Exempt the Service Thread from BLOCK_SHUTDOWN DCHECKs

base/task_scheduler/task_tracker_posix.h

Index: base/task_scheduler/task_tracker_posix.h
diff --git a/base/task_scheduler/task_tracker_posix.h b/base/task_scheduler/task_tracker_posix.h
index 53859b21ab63654d9ef7d7a7e68580acd4558b0b..126bbb9a0e8f2b4b8314792e79228550eb2e6c8c 100644
--- a/base/task_scheduler/task_tracker_posix.h
+++ b/base/task_scheduler/task_tracker_posix.h
@@ -8,8 +8,10 @@
 #include <memory>
 
 #include "base/base_export.h"
+#include "base/logging.h"
 #include "base/macros.h"
 #include "base/task_scheduler/task_tracker.h"
+#include "base/threading/platform_thread.h"
 
 namespace base {
 
@@ -37,12 +39,31 @@ class BASE_EXPORT TaskTrackerPosix : public TaskTracker {
     watch_file_descriptor_message_loop_ = watch_file_descriptor_message_loop;
   }
 
+#if DCHECK_IS_ON()
+  // TODO(robliao): http://crbug.com/698140. This addresses service thread tasks
+  // that could run after the task scheduler has shut down. Anything from the
+  // service thread is exempted from the task scheduler shutdown DCHECKs.
+  void set_service_thread_handle(

[gab, 2017/05/04 17:36:11]

Can you set a bit in TLS in TaskTracker instead of using the
PlatformThreadHandle? I have a TODO in thread.cc to get rid of |thread_lock_|
and would be sad to add a valid use case of it...

[robliao, 2017/05/04 17:43:24]

That would create a race between starting the thread and shutting down the task
tracker. Since the handle is known at start, this avoids the race.

[robliao, 2017/05/04 17:44:06]

One more note, this code will go away once we fix the shutdown issue with
regards to Service thread, so this legitimate use is temporary.

+      const PlatformThreadHandle& service_thread_handle) {
+    DCHECK(!service_thread_handle.is_null());
+    service_thread_handle_ = service_thread_handle;
+  }
+#endif
+
  private:
   // TaskTracker:
   void PerformRunTask(std::unique_ptr<Task> task) override;
 
+#if DCHECK_IS_ON()
+  bool IsExemptFromBlockingShutdownChecks() override;

[gab, 2017/05/04 17:36:11]

Why is this POSIX only?

[robliao, 2017/05/04 17:43:24]

FileDescriptorWatchers only exist on POSIX. Delayed tasks aren't subject to this
because they go through the task tracker. FileDescriptorWatchers fire and
execute directly.

+#endif
+
   MessageLoopForIO* watch_file_descriptor_message_loop_ = nullptr;
 
+#if DCHECK_IS_ON()
+  PlatformThreadHandle service_thread_handle_;
+#endif
+
   DISALLOW_COPY_AND_ASSIGN(TaskTrackerPosix);
 };