Create base::WorkerPoolJob. Use it for HostResolverImpl and DirectoryLister.

base/worker_pool_job.h

+// Copyright (c) 2010 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_WORKER_POOL_JOB_H_
+#define BASE_WORKER_POOL_JOB_H_
+#pragma once
+
+#include "base/basictypes.h"
+#include "base/lock.h"
+#include "base/message_loop_proxy.h"
+#include "base/ref_counted.h"
+#include "base/thread_checker.h"
+
+namespace base {
+
+// WARNING: Are you sure you want to use WorkerPool in the first place?  The
+// thread will _not_ be joined, so the job may still be running during shutdown.
+// Global objects that are deleted by AtExitManager should not be accessed on
+// the WorkerPool thread, otherwise there is potential for shutdown crashes.
+//
+// WorkerPoolJob is a helper class for implementing jobs that get created on an
+// origin thread, get posted to run on a WorkerPool thread, and then run back on
+// the origin thread upon completion.  WorkerPoolJob also handles cancellation
+// of a job which prevents it from calling back.  Note that we use inheritance
+// rather than composition here, since WorkerPoolJob also handles lifetime
+// management via refcounting.
+class WorkerPoolJob : public base::RefCountedThreadSafe<WorkerPoolJob> {
+ public:
+  // ScopedHandle is provided to automate cancellation of the WorkerPoolJob.
+  template <typename JobType>
+  class ScopedHandle {
+   public:
+    explicit ScopedHandle(JobType* job) : job_(job) {}
+
+    ~ScopedHandle() {
+      if (!job_->canceled() && job_->state_ == RUNNING)
+        job_->CancelJob();
+    }
+
+    JobType* job() const { return job_.get(); }
+
+   private:
+    const scoped_refptr<JobType> job_;
+    DISALLOW_COPY_AND_ASSIGN(ScopedHandle);
+  };
+
+ protected:
+  WorkerPoolJob();
+
+  // This may be deleted on the origin or worker thread.
+  virtual ~WorkerPoolJob();
+
+  // Starts the job.  Will post itself to the WorkerPool, where it will invoke
+  // RunJob().  After RunJob() runs, it will post itself back to the origin
+  // thread, where it will invoke OnJobCompleted().  This can only be called
+  // once and must be called on the origin thread.
+  void StartJob();
+
+  // Cancels the job.  OnJobCompleted() is guaranteed not to be called when this
+  // happens.  May only be called once, and only on the origin thread.
+  void CancelJob();
+
+  // Returns true if CancelJob() has been called.
+  bool canceled() const;
+
+  // NOTE(willchan): I've restricted this to unit tests only, primarily because
+  // I haven't found a good reason for allowing subtypes to access this.  If
+  // there is a motivating reason to do so, I am open to it.
+#if defined(UNIT_TEST)
+  scoped_refptr<MessageLoopProxy> origin_loop() const {
+    return origin_loop_;
+  }
+
+  // Returns true if OnJobCompleted() has not been called yet.
+  bool is_running() const {
+    DCHECK(thread_checker_.CalledOnValidThread());
+    DCHECK_NE(state_, NONE);
+    return state_ == RUNNING;
+  }
+#endif  // defined(UNIT_TEST)
+
+ private:
+  friend class base::RefCountedThreadSafe<WorkerPoolJob>;
+
+  // State machine, primarily for enforcing that certain events only happen
+  // once.  Also lets us implement is_running().
+  enum State {
+    NONE,
+    RUNNING,
+    DONE,
+  };
+
+  // API for subtypes to implement.
+
+  // RunJob() runs on the worker thread.  If CancelJob() is called on the origin
+  // thread , then it may never be invoked, but that's a race.
+  virtual void RunJob() = 0;
+
+  // OnJobCompleted() runs on the origin thread.  If CancelJob() is called, then
+  // it may never be invoked.

[eroman, 2011/01/05 01:24:52]

This wording can be stronger. If CancelJob() was called, then OnJobCompleted()
will definitely not be called (since both run on the origin thread).

+  virtual void OnJobCompleted() = 0;
+
+  void RunJobOnWorkerPool();
+  void CompleteJobOnOriginLoop();
+
+  ThreadChecker thread_checker_;
+  const scoped_refptr<MessageLoopProxy> origin_loop_;
+
+  bool canceled_;
+  State state_;
+
+  DISALLOW_COPY_AND_ASSIGN(WorkerPoolJob);
+};
+
+}  // namespace base
+
+#endif  // BASE_WORKER_POOL_JOB_H_