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* Copyright 2020 The WebRTC Project Authors. All rights reserved.
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
#include "api/ref_counted_base.h"
#include "api/scoped_refptr.h"
#include "api/sequence_checker.h"
#include "rtc_base/checks.h"
#include "rtc_base/system/no_unique_address.h"
namespace webrtc {
// The PendingTaskSafetyFlag and the ScopedTaskSafety are designed to address
// the issue where you have a task to be executed later that has references,
// but cannot guarantee that the referenced object is alive when the task is
// executed.
// This mechanism can be used with tasks that are created and destroyed
// on a single thread / task queue, and with tasks posted to the same
// thread/task queue, but tasks can be posted from any thread/TQ.
// Typical usage:
// When posting a task, post a copy (capture by-value in a lambda) of the flag
// reference and before performing the work, check the |alive()| state. Abort if
// alive() returns |false|:
// class ExampleClass {
// ....
// my_task_queue_->PostTask(ToQueuedTask(
// [safety = pending_task_safety_flag_, this]() {
// // Now running on the main thread.
// if (!safety->alive())
// return;
// MyMethod();
// }));
// ....
// ~ExampleClass() {
// pending_task_safety_flag_->SetNotAlive();
// }
// scoped_refptr<PendingTaskSafetyFlag> pending_task_safety_flag_
// = PendingTaskSafetyFlag::Create();
// }
// ToQueuedTask has an overload that makes this check automatic:
// my_task_queue_->PostTask(ToQueuedTask(pending_task_safety_flag_,
// [this]() { MyMethod(); }));
class PendingTaskSafetyFlag final
: public rtc::RefCountedNonVirtual<PendingTaskSafetyFlag> {
static rtc::scoped_refptr<PendingTaskSafetyFlag> Create();
// Creates a flag, but with its SequenceChecker initially detached. Hence, it
// may be created on a different thread than the flag will be used on.
static rtc::scoped_refptr<PendingTaskSafetyFlag> CreateDetached();
// Same as `CreateDetached()` except the initial state of the returned flag
// will be `!alive()`.
static rtc::scoped_refptr<PendingTaskSafetyFlag> CreateDetachedInactive();
~PendingTaskSafetyFlag() = default;
void SetNotAlive();
// The SetAlive method is intended to support Start/Stop/Restart usecases.
// When a class has called SetNotAlive on a flag used for posted tasks, and
// decides it wants to post new tasks and have them run, there are two
// reasonable ways to do that:
// (i) Use the below SetAlive method. One subtlety is that any task posted
// prior to SetNotAlive, and still in the queue, is resurrected and will
// run.
// (ii) Create a fresh flag, and just drop the reference to the old one. This
// avoids the above problem, and ensures that tasks poster prior to
// SetNotAlive stay cancelled. Instead, there's a potential data race on
// the flag pointer itself. Some synchronization is required between the
// thread overwriting the flag pointer, and the threads that want to post
// tasks and therefore read that same pointer.
void SetAlive();
bool alive() const;
explicit PendingTaskSafetyFlag(bool alive) : alive_(alive) {}
bool alive_ = true;
RTC_NO_UNIQUE_ADDRESS SequenceChecker main_sequence_;
// The ScopedTaskSafety makes using PendingTaskSafetyFlag very simple.
// It does automatic PTSF creation and signalling of destruction when the
// ScopedTaskSafety instance goes out of scope.
// ToQueuedTask has an overload that takes a ScopedTaskSafety too, so there
// is no need to explicitly call the "flag" method.
// Example usage:
// my_task_queue->PostTask(ToQueuedTask(scoped_task_safety,
// [this]() {
// // task goes here
// }
// This should be used by the class that wants tasks dropped after destruction.
// The requirement is that the instance has to be constructed and destructed on
// the same thread as the potentially dropped tasks would be running on.
class ScopedTaskSafety final {
ScopedTaskSafety() = default;
~ScopedTaskSafety() { flag_->SetNotAlive(); }
// Returns a new reference to the safety flag.
rtc::scoped_refptr<PendingTaskSafetyFlag> flag() const { return flag_; }
rtc::scoped_refptr<PendingTaskSafetyFlag> flag_ =
// Like ScopedTaskSafety, but allows construction on a different thread than
// where the flag will be used.
class ScopedTaskSafetyDetached final {
ScopedTaskSafetyDetached() = default;
~ScopedTaskSafetyDetached() { flag_->SetNotAlive(); }
// Returns a new reference to the safety flag.
rtc::scoped_refptr<PendingTaskSafetyFlag> flag() const { return flag_; }
rtc::scoped_refptr<PendingTaskSafetyFlag> flag_ =
} // namespace webrtc