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

base/single_thread_task_runner.cc

Index: base/single_thread_task_runner.cc
diff --git a/base/single_thread_task_runner.cc b/base/single_thread_task_runner.cc
new file mode 100644
index 0000000000000000000000000000000000000000..c0d0d50242701a0269b817480363fa65e5bd02b4
--- /dev/null
+++ b/base/single_thread_task_runner.cc
@@ -0,0 +1,30 @@
+// 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/single_thread_task_runner.h"
+
+#include "base/lazy_instance.h"
+#include "base/threading/thread_local.h"
+
+namespace base {
+
+namespace {
+
+base::LazyInstance<base::ThreadLocalPointer<SingleThreadTaskRunner> >
+    lazy_tls_ptr = LAZY_INSTANCE_INITIALIZER;
+
+}  // namespace
+
+// static
+scoped_refptr<SingleThreadTaskRunner> SingleThreadTaskRunner::current() {
+  return lazy_tls_ptr.Pointer()->Get();

[Ami GONE FROM CHROMIUM, 2012/04/25 17:27:55]

Can this DCHECK the ret value (see the TODO(darin) in MessageLoop::current()'s
impl).

[Sergey Ulanov, 2012/04/30 20:02:10]

Done.

+}
+
+// static
+void SingleThreadTaskRunner::SetCurrent(SingleThreadTaskRunner* new_current) {
+  DCHECK(!current() || !new_current);

[Ami GONE FROM CHROMIUM, 2012/04/25 17:27:55]

WDYT of xor instead of !||! ?
DCHECK(current() ^ new_current);

[Sergey Ulanov, 2012/04/30 20:02:10]

^ is bitwise xor, so I'm not sure it would work with pointers properly here. In
any case, I don't see a good reason to prohibit SetCurrent(NULL) when current()
is already set to NULL.

+  lazy_tls_ptr.Pointer()->Set(new_current);
+}
+
+}  // namespace base