point in the future.
+// -----------------------------------------------------------------------------
+void WaitableEventWatcher::WillDestroyCurrentMessageLoop() {
+  StopWatching();
+}
+
+}  // namespace base