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"
+#include "src/base/platform/condition-variable.h"
+#include "src/hashmap.h"
 
 namespace v8 {
 namespace internal {
 
+class Cancelable;
 class Isolate;
 
 
+// Keeps track of cancelable fore- and background tasks.
+class CancelableTaskManager {
+ public:
+  CancelableTaskManager();
+
+  // Registers a new {Cancelable} task.

[Hannes Payer (out of office), 2015/11/12 22:01:50]

Specify what the return value is.

[Michael Lippautz, 2015/11/12 22:15:21]

Done.

+  uint32_t Register(Cancelable* task);
+
+  // Remove a given task by id if it is still managed. Only returns true if the
+  // task has not been started yet and could be canceled.

[Hannes Payer (out of office), 2015/11/12 22:01:50]

It is not clear to me what the semantics of this API are, could you be more
precise here?

[Michael Lippautz, 2015/11/12 22:15:21]

Done. Hope the more elaborate comment makes it clear.

+  bool Remove(uint32_t id);
+
+  // Cancel all registered tasks and wait for tasks that are already running.
+  void CancelAndWait();
+
+ private:
+  // To mitigate the ABA problem, the api refers to tasks through an id.
+  uint32_t task_id_counter_;
+
+  // A set of cancelable tasks that are currently registered.
+  HashMap 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_;
+
+  DISALLOW_COPY_AND_ASSIGN(CancelableTaskManager);
+};
+
+
 class Cancelable {
  public:
-  explicit Cancelable(Isolate* isolate);
+  explicit Cancelable(CancelableTaskManager* parent);
   virtual ~Cancelable();
 
-  virtual void Cancel() { is_cancelled_ = true; }
+  // Never invoke after handing over the task to the platform!

[Hannes Payer (out of office), 2015/11/12 22:01:50]

Please explain why.

[Michael Lippautz, 2015/11/12 22:15:21]

Done.

+  uint32_t id() { return id_; }
+
+  // Always invoke indirectly through the connected {CancelableTaskManager}.

[Hannes Payer (out of office), 2015/11/12 22:01:50]

Should this really be public then? Or should it be private and friend with the
manager.

[Michael Lippautz, 2015/11/12 22:15:21]

Done.

+  bool Cancel() {
+    if (status_.TrySetValue(kWaiting, kCanceled)) {
+      return true;
+    }
+    cancel_counter_.Increment(1);
+    return false;
+  }
 
  protected:
-  Isolate* isolate_;
-  bool is_cancelled_;
+  bool TryRun() { return status_.TrySetValue(kWaiting, kRunning); }
+  bool IsRunning() { return status_.Value() == kRunning; }
+  intptr_t CancelAttempts() { return cancel_counter_.Value(); }
 
  private:
+  enum Status {

[Hannes Payer (out of office), 2015/11/12 22:01:50]

Please explain the states.

+    kWaiting,
+    kCanceled,
+    kRunning,
+  };
+
+  CancelableTaskManager* parent_;
+  AtomicValue<Status> status_;
+  uint32_t id_;
+  AtomicNumber<intptr_t> cancel_counter_;
+
   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) {}
+  explicit CancelableTask(Isolate* isolate);
 
   // Task overrides.
   void Run() final {
-    if (!is_cancelled_) {
+    if (TryRun()) {
       RunInternal();
     }
   }
 
   virtual void RunInternal() = 0;
 
+  Isolate* isolate() { return isolate_; }
+
  private:
+  Isolate* isolate_;
   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) {}
+  explicit CancelableIdleTask(Isolate* isolate);
 
   // IdleTask overrides.
   void Run(double deadline_in_seconds) final {
-    if (!is_cancelled_) {
+    if (TryRun()) {
       RunInternal(deadline_in_seconds);
     }
   }
 
   virtual void RunInternal(double deadline_in_seconds) = 0;
 
+  Isolate* isolate() { return isolate_; }
+
  private:
+  Isolate* isolate_;
   DISALLOW_COPY_AND_ASSIGN(CancelableIdleTask);
 };
 
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_CANCELABLE_TASK_H_