Do not inherit TaskPriority in TaskTraits.

base/task_scheduler/task_traits.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_TRAITS_H_
 #define BASE_TASK_SCHEDULER_TASK_TRAITS_H_
 
 #include <stdint.h>
 
 #include <iosfwd>
@@ skipping 124 matching lines @@
 
   // Applies |shutdown_behavior| to tasks with these traits.
   TaskTraits& WithShutdownBehavior(TaskShutdownBehavior shutdown_behavior);
 
   // Returns true if tasks with these traits may block.
   bool may_block() const { return may_block_; }
 
   // Returns true if tasks with these traits may use base/ sync primitives.
   bool with_base_sync_primitives() const { return with_base_sync_primitives_; }
 
+  // Returns true if the priority was set explicitly.
+  bool priority_set_explicitly() const { return priority_set_explicitly_; }
+
   // Returns the priority of tasks with these traits.
   TaskPriority priority() const { return priority_; }
 
   // Returns the shutdown behavior of tasks with these traits.
   TaskShutdownBehavior shutdown_behavior() const { return shutdown_behavior_; }
 
  private:
-  bool may_block_;
-  bool with_base_sync_primitives_;
-  TaskPriority priority_;
-  TaskShutdownBehavior shutdown_behavior_;
+  // Do not rely on defaults hard-coded below beyond the guarantees described on
+  // the constructor; anything else is subject to change. Tasks should
+  // explicitly request defaults if the behavior is critical to the task.
+  bool may_block_ = false;
+  bool with_base_sync_primitives_ = false;
+  bool priority_set_explicitly_ = false;
+  TaskPriority priority_ = TaskPriority::BACKGROUND;

[gab, 2017/04/25 18:50:42]

GetTaskPriorityForCurrentThread() defaults to USER_VISIBLE so this should too.

[robliao, 2017/04/25 20:06:47]

+1

[fdoray, 2017/04/25 21:20:24]

Done.

+  TaskShutdownBehavior shutdown_behavior_ =
+      TaskShutdownBehavior::SKIP_ON_SHUTDOWN;
 };
 
 // Returns string literals for the enums defined in this file. These methods
 // should only be used for tracing and debugging.
 BASE_EXPORT const char* TaskPriorityToString(TaskPriority task_priority);
 BASE_EXPORT const char* TaskShutdownBehaviorToString(
     TaskShutdownBehavior task_priority);
 
 // Stream operators so that the enums defined in this file can be used in
 // DCHECK and EXPECT statements.
 BASE_EXPORT std::ostream& operator<<(std::ostream& os,
                                      const TaskPriority& shutdown_behavior);
 BASE_EXPORT std::ostream& operator<<(
     std::ostream& os,
     const TaskShutdownBehavior& shutdown_behavior);
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_TRAITS_H_