Enforce singleton not allowed behavior for CONTINUE_ON_SHUTDOWN tasks

base/threading/sequenced_worker_pool.cc

Index: base/threading/sequenced_worker_pool.cc
diff --git a/base/threading/sequenced_worker_pool.cc b/base/threading/sequenced_worker_pool.cc
index 24a07511a9847a4faba4dc855470cbda3fbb347b..4a82b7b95cfb2c9471d2352170f945647d6c0417 100644
--- a/base/threading/sequenced_worker_pool.cc
+++ b/base/threading/sequenced_worker_pool.cc
@@ -246,6 +246,12 @@ class SequencedWorkerPool::Worker : public SimpleThread {
     is_processing_task_ = true;
     task_sequence_token_ = token;
     task_shutdown_behavior_ = shutdown_behavior;
+
+    // It is dangerous for tasks with CONTINUE_ON_SHUTDOWN to access a class
+    // that implements base::Singleton. This will trigger a DCHECK to warn of

[robliao, 2016/04/25 23:03:30]

Put why this is the case in the comment as well. (e.g. the singleton is
generally destroyed at exit).

[manzagop (departed), 2016/04/26 13:13:44]

Also mention leaky versions are fine to access from nonjoinable threads?

[Patrick Monette, 2016/04/26 19:05:27]

Done.

[Patrick Monette, 2016/04/26 19:05:27]

Done.

+    // such cases. See the comment about CONTINUE_ON_SHUTDOWN for more details.
+    ThreadRestrictions::SetSingletonAllowed(task_shutdown_behavior_ !=
+                                            CONTINUE_ON_SHUTDOWN);
   }
 
   // Indicates that the task has finished running.