Move Task Scheduler Single Thread Task Runners to Dedicated Threads

base/task_scheduler/task_scheduler_impl_unittest.cc

 // 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.
 
 #include "base/task_scheduler/task_scheduler_impl.h"
 
 #include <stddef.h>
 
 #include <string>
 #include <utility>
@@ skipping 259 matching lines @@
       GetParam().execution_mode);
   EXPECT_FALSE(factory.task_runner()->RunsTasksOnCurrentThread());
 
   const size_t kNumTasksPerTest = 150;
   for (size_t i = 0; i < kNumTasksPerTest; ++i) {
     factory.PostTask(test::TestTaskFactory::PostNestedTask::NO,
                      Bind(&VerifyTaskEnvironment, GetParam().traits));
   }
 
   factory.WaitForAllTasksToRun();
+
+  // State like Single Thread Task Runners may be lingering around at this point
+  // attached to tasks on their running thread even though
+  // WaitForAllTasksToRun() returned. This will cause issues when Single Thread
+  // Task Runner unregistration occurs during JoinForTesting(). Flushing out the
+  // tasks prevents this state.
+  scheduler_->FlushForTesting();

[fdoray, 2017/01/27 16:47:35]

Is the issue that this DCHECK can be hit
https://codereview.chromium.org/2650383007/diff/60001/base/task_scheduler/sch...
if a SingleThreadTaskRunner is destroyed during
SchedulerSingleThreadWorkerPoolManager::JoinForTesting()?

If so, could you instead add a |join_for_testing_called_| flag in
SchedulerSingleThreadWorkerPoolManager and return immediately in
UnregisterSingleThreadWorkerPool() when this flag is set? That way, we won't
have to think about this issue every time we join TaskScheduler.

[robliao, 2017/01/27 21:25:41]

sgtm. Done. Added a reinforcing comment in task_scheduler.h to say that after
JoinForTesting() returns, that's the end for that task scheduler instance.

 }
 
 INSTANTIATE_TEST_CASE_P(OneTraitsExecutionModePair,
                         TaskSchedulerImplTest,
                         ::testing::ValuesIn(GetTraitsExecutionModePairs()));
 
 // Spawns threads that simultaneously post Tasks to TaskRunners with various
 // TaskTraits and ExecutionModes. Verifies that each Task runs on a thread with
 // the expected priority and I/O restrictions and respects the characteristics
 // of its ExecutionMode.
@@ skipping 10 matching lines @@
     thread->WaitForAllTasksToRun();
     thread->Join();
   }
 }
 
 // TODO(fdoray): Add tests with Sequences that move around worker pools once
 // child TaskRunners are supported.
 
 }  // namespace internal
 }  // namespace base