michael@0: // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
michael@0: // Use of this source code is governed by a BSD-style license that can be
michael@0: // found in the LICENSE file.
michael@0: 
michael@0: #include "base/object_watcher.h"
michael@0: 
michael@0: #include "base/logging.h"
michael@0: 
michael@0: namespace base {
michael@0: 
michael@0: //-----------------------------------------------------------------------------
michael@0: 
michael@0: struct ObjectWatcher::Watch : public Task {
michael@0:   ObjectWatcher* watcher;    // The associated ObjectWatcher instance
michael@0:   HANDLE object;             // The object being watched
michael@0:   HANDLE wait_object;        // Returned by RegisterWaitForSingleObject
michael@0:   MessageLoop* origin_loop;  // Used to get back to the origin thread
michael@0:   Delegate* delegate;        // Delegate to notify when signaled
michael@0:   bool did_signal;           // DoneWaiting was called
michael@0: 
michael@0:   virtual void Run() {
michael@0:     // The watcher may have already been torn down, in which case we need to
michael@0:     // just get out of dodge.
michael@0:     if (!watcher)
michael@0:       return;
michael@0: 
michael@0:     DCHECK(did_signal);
michael@0:     watcher->StopWatching();
michael@0: 
michael@0:     delegate->OnObjectSignaled(object);
michael@0:   }
michael@0: };
michael@0: 
michael@0: //-----------------------------------------------------------------------------
michael@0: 
michael@0: ObjectWatcher::ObjectWatcher() : watch_(NULL) {
michael@0: }
michael@0: 
michael@0: ObjectWatcher::~ObjectWatcher() {
michael@0:   StopWatching();
michael@0: }
michael@0: 
michael@0: bool ObjectWatcher::StartWatching(HANDLE object, Delegate* delegate) {
michael@0:   if (watch_) {
michael@0:     NOTREACHED() << "Already watching an object";
michael@0:     return false;
michael@0:   }
michael@0: 
michael@0:   Watch* watch = new Watch;
michael@0:   watch->watcher = this;
michael@0:   watch->object = object;
michael@0:   watch->origin_loop = MessageLoop::current();
michael@0:   watch->delegate = delegate;
michael@0:   watch->did_signal = false;
michael@0: 
michael@0:   // Since our job is to just notice when an object is signaled and report the
michael@0:   // result back to this thread, we can just run on a Windows wait thread.
michael@0:   DWORD wait_flags = WT_EXECUTEDEFAULT | WT_EXECUTEONLYONCE;
michael@0: 
michael@0:   if (!RegisterWaitForSingleObject(&watch->wait_object, object, DoneWaiting,
michael@0:                                    watch, INFINITE, wait_flags)) {
michael@0:     NOTREACHED() << "RegisterWaitForSingleObject failed: " << GetLastError();
michael@0:     delete watch;
michael@0:     return false;
michael@0:   }
michael@0: 
michael@0:   watch_ = watch;
michael@0: 
michael@0:   // We need to know if the current message loop is going away so we can
michael@0:   // prevent the wait thread from trying to access a dead message loop.
michael@0:   MessageLoop::current()->AddDestructionObserver(this);
michael@0:   return true;
michael@0: }
michael@0: 
michael@0: bool ObjectWatcher::StopWatching() {
michael@0:   if (!watch_)
michael@0:     return false;
michael@0: 
michael@0:   // Make sure ObjectWatcher is used in a single-threaded fashion.
michael@0:   DCHECK(watch_->origin_loop == MessageLoop::current());
michael@0: 
michael@0:   // If DoneWaiting is in progress, we wait for it to finish.  We know whether
michael@0:   // DoneWaiting happened or not by inspecting the did_signal flag.
michael@0:   if (!UnregisterWaitEx(watch_->wait_object, INVALID_HANDLE_VALUE)) {
michael@0:     NOTREACHED() << "UnregisterWaitEx failed: " << GetLastError();
michael@0:     return false;
michael@0:   }
michael@0: 
michael@0:   // Make sure that we see any mutation to did_signal.  This should be a no-op
michael@0:   // since we expect that UnregisterWaitEx resulted in a memory barrier, but
michael@0:   // just to be sure, we're going to be explicit.
michael@0:   MemoryBarrier();
michael@0: 
michael@0:   // If the watch has been posted, then we need to make sure it knows not to do
michael@0:   // anything once it is run.
michael@0:   watch_->watcher = NULL;
michael@0: 
michael@0:   // If DoneWaiting was called, then the watch would have been posted as a
michael@0:   // task, and will therefore be deleted by the MessageLoop.  Otherwise, we
michael@0:   // need to take care to delete it here.
michael@0:   if (!watch_->did_signal)
michael@0:     delete watch_;
michael@0: 
michael@0:   watch_ = NULL;
michael@0: 
michael@0:   MessageLoop::current()->RemoveDestructionObserver(this);
michael@0:   return true;
michael@0: }
michael@0: 
michael@0: HANDLE ObjectWatcher::GetWatchedObject() {
michael@0:   if (!watch_)
michael@0:     return NULL;
michael@0: 
michael@0:   return watch_->object;
michael@0: }
michael@0: 
michael@0: // static
michael@0: void CALLBACK ObjectWatcher::DoneWaiting(void* param, BOOLEAN timed_out) {
michael@0:   DCHECK(!timed_out);
michael@0: 
michael@0:   Watch* watch = static_cast<Watch*>(param);
michael@0: 
michael@0:   // Record that we ran this function.
michael@0:   watch->did_signal = true;
michael@0: 
michael@0:   // We rely on the locking in PostTask() to ensure that a memory barrier is
michael@0:   // provided, which in turn ensures our change to did_signal can be observed
michael@0:   // on the target thread.
michael@0:   watch->origin_loop->PostTask(FROM_HERE, watch);
michael@0: }
michael@0: 
michael@0: void ObjectWatcher::WillDestroyCurrentMessageLoop() {
michael@0:   // Need to shutdown the watch so that we don't try to access the MessageLoop
michael@0:   // after this point.
michael@0:   StopWatching();
michael@0: }
michael@0: 
michael@0: }  // namespace base