Make MessageLoopProxy::current() usable on threads that don't have MessageLoop.

base/thread_main_task_runner.cc

+// Copyright (c) 2012 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 "base/thread_main_task_runner.h"
+
+#include "base/lazy_instance.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+
+namespace {
+
+base::LazyInstance<base::ThreadLocalPointer<ThreadMainTaskRunner> >
+    lazy_tls_ptr = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+// static
+scoped_refptr<ThreadMainTaskRunner> ThreadMainTaskRunner::current() {
+  ThreadMainTaskRunner* result = lazy_tls_ptr.Pointer()->Get();
+  DCHECK(result);
+  return result;
+}
+
+ThreadMainTaskRunner::ThreadMainTaskRunner()
+    : detached_(false) {
+  DCHECK(!lazy_tls_ptr.Pointer()->Get());
+  lazy_tls_ptr.Pointer()->Set(this);
+}
+
+ThreadMainTaskRunner::~ThreadMainTaskRunner() {
+  DCHECK(detached_);
+}
+
+void ThreadMainTaskRunner::Detach() {
+  DCHECK(BelongsToCurrentThread());
+  DCHECK_EQ(current(), this);
+  lazy_tls_ptr.Pointer()->Set(NULL);
+  detached_ = true;
+}
+
+}  // namespace base