Support calling BackgroundSyncProviderThreadProxy::GetThreadInstance after the worker has stopped

content/child/background_sync/background_sync_provider_thread_proxy.cc

 // Copyright 2015 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 "content/child/background_sync/background_sync_provider_thread_proxy.h"
 
 #include "base/bind.h"
 #include "base/bind_helpers.h"
 #include "base/lazy_instance.h"
 #include "base/location.h"
@@ skipping 44 matching lines @@
  private:
   scoped_ptr<blink::WebCallbacks<S, T>> callbacks_;
   int worker_thread_id_;
 };
 
 LazyInstance<ThreadLocalPointer<BackgroundSyncProviderThreadProxy>>::Leaky
     g_sync_provider_tls = LAZY_INSTANCE_INITIALIZER;
 
 }  // anonymous namespace
 
+// static
 BackgroundSyncProviderThreadProxy*
 BackgroundSyncProviderThreadProxy::GetThreadInstance(
     base::SingleThreadTaskRunner* main_thread_task_runner,
     BackgroundSyncProvider* sync_provider) {
   if (g_sync_provider_tls.Pointer()->Get())
     return g_sync_provider_tls.Pointer()->Get();
 
+  if (!WorkerTaskRunner::Instance()->CurrentWorkerId()) {
+    // This could happen if GetThreadInstance is called very late (say by a
+    // garbage collected SyncRegistration).
+    return nullptr;
+  }
+
   BackgroundSyncProviderThreadProxy* instance =
       new BackgroundSyncProviderThreadProxy(main_thread_task_runner,
                                             sync_provider);
-  DCHECK(WorkerTaskRunner::Instance()->CurrentWorkerId());
   WorkerTaskRunner::Instance()->AddStopObserver(instance);
   return instance;
 }
 
 void BackgroundSyncProviderThreadProxy::registerBackgroundSync(
     const blink::WebSyncRegistration* options,
     blink::WebServiceWorkerRegistration* service_worker_registration,
     blink::WebSyncRegistrationCallbacks* callbacks) {
   DCHECK(options);
   DCHECK(service_worker_registration);
@@ skipping 94 matching lines @@
     : main_thread_task_runner_(main_thread_task_runner),
       sync_provider_(sync_provider) {
   g_sync_provider_tls.Pointer()->Set(this);
 }
 
 BackgroundSyncProviderThreadProxy::~BackgroundSyncProviderThreadProxy() {
   g_sync_provider_tls.Pointer()->Set(nullptr);
 }
 
 }  // namespace content