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

base/thread_main_task_runner.cc

Index: base/thread_main_task_runner.cc
diff --git a/base/thread_main_task_runner.cc b/base/thread_main_task_runner.cc
new file mode 100644
index 0000000000000000000000000000000000000000..ca68bb7d7f01877629cf8cde224ef701711e3a11
--- /dev/null
+++ b/base/thread_main_task_runner.cc
@@ -0,0 +1,32 @@
+// 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;
+}
+
+// static
+void ThreadMainTaskRunner::SetCurrent(ThreadMainTaskRunner* new_current) {
+  DCHECK(!current() || !new_current);

[Ami GONE FROM CHROMIUM, 2012/04/30 23:34:05]

I'm surprised this doesn't DCHECK-fail the first time SetCurrent() is called,
since current() DCHECK's its own result.

[Ami GONE FROM CHROMIUM, 2012/04/30 23:34:05]

(assuming you s/current()/lazy_tls_ptr.Pointer()->Get()/ for the previous
comment)
Can you think of a case where it's not a programmer error to call
SetCurrent(NULL) without a previous not-NULL value being set?

[Sergey Ulanov, 2012/05/01 00:34:54]

Oops, I didn't test this code after adding DCHECK in current(). Fixed now.

[Sergey Ulanov, 2012/05/01 00:34:54]

No. Ok, I changed this code to check this case too, but without XOR.

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