TaskScheduler: Adjust shutdown behavior of delayed tasks.

base/task_scheduler/task.h

 // Copyright 2016 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 BASE_TASK_SCHEDULER_TASK_H_
 #define BASE_TASK_SCHEDULER_TASK_H_
 
 #include "base/base_export.h"
 #include "base/callback_forward.h"
 #include "base/location.h"
 #include "base/macros.h"
 #include "base/memory/ref_counted.h"
 #include "base/pending_task.h"
 #include "base/sequenced_task_runner.h"
 #include "base/single_thread_task_runner.h"
 #include "base/task_scheduler/task_traits.h"
 #include "base/time/time.h"
 
 namespace base {
 namespace internal {
 
 // A task is a unit of work inside the task scheduler. Support for tracing and
 // profiling inherited from PendingTask.
 struct BASE_EXPORT Task : public PendingTask {
   // |posted_from| is the site the task was posted from. |task| is the closure
   // to run. |traits| is metadata about the task. |delay| is a delay that must
-  // expire before the Task runs.
+  // expire before the Task runs. If |delay| is non-zero and the shutdown
+  // behavior in |traits| is BLOCK_SHUTDOWN, the shutdown behavior is
+  // automatically changed to SKIP_ON_SHUTDOWN.
   Task(const tracked_objects::Location& posted_from,
        const Closure& task,
        const TaskTraits& traits,
-       const TimeDelta& delay);
+       TimeDelta delay);

[gab, 2016/09/30 19:01:26]

Is passing TimeDelta by value common practice? I know it's fine in practice
because it's merely an int64_t but style is usually still to pass classes by
const&.

[fdoray, 2016/09/30 19:10:11]

See https://cs.chromium.org/chromium/src/base/time/time.h?l=24
TaskRunner takes a TimeDelta by value.

[robliao, 2016/09/30 19:16:56]

I don't think there is a style ruling on passing by reference vs by value. The
only requirement is if you pass by reference, you make it a const reference.

There is a strong convention that generally input args are const reference, but
not absolutely required.

In these cases, I sometimes let the code vote, and it looks like TimeDelta (by
value) wins.

[gab, 2016/09/30 19:34:45]

IMO by value only wins because of
https://cs.chromium.org/chromium/src/base/time/time.h?l=24. i.e. only if the
class provides documented guarantees, otherwise you can't assume it won't grow
beyond POD types only and become inefficient to pass by value.

   ~Task();
 
   // The TaskTraits of this task.
   const TaskTraits traits;
 
   // The time at which the task was inserted in its sequence. For an undelayed
   // task, this happens at post time. For a delayed task, this happens some
   // time after the task's delay has expired. If the task hasn't been inserted
   // in a sequence yet, this defaults to a null TimeTicks.
   TimeTicks sequenced_time;
@@ skipping 13 matching lines @@
  private:
   // Disallow copies to make sure no unnecessary ref-bumps are incurred. Making
   // it move-only would be an option, but isn't necessary for now.
   DISALLOW_COPY_AND_ASSIGN(Task);
 };
 
 }  // namespace internal
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_H_