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 // Copyright (c) 2009 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.

 //

 // The thread pool used in the Linux implementation of WorkerPool dynamically

 // adds threads as necessary to handle all tasks.  It keeps old threads around

 // for a period of time to allow them to be reused.  After this waiting period,

 // the threads exit.  This thread pool uses non-joinable threads, therefore

 // worker threads are not joined during process shutdown.  This means that

 // potentially long running tasks (such as DNS lookup) do not block process

 // shutdown, but also means that process shutdown may "leak" objects.  Note that

 // although LinuxDynamicThreadPool spawns the worker threads and manages the

 // task queue, it does not own the worker threads.  The worker threads ask the

 // LinuxDynamicThreadPool for work and eventually clean themselves up.  The

 // worker threads all maintain scoped_refptrs to the LinuxDynamicThreadPool

 // instance, which prevents LinuxDynamicThreadPool from disappearing before all

 // worker threads exit.  The owner of LinuxDynamicThreadPool should likewise

 // maintain a scoped_refptr to the LinuxDynamicThreadPool instance.

 //

 // NOTE: The classes defined in this file are only meant for use by the Linux

 // implementation of WorkerPool.  No one else should be using these classes.

 // These symbols are exported in a header purely for testing purposes.

 #ifndef BASE_WORKER_POOL_LINUX_H_

 #define BASE_WORKER_POOL_LINUX_H_

 #include <queue>

 #include <string>

 #include "base/basictypes.h"

 #include "base/condition_variable.h"

 #include "base/lock.h"

 #include "base/platform_thread.h"

 #include "base/ref_counted.h"

 #include "base/scoped_ptr.h"

 class Task;

 namespace base {

 class LinuxDynamicThreadPool

     : public RefCountedThreadSafe<LinuxDynamicThreadPool> {

  public:

   class LinuxDynamicThreadPoolPeer;

   // All worker threads will share the same |name_prefix|.  They will exit after

   // |idle_seconds_before_exit|.

   LinuxDynamicThreadPool(const std::string& name_prefix,

                          int idle_seconds_before_exit);

   ~LinuxDynamicThreadPool();

   // Indicates that the thread pool is going away.  Stops handing out tasks to

   // worker threads.  Wakes up all the idle threads to let them exit.

   void Terminate();

   // Adds |task| to the thread pool.  LinuxDynamicThreadPool assumes ownership

   // of |task|.

   void PostTask(Task* task);

   // Worker thread method to wait for up to |idle_seconds_before_exit| for more

   // work from the thread pool.  Returns NULL if no work is available.

   Task* WaitForTask();

  private:

   friend class LinuxDynamicThreadPoolPeer;

   const std::string name_prefix_;

   const int idle_seconds_before_exit_;

   Lock lock_;  // Protects all the variables below.

   // Signal()s worker threads to let them know more tasks are available.

   // Also used for Broadcast()'ing to worker threads to let them know the pool

   // is being deleted and they can exit.

   ConditionVariable tasks_available_cv_;

   int num_idle_threads_;

   std::queue<Task*> tasks_;

   bool terminated_;

   // Only used for tests to ensure correct thread ordering.  It will always be

   // NULL in non-test code.

   scoped_ptr<ConditionVariable> num_idle_threads_cv_;

   DISALLOW_COPY_AND_ASSIGN(LinuxDynamicThreadPool);

 };

 }  // namespace base

 #endif  // BASE_WORKER_POOL_LINUX_H_