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114 lines
4.0 KiB
C++
114 lines
4.0 KiB
C++
// Copyright 2017 The Chromium Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style license that can be
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// found in the LICENSE file.
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#include "base/synchronization/waitable_event_watcher.h"
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#include "base/bind.h"
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#include "base/callback.h"
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#include "base/threading/sequenced_task_runner_handle.h"
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namespace base {
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WaitableEventWatcher::WaitableEventWatcher() : weak_ptr_factory_(this) {}
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WaitableEventWatcher::~WaitableEventWatcher() {
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StopWatching();
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}
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bool WaitableEventWatcher::StartWatching(
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WaitableEvent* event,
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EventCallback callback,
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scoped_refptr<SequencedTaskRunner> task_runner) {
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DCHECK(task_runner->RunsTasksInCurrentSequence());
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DCHECK(!source_ || dispatch_source_testcancel(source_));
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// Keep a reference to the receive right, so that if the event is deleted
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// out from under the watcher, a signal can still be observed.
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receive_right_ = event->receive_right_;
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callback_ = BindOnce(std::move(callback), event);
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// Locals for capture by the block. Accessing anything through the |this| or
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// |event| pointers is not safe, since either may have been deleted by the
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// time the handler block is invoked.
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WeakPtr<WaitableEventWatcher> weak_this = weak_ptr_factory_.GetWeakPtr();
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const bool auto_reset =
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event->policy_ == WaitableEvent::ResetPolicy::AUTOMATIC;
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// Auto-reset events always use a dispatch source. Manual-reset events
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// only do so if dispatch provides reliable delivery, otherwise a manual
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// watcher list is used.
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if (!WaitableEvent::UseSlowWatchList(event->policy_)) {
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// Use the global concurrent queue here, since it is only used to thunk
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// to the real callback on the target task runner.
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source_.reset(dispatch_source_create(
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DISPATCH_SOURCE_TYPE_MACH_RECV, receive_right_->Name(), 0,
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dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)));
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// Additional locals for block capture.
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dispatch_source_t source = source_.get();
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mach_port_t name = receive_right_->Name();
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dispatch_source_set_event_handler(source_, ^{
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// For automatic-reset events, only fire the callback if this watcher
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// can claim/dequeue the event. For manual-reset events, all watchers can
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// be called back.
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if (auto_reset && !WaitableEvent::PeekPort(name, true)) {
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return;
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}
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// The event has been consumed. A watcher is one-shot, so cancel the
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// source to prevent receiving future event signals.
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dispatch_source_cancel(source);
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task_runner->PostTask(
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FROM_HERE,
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BindOnce(&WaitableEventWatcher::InvokeCallback, weak_this));
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});
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dispatch_resume(source_);
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} else {
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// The |event->watch_list_| closures can be run from any thread, so bind
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// the callback as an invocation of PostTask.
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OnceClosure watcher =
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BindOnce(IgnoreResult(&TaskRunner::PostTask), task_runner, FROM_HERE,
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BindOnce(&WaitableEventWatcher::InvokeCallback, weak_this));
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// Hold an additional reference to the ReceiveRight, in case |watcher|
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// runs and deletes the event while the lock is held.
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// Hold the lock for the duration of IsSignaled() so that if Signal()
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// is called by another thread, it waits for this to be added to the
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// watch list.
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scoped_refptr<WaitableEvent::ReceiveRight> receive_right(receive_right_);
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AutoLock lock(receive_right->SlowWatchList()->lock);
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if (event->IsSignaled()) {
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std::move(watcher).Run();
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return true;
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}
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receive_right_->SlowWatchList()->list.push_back(std::move(watcher));
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}
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return true;
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}
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void WaitableEventWatcher::StopWatching() {
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callback_.Reset();
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receive_right_ = nullptr;
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if (source_) {
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dispatch_source_cancel(source_);
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source_.reset();
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}
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}
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void WaitableEventWatcher::InvokeCallback() {
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// The callback can be null if StopWatching() is called between signaling
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// and the |callback_| getting run on the target task runner.
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if (callback_.is_null())
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return;
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source_.reset();
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receive_right_ = nullptr;
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std::move(callback_).Run();
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}
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} // namespace base
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