Make tasks cancellable inside the blink scheduler.

third_party/WebKit/Source/platform/scheduler/base/work_queue.h

 // Copyright 2015 The Chromium 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 THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_WORK_QUEUE_H_
 #define THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_WORK_QUEUE_H_
 
 #include <stddef.h>
 
 #include <set>
 
 #include "base/trace_event/trace_event.h"
 #include "base/trace_event/trace_event_argument.h"
 #include "platform/scheduler/base/enqueue_order.h"
 #include "platform/scheduler/base/task_queue_impl.h"
 
 namespace blink {
 namespace scheduler {
 namespace internal {
 class WorkQueueSets;
 
+// This class keeps track of immediate and delayed tasks which are due to run
+// now. It interfaces deeply with WorkQueueSets which keeps track of which queue
+// (with a given priority) contains the oldest task.
 class BLINK_PLATFORM_EXPORT WorkQueue {
  public:
-  WorkQueue(TaskQueueImpl* task_queue, const char* name);
+  WorkQueue(TaskQueueImpl* task_queue,
+            const char* name,
+            TaskQueueImpl::Task::ComparatorFn queue_comparator);
   ~WorkQueue();
 
   // Associates this work queue with the given work queue sets. This must be
   // called before any tasks can be inserted into this work queue.
   void AssignToWorkQueueSets(WorkQueueSets* work_queue_sets);
 
   // Assigns the current set index.
   void AssignSetIndex(size_t work_queue_set_index);
 
   void AsValueInto(base::trace_event::TracedValue* state) const;
 
   // Clears the |work_queue_|.
   void Clear();
 
   // returns true if the |work_queue_| is empty.
   bool Empty() const { return work_queue_.empty(); }
 
   // If the |work_queue_| isn't empty, |enqueue_order| gets set to the enqueue
   // order of the front task and the function returns true.  Otherwise the
   // function returns false.
   bool GetFrontTaskEnqueueOrder(EnqueueOrder* enqueue_order) const;
 
   // Returns the first task in this queue or null if the queue is empty.
   const TaskQueueImpl::Task* GetFrontTask() const;
 
   // Pushes the task onto the |work_queue_| and informs the WorkQueueSets if
   // the head changed.
   void Push(TaskQueueImpl::Task task);
 
-  // Pushes the task onto the |work_queue_|, sets the |enqueue_order| and
-  // informs the WorkQueueSets if the head changed.
-  void PushAndSetEnqueueOrder(TaskQueueImpl::Task task,
-                              EnqueueOrder enqueue_order);
+  // Removes a cancelled task from the |work_queue_|. Note |key| isn't required
+  // to be the origional task posted, rather it must match the key of the task

[haraken, 2016/08/19 10:42:51]

original

[haraken, 2016/08/19 10:42:51]

I don't fully get the meaning of this comment. What's a difference between "the
original task posted" and "the task posted"?

[alex clarke (OOO till 29th), 2016/08/19 11:02:59]

The handle doesn't contain all the information in TaskQueueImpl::Task, and we
use 
TaskQueueImpl::Task::CreateFakeTaskFromHandle to create a fake task which
matches the
key in so far as the comparators are concerned.

[alex clarke (OOO till 29th), 2016/08/19 11:02:59]

Done.

+  // posted.
+  bool CancelTask(const TaskQueueImpl::Task& key);
+
+  // Returns true if |work_queue_| contains a task matching |key|.
+  bool IsTaskPending(const TaskQueueImpl::Task& key) const;
 
   // Swap the |work_queue_| with |incoming_queue| and informs the
   // WorkQueueSets if the head changed. Assumes |task_queue_->any_thread_lock_|
   // is locked.
-  void SwapLocked(std::queue<TaskQueueImpl::Task>& incoming_queue);
+  void SwapLocked(TaskQueueImpl::ComparatorQueue& incoming_queue);
 
   size_t Size() const { return work_queue_.size(); }
 
   // Pulls a task off the |work_queue_| and informs the WorkQueueSets.
   TaskQueueImpl::Task TakeTaskFromWorkQueue();
 
   const char* name() const { return name_; }
 
   TaskQueueImpl* task_queue() const { return task_queue_; }
 
   WorkQueueSets* work_queue_sets() const { return work_queue_sets_; }
 
   size_t work_queue_set_index() const { return work_queue_set_index_; }
 
   // Test support function. This should not be used in production code.
   void PopTaskForTest();
 
   // Returns true if the front task in this queue has an older enqueue order
   // than the front task of |other_queue|. Both queue are assumed to be
   // non-empty.
   bool ShouldRunBefore(const WorkQueue* other_queue) const;
 
  private:
-  std::queue<TaskQueueImpl::Task> work_queue_;
+  TaskQueueImpl::ComparatorQueue work_queue_;
   WorkQueueSets* work_queue_sets_;  // NOT OWNED.
   TaskQueueImpl* task_queue_;       // NOT OWNED.
   size_t work_queue_set_index_;
   const char* name_;
 
   DISALLOW_COPY_AND_ASSIGN(WorkQueue);
 };
 
 }  // namespace internal
 }  // namespace scheduler
 }  // namespace blink
 
 #endif  // THIRD_PARTY_WEBKIT_SOURCE_PLATFORM_SCHEDULER_BASE_WORK_QUEUE_H_