Make new TaskRunner, SequencedTaskRunner, and SingleThreadTaskRunner interfaces

base/serial_executor.h

+// 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.
+
+#ifndef BASE_SERIAL_EXECUTOR_H_
+#define BASE_SERIAL_EXECUTOR_H_
+#pragma once
+
+#include "base/base_export.h"
+#include "base/executor.h"
+
+namespace base {
+
+// A SerialExecutor is an Executor with more guarantees.  In
+// particular, it guarantees that all submitted tasks, if executed,
+// will be executed on a single specific thread.  Furthermore, it

[darin (slow to review), 2012/01/25 07:25:56]

It wasn't clear to me that we needed this interface to guarantee same-thread
execution.  The main requirement was FIFO execution, I thought, for tasks posted
from the same thread.  That way, the user can know that their tasks won't ever
overlap or run non-FIFO.

+// guarantees that tasks with the same delay and same nestable state
+// (i.e., whether the task was posted via Post*Task or
+// PostNonNestable*Task) get processed in FIFO order, with Post*Task
+// being equivalent to Post*DelayedTask with zero delay.
+class BASE_EXPORT SerialExecutor : public Executor {
+ public:
+  // Returns true iff the current thread is the thread which tasks
+  // submitted to this object will run on.
+  //
+  // TODO(akalin): Make this a const function.
+  virtual bool BelongsToCurrentThread() = 0;
+};
+
+}  // namespace base
+
+#endif  // BASE_SERIAL_EXECUTOR_H_