Add function to CancelableTaskTracker and convert BootTimeLoader (take 2)

chrome/common/cancelable_task_tracker.h

 // Copyright (c) 2012 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.
 
 // CancelableTaskTracker posts tasks (in the form of a Closure) to a TaskRunner,
 // and is able to cancel the task later if it's not needed anymore. On
 // destruction, CancelableTaskTracker will cancel all tracked tasks.
 //
 // Each cancelable task can be associated with a reply (also a Closure). After
 // the task is run on the TaskRunner, |reply| will be posted back to originating
@@ skipping 10 matching lines @@
 // use one of the other mechanisms.
 //
 // THREAD-SAFETY:
 //
 // 1. CancelableTaskTracker objects are not thread safe. They must be created,
 // used, and destroyed on the originating thread that posts the task. It's safe
 // to destroy a CancelableTaskTracker while there are outstanding tasks. This is
 // commonly used to cancel all outstanding tasks.
 //
 // 2. Both task and reply are deleted on the originating thread.
+//
+// 3. IsCanceledCallback is thread safe and can be run or deleted on any thread.
 
 #ifndef CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
 #define CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_
 
 #include "base/basictypes.h"
 #include "base/callback.h"
 #include "base/hash_tables.h"
 #include "base/memory/weak_ptr.h"
 #include "base/threading/thread_checker.h"
 
 namespace base {
 class CancellationFlag;
 class TaskRunner;
 }  // namespace base
 
 namespace tracked_objects {
 class Location;
 }  // namespace tracked_objects
 
 class CancelableTaskTracker {
  public:
   // All values except kBadTaskId are valid.
   typedef int64 TaskId;
   static const TaskId kBadTaskId;
 
+  typedef base::Callback<bool()> IsCanceledCallback;
+
   CancelableTaskTracker();
 
   // Cancels all tracked tasks.
   ~CancelableTaskTracker();
 
   TaskId PostTask(base::TaskRunner* task_runner,
                   const tracked_objects::Location& from_here,
                   const base::Closure& task);
 
   TaskId PostTaskAndReply(base::TaskRunner* task_runner,
                           const tracked_objects::Location& from_here,
                           const base::Closure& task,
                           const base::Closure& reply);
 
+  // Creates a tracked TaskId and an associated IsCanceledCallback. Client can
+  // later call TryCancel() with the returned TaskId, and run |is_canceled_cb|
+  // to check whether the TaskId is canceled.
+  //
+  // Note. This function is used to address some special cancelation requirement
+  // in existing code. You SHOULD NOT need this function in new code.
+  TaskId NewTrackedTaskId(IsCanceledCallback* is_canceled_cb);
+
   // After calling this function, |task| and |reply| will not run. If the
   // cancelation happens when |task| is running or has finished running, |reply|
   // will not run. If |reply| is running or has finished running, cancellation
   // is a noop.
   //
   // Note. It's OK to cancel a |task| for more than once. The later calls are
   // noops.
   void TryCancel(TaskId id);
 
   // It's OK to call this function for more than once. The later calls are
   // noops.
   void TryCancelAll();
 
  private:
   void Track(TaskId id, base::CancellationFlag* flag);
   void Untrack(TaskId id);
 
   base::hash_map<TaskId, base::CancellationFlag*> task_flags_;
   base::WeakPtrFactory<CancelableTaskTracker> weak_factory_;
 
   TaskId next_id_;
   base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(CancelableTaskTracker);
 };
 
 #endif  // CHROME_COMMON_CANCELABLE_TASK_TRACKER_H_