Add {CancelableTaskManager} to handle {Cancelable} concurrent tasks.

src/cancelable-task.h

 // 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_CANCELABLE_TASK_H_
 #define V8_CANCELABLE_TASK_H_
 
 #include "include/v8-platform.h"
+#include "src/atomic-utils.h"
 #include "src/base/macros.h"
 
 namespace v8 {
 namespace internal {
 
 class Isolate;
 
-
+// {Cancelable} can be used to create tasks that can run on foreground and
+// background workers and can be canceled from any of those.
 class Cancelable {
  public:
   explicit Cancelable(Isolate* isolate);
   virtual ~Cancelable();
 
-  virtual void Cancel() { is_cancelled_ = true; }
+  bool Cancel() { return status_.TrySetValue(kWaiting, kCanceled); }
+  bool IsCanceled() { return status_.Value() == kCanceled; }
+  bool IsRunning() { return status_.Value() == kRunning; }
 
  protected:
+  // Before actually running, the task needs to call {CanRun} to indicate that
+  // it cannot be canceled anymore.
+  bool CanRun() { return status_.TrySetValue(kWaiting, kRunning); }

[Hannes Payer (out of office), 2015/11/04 23:19:00]

I think TryRun matches better the semantics.

+
   Isolate* isolate_;
-  bool is_cancelled_;
 
  private:
+  enum Status {
+    kWaiting,
+    kCanceled,
+    kRunning,
+    kCanceledForShutdown,
+  };
+
+  bool CancelForShutdown() {
+    return status_.TrySetValue(kWaiting, kCanceledForShutdown);
+  }
+
+  AtomicValue<Status> status_;
+
+  friend class Isolate;

[Hannes Payer (out of office), 2015/11/04 23:19:00]

Why friends?

+
   DISALLOW_COPY_AND_ASSIGN(Cancelable);
 };
 
 
 // Multiple inheritance can be used because Task is a pure interface.
 class CancelableTask : public Cancelable, public Task {
  public:
   explicit CancelableTask(Isolate* isolate) : Cancelable(isolate) {}
 
   // Task overrides.
   void Run() final {
-    if (!is_cancelled_) {
+    if (CanRun()) {
       RunInternal();
     }
   }
 
   virtual void RunInternal() = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CancelableTask);
 };
 
 
 // Multiple inheritance can be used because IdleTask is a pure interface.
 class CancelableIdleTask : public Cancelable, public IdleTask {
  public:
   explicit CancelableIdleTask(Isolate* isolate) : Cancelable(isolate) {}
 
   // IdleTask overrides.
   void Run(double deadline_in_seconds) final {
-    if (!is_cancelled_) {
+    if (CanRun()) {
       RunInternal(deadline_in_seconds);
     }
   }
 
   virtual void RunInternal(double deadline_in_seconds) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CancelableIdleTask);
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CANCELABLE_TASK_H_