Exempt the Service Thread from BLOCK_SHUTDOWN DCHECKs

base/task_scheduler/task_tracker.cc

Index: base/task_scheduler/task_tracker.cc
diff --git a/base/task_scheduler/task_tracker.cc b/base/task_scheduler/task_tracker.cc
index da21d5ea8ef510c33636cc993b34e3501aa09101..06a0553280d16976fc074a7c018d1752ee3d2604 100644
--- a/base/task_scheduler/task_tracker.cc
+++ b/base/task_scheduler/task_tracker.cc
@@ -10,7 +10,6 @@
 #include "base/callback.h"
 #include "base/debug/task_annotator.h"
 #include "base/json/json_writer.h"
-#include "base/logging.h"
 #include "base/memory/ptr_util.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/sequence_token.h"
@@ -379,6 +378,12 @@ void TaskTracker::PerformShutdown() {
   }
 }
 
+#if DCHECK_IS_ON()
+bool TaskTracker::IsExemptFromBlockingShutdownChecks() {
+  return false;
+}
+#endif
+
 bool TaskTracker::BeforePostTask(TaskShutdownBehavior shutdown_behavior) {
   if (shutdown_behavior == TaskShutdownBehavior::BLOCK_SHUTDOWN) {
     // BLOCK_SHUTDOWN tasks block shutdown between the moment they are posted
@@ -391,7 +396,16 @@ bool TaskTracker::BeforePostTask(TaskShutdownBehavior shutdown_behavior) {
       // A BLOCK_SHUTDOWN task posted after shutdown has completed is an
       // ordering bug. This aims to catch those early.
       DCHECK(shutdown_event_);
-      DCHECK(!shutdown_event_->IsSignaled());
+      if (shutdown_event_->IsSignaled()) {
+        // TODO(robliao): http://crbug.com/698140. Since the service thread
+        // doesn't stop processing its own tasks at shutdown, we may still
+        // attempt to post a BLOCK_SHUTDOWN task in response to a
+        // FileDescriptorWatcher. Delayed tasks are fine because they're marked

[fdoray, 2017/05/03 20:34:33]

Delayed tasks go through TaskTracker::BeforePostTask() when they are posted, not
when their delay expires on the service thread. Therefore, I'm not sure it is
necessary to comment on delayed tasks here.

[robliao, 2017/05/03 21:11:17]

Indeed. I was thinking of the general case. Removed.

+        // as SKIP_ON_SHUTDOWN and are accounted for shutdown purposes.
+        DCHECK(IsExemptFromBlockingShutdownChecks());
+        state_->DecrementNumTasksBlockingShutdown();
+        return false;
+      }
 
       ++num_block_shutdown_tasks_posted_during_shutdown_;