Speculative patch: Make it safe to delete the TQM inside a Task

content/child/scheduler/task_queue_manager.cc

Index: content/child/scheduler/task_queue_manager.cc
diff --git a/content/child/scheduler/task_queue_manager.cc b/content/child/scheduler/task_queue_manager.cc
index 386ebd0c955cc642f6ea09c8db56d004d33e9a41..d8228c82114ac121e62e0f8bec33fdbde7dad3fa 100644
--- a/content/child/scheduler/task_queue_manager.cc
+++ b/content/child/scheduler/task_queue_manager.cc
@@ -464,6 +464,7 @@ TaskQueueManager::TaskQueueManager(
       time_source_(nullptr),
       disabled_by_default_tracing_category_(
           disabled_by_default_tracing_category),
+      deletion_sentinel_(new DeletionSentinel()),
       weak_factory_(this) {
   DCHECK(main_task_runner->RunsTasksOnCurrentThread());
   TRACE_EVENT_OBJECT_CREATED_WITH_ID(disabled_by_default_tracing_category,
@@ -596,7 +597,9 @@ void TaskQueueManager::DoWork(bool posted_from_main_thread) {
     // Note that this function won't post another call to DoWork if one is
     // already pending, so it is safe to call it in a loop.
     MaybePostDoWorkOnMainRunner();
-    ProcessTaskFromWorkQueue(queue_index, i > 0, &previous_task);
+
+    if (ProcessTaskFromWorkQueue(queue_index, i > 0, &previous_task))
+      return;
 
     if (!UpdateWorkQueues(&previous_task))
       return;
@@ -616,11 +619,12 @@ void TaskQueueManager::DidQueueTask(base::PendingTask* pending_task) {
   task_annotator_.DidQueueTask("TaskQueueManager::PostTask", *pending_task);
 }
 
-void TaskQueueManager::ProcessTaskFromWorkQueue(
+bool TaskQueueManager::ProcessTaskFromWorkQueue(
     size_t queue_index,
     bool has_previous_task,
     base::PendingTask* previous_task) {
   DCHECK(main_thread_checker_.CalledOnValidThread());
+  scoped_refptr<DeletionSentinel> protect(deletion_sentinel_);
   internal::TaskQueue* queue = Queue(queue_index);
   base::PendingTask pending_task = queue->TakeTaskFromWorkQueue();
   if (!pending_task.nestable && main_task_runner_->IsNested()) {
@@ -640,9 +644,15 @@ void TaskQueueManager::ProcessTaskFromWorkQueue(
     }
     task_annotator_.RunTask("TaskQueueManager::PostTask",
                             "TaskQueueManager::RunTask", pending_task);
+
+    // Detect if the TaskQueueManager just got deleted.

[Sami, 2015/04/08 14:45:56]

Could you add a note here that we mustn't access any member variables after this
point?

[alex clarke (OOO till 29th), 2015/04/08 14:59:40]

Done.

+    if (protect->HasOneRef())
+      return true;
+
     pending_task.task.Reset();
     *previous_task = pending_task;
   }
+  return false;
 }
 
 bool TaskQueueManager::RunsTasksOnCurrentThread() const {