cancelable-task.h

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// Copyright 2015 the V8 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.
 
#ifndef V8_TASKS_CANCELABLE_TASK_H_
#define V8_TASKS_CANCELABLE_TASK_H_
 
#include <atomic>
#include <unordered_map>
 
#include "include/v8-platform.h"
#include "src/base/macros.h"
#include "src/base/platform/condition-variable.h"
 
namespace v8 {
namespace internal {
 
class Cancelable;
class Isolate;
 
// The possible outcomes of trying to abort a job are:
// (1) The task is already finished running or was canceled before and
//     thus has been removed from the manager.
// (2) The task is currently running and cannot be canceled anymore.
// (3) The task is not yet running (or finished) so it is canceled and
//     removed.
enum class TryAbortResult { kTaskRemoved, kTaskRunning, kTaskAborted };
 
// Keeps track of cancelable tasks. It is possible to register and remove tasks
// from any fore- and background task/thread.
class V8_EXPORT_PRIVATE CancelableTaskManager {
 public:
  using Id = uint64_t;
  static constexpr Id kInvalidTaskId = 0;
 
  CancelableTaskManager();
 
  ~CancelableTaskManager();
  CancelableTaskManager(const CancelableTaskManager&) = delete;
  CancelableTaskManager& operator=(const CancelableTaskManager&) = delete;
 
  // Registers a new cancelable {task}. Returns the unique {id} of the task that
  // can be used to try to abort a task by calling {Abort}.
  // If {Register} is called after {CancelAndWait}, then the task will be
  // aborted immediately.
  // {Register} should only be called by the thread which owns the
  // {CancelableTaskManager}, or by a task which is managed by the
  // {CancelableTaskManager}.
  Id Register(Cancelable* task);
 
  // Try to abort running a task identified by {id}.
  TryAbortResult TryAbort(Id id);
 
  // Tries to cancel all remaining registered tasks. The return value indicates
  // whether
  //
  // 1) No tasks were registered (kTaskRemoved), or
  //
  // 2) There is at least one remaining task that couldn't be cancelled
  // (kTaskRunning), or
  //
  // 3) All registered tasks were cancelled (kTaskAborted).
  TryAbortResult TryAbortAll();
 
  // Cancels all remaining registered tasks and waits for tasks that are
  // already running. This disallows subsequent Register calls.
  void CancelAndWait();
 
  // Returns true of the task manager has been cancelled.
  bool canceled() const { return canceled_; }
 
 private:
  // Only called by {Cancelable} destructor. The task is done with executing,
  // but needs to be removed.
  void RemoveFinishedTask(Id id);
 
  // To mitigate the ABA problem, the api refers to tasks through an id.
  Id task_id_counter_;
 
  // A set of cancelable tasks that are currently registered.
  std::unordered_map<Id, Cancelable*> cancelable_tasks_;
 
  // Mutex and condition variable enabling concurrent register and removing, as
  // well as waiting for background tasks on {CancelAndWait}.
  base::ConditionVariable cancelable_tasks_barrier_;
  base::Mutex mutex_;
 
  bool canceled_;
 
  friend class Cancelable;
};
 
class V8_EXPORT_PRIVATE Cancelable {
 public:
  explicit Cancelable(CancelableTaskManager* parent)
      : parent_(parent), id_(parent->Register(this)) {}
 
  virtual ~Cancelable();
  Cancelable(const Cancelable&) = delete;
  Cancelable& operator=(const Cancelable&) = delete;
 
  // Never invoke after handing over the task to the platform! The reason is
  // that {Cancelable} is used in combination with {v8::Task} and handed to
  // a platform. This step transfers ownership to the platform, which destroys
  // the task after running it. Since the exact time is not known, we cannot
  // access the object after handing it to a platform.
  CancelableTaskManager::Id id() { return id_; }
 
 protected:
  // Identifies the state a cancelable task is in:
  // |kWaiting|: The task is scheduled and waiting to be executed. {TryRun} will
  //   succeed.
  // |kCanceled|: The task has been canceled. {TryRun} will fail.
  // |kRunning|: The task is currently running and cannot be canceled anymore.
  enum Status { kWaiting, kCanceled, kRunning };
 
  bool TryRun(Status* previous = nullptr) {
    return CompareExchangeStatus(kWaiting, kRunning, previous);
  }
 
 private:
  friend class CancelableTaskManager;
 
  // Use {CancelableTaskManager} to abort a task that has not yet been
  // executed.
  bool Cancel() { return CompareExchangeStatus(kWaiting, kCanceled); }
 
  bool CompareExchangeStatus(Status expected, Status desired,
                             Status* previous = nullptr) {
    // {compare_exchange_strong} updates {expected}.
    bool success = status_.compare_exchange_strong(expected, desired,
                                                   std::memory_order_acq_rel,
                                                   std::memory_order_acquire);
    if (previous) *previous = expected;
    return success;
  }
 
  CancelableTaskManager* const parent_;
  std::atomic<Status> status_{kWaiting};
  const CancelableTaskManager::Id id_;
};
 
// Multiple inheritance can be used because Task is a pure interface.
class V8_EXPORT_PRIVATE CancelableTask : public Cancelable,
                                         NON_EXPORTED_BASE(public Task) {
 public:
  explicit CancelableTask(Isolate* isolate);
  explicit CancelableTask(CancelableTaskManager* manager);
  CancelableTask(const CancelableTask&) = delete;
  CancelableTask& operator=(const CancelableTask&) = delete;
 
  // Task overrides.
  void Run() final {
    if (TryRun()) {
      RunInternal();
    }
  }
 
  virtual void RunInternal() = 0;
};
 
// Multiple inheritance can be used because IdleTask is a pure interface.
class CancelableIdleTask : public Cancelable, public IdleTask {
 public:
  explicit CancelableIdleTask(Isolate* isolate);
  explicit CancelableIdleTask(CancelableTaskManager* manager);
  CancelableIdleTask(const CancelableIdleTask&) = delete;
  CancelableIdleTask& operator=(const CancelableIdleTask&) = delete;
 
  // IdleTask overrides.
  void Run(double deadline_in_seconds) final {
    if (TryRun()) {
      RunInternal(deadline_in_seconds);
    }
  }
 
  virtual void RunInternal(double deadline_in_seconds) = 0;
};
 
}  // namespace internal
}  // namespace v8
 
#endif  // V8_TASKS_CANCELABLE_TASK_H_