Introduce SingleThreadTaskRunnerThreadMode

base/task_scheduler/task_scheduler.h

Index: base/task_scheduler/task_scheduler.h
diff --git a/base/task_scheduler/task_scheduler.h b/base/task_scheduler/task_scheduler.h
index e36f7de53910db734ccf14bb755943418c081bfc..199a58c5a07150cc7ecb1ccfff84694429f200c4 100644
--- a/base/task_scheduler/task_scheduler.h
+++ b/base/task_scheduler/task_scheduler.h
@@ -61,6 +61,19 @@ class BASE_EXPORT TaskScheduler {
     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.
@@ -92,7 +105,10 @@ class BASE_EXPORT TaskScheduler {
   // 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
@@ -104,7 +120,10 @@ class BASE_EXPORT TaskScheduler {
   // 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.