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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.
*/
#ifndef API_TASK_QUEUE_PENDING_TASK_SAFETY_FLAG_H_
#define API_TASK_QUEUE_PENDING_TASK_SAFETY_FLAG_H_
#include <utility>
#include "absl/functional/any_invocable.h"
#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 {
// ....
// rtc::scoped_refptr<PendingTaskSafetyFlag> flag = safety_flag_;
// my_task_queue_->PostTask(
// [flag = std::move(flag), this] {
// // Now running on the main thread.
// if (!flag->alive())
// return;
// MyMethod();
// });
// ....
// ~ExampleClass() {
// safety_flag_->SetNotAlive();
// }
// scoped_refptr<PendingTaskSafetyFlag> safety_flag_
// = PendingTaskSafetyFlag::Create();
// }
//
// SafeTask makes this check automatic:
//
// my_task_queue_->PostTask(SafeTask(safety_flag_, [this] { MyMethod(); }));
//
class PendingTaskSafetyFlag final
: public rtc::RefCountedNonVirtual<PendingTaskSafetyFlag> {
public:
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;
protected:
explicit PendingTaskSafetyFlag(bool alive) : alive_(alive) {}
private:
static rtc::scoped_refptr<PendingTaskSafetyFlag> CreateInternal(bool 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.
//
// Example usage:
//
// my_task_queue->PostTask(SafeTask(scoped_task_safety.flag(),
// [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 {
public:
ScopedTaskSafety() = default;
explicit ScopedTaskSafety(rtc::scoped_refptr<PendingTaskSafetyFlag> flag)
: flag_(std::move(flag)) {}
~ScopedTaskSafety() { flag_->SetNotAlive(); }
// Returns a new reference to the safety flag.
rtc::scoped_refptr<PendingTaskSafetyFlag> flag() const { return flag_; }
// Marks the current flag as not-alive and attaches to a new one.
void reset(rtc::scoped_refptr<PendingTaskSafetyFlag> new_flag =
PendingTaskSafetyFlag::Create()) {
flag_->SetNotAlive();
flag_ = std::move(new_flag);
}
private:
rtc::scoped_refptr<PendingTaskSafetyFlag> flag_ =
PendingTaskSafetyFlag::Create();
};
// Like ScopedTaskSafety, but allows construction on a different thread than
// where the flag will be used.
class ScopedTaskSafetyDetached final {
public:
ScopedTaskSafetyDetached() = default;
~ScopedTaskSafetyDetached() { flag_->SetNotAlive(); }
// Returns a new reference to the safety flag.
rtc::scoped_refptr<PendingTaskSafetyFlag> flag() const { return flag_; }
private:
rtc::scoped_refptr<PendingTaskSafetyFlag> flag_ =
PendingTaskSafetyFlag::CreateDetached();
};
inline absl::AnyInvocable<void() &&> SafeTask(
rtc::scoped_refptr<PendingTaskSafetyFlag> flag,
absl::AnyInvocable<void() &&> task) {
return [flag = std::move(flag), task = std::move(task)]() mutable {
if (flag->alive()) {
std::move(task)();
}
};
}
} // namespace webrtc
#endif // API_TASK_QUEUE_PENDING_TASK_SAFETY_FLAG_H_
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