Introduce SingleThreadTaskRunnerThreadMode

base/task_scheduler/task_scheduler.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_SCHEDULER_H_
 #define BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_
 
 #include <memory>
 #include <vector>
 
@@ skipping 43 matching lines @@
         const SchedulerWorkerPoolParams&
             foreground_blocking_worker_pool_params_in);
     ~InitParams();
 
     const SchedulerWorkerPoolParams background_worker_pool_params;
     const SchedulerWorkerPoolParams background_blocking_worker_pool_params;
     const SchedulerWorkerPoolParams foreground_worker_pool_params;
     const SchedulerWorkerPoolParams foreground_blocking_worker_pool_params;
   };
 
+  enum class SingleThreadTaskRunnerThreadMode {

[fdoray, 2017/05/10 12:32:23]

Declare this in a separate file, so that post_task.h doesn't have to include
task_scheduler.h to support this. We wouldn't want task_scheduler.h to be
included in every file that includes post_task.h.

[gab, 2017/05/10 15:20:47]

Agreed, feels like this belongs in task_traits.h or post_task.h, people using
post_task.h shouldn't have to include extra headers to be able to use the API
(and as Francois said we don't wait task_scheduler.h to be implicitly included
everywhere)

[robliao, 2017/05/11 05:30:12]

Went with a new file. This isn't really part of Task Traits by design, so it
didn't really make sense for it to go there. Having TaskScheduler* include
post_task.h also seemed circular, resulting in the new file.

+    // Allow the SingleThreadTaskRunner's thread to be shared with others,
+    // allowing for efficient use of thread resources when this
+    // SingleThreadTaskRunner is idle. This is the default mode and is
+    // recommended for most code.
+    SHARED,
+    // Dedicate a single thread for this SingleThreadTaskRunner. No other tasks
+    // from any other source will be scheduled on the thread backing
+    // the SingleThreadTaskRunner. Use sparingly as this reserves an entire
+    // thread for this SingleThreadTaskRunner.
+    DEDICATED,
+  };
+
   // Destroying a TaskScheduler is not allowed in production; it is always
   // leaked. In tests, it should only be destroyed after JoinForTesting() has
   // returned.
   virtual ~TaskScheduler() = default;
 
   // Allows the task scheduler to create threads and run tasks following the
   // |init_params| specification. CHECKs on failure.
   virtual void Start(const InitParams& init_params) = 0;
 
   // Posts |task| with a |delay| and specific |traits|. |delay| can be zero.
@@ skipping 11 matching lines @@
 
   // Returns a SequencedTaskRunner whose PostTask invocations result in
   // scheduling tasks using |traits|. Tasks run one at a time in posting order.
   virtual scoped_refptr<SequencedTaskRunner>
   CreateSequencedTaskRunnerWithTraits(const TaskTraits& traits) = 0;
 
   // Returns a SingleThreadTaskRunner whose PostTask invocations result in
   // scheduling tasks using |traits|. Tasks run on a single thread in posting
   // order.
   virtual scoped_refptr<SingleThreadTaskRunner>
-  CreateSingleThreadTaskRunnerWithTraits(const TaskTraits& traits) = 0;
+  CreateSingleThreadTaskRunnerWithTraits(
+      const TaskTraits& traits,
+      SingleThreadTaskRunnerThreadMode thread_mode =
+          SingleThreadTaskRunnerThreadMode::SHARED) = 0;
 
 #if defined(OS_WIN)
   // Returns a SingleThreadTaskRunner whose PostTask invocations result in
   // scheduling tasks using |traits| in a COM Single-Threaded Apartment. Tasks
   // run in the same Single-Threaded Apartment in posting order for the returned
   // SingleThreadTaskRunner. There is not necessarily a one-to-one
   // correspondence between SingleThreadTaskRunners and Single-Threaded
   // Apartments. The implementation is free to share apartments or create new
   // apartments as necessary. In either case, care should be taken to make sure
   // COM pointers are not smuggled across apartments.
   virtual scoped_refptr<SingleThreadTaskRunner>
-  CreateCOMSTATaskRunnerWithTraits(const TaskTraits& traits) = 0;
+  CreateCOMSTATaskRunnerWithTraits(
+      const TaskTraits& traits,
+      SingleThreadTaskRunnerThreadMode thread_mode =
+          SingleThreadTaskRunnerThreadMode::SHARED) = 0;
 #endif  // defined(OS_WIN)
 
   // Returns a vector of all histograms available in this task scheduler.
   virtual std::vector<const HistogramBase*> GetHistograms() const = 0;
 
   // Synchronously shuts down the scheduler. Once this is called, only tasks
   // posted with the BLOCK_SHUTDOWN behavior will be run. When this returns:
   // - All SKIP_ON_SHUTDOWN tasks that were already running have completed their
   //   execution.
   // - All posted BLOCK_SHUTDOWN tasks have completed their execution.
@@ skipping 78 matching lines @@
   // each process n/GetMaxConcurrentTasksWithTraitsDeprecated() items.
   //
   // TODO(fdoray): Remove this method. https://crbug.com/687264
   virtual int GetMaxConcurrentTasksWithTraitsDeprecated(
       const TaskTraits& traits) const = 0;
 };
 
 }  // namespace base
 
 #endif  // BASE_TASK_SCHEDULER_TASK_SCHEDULER_H_