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chrome_thread.h

// 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.

#ifndef CHROME_BROWSER_CHROME_THREAD_H_
#define CHROME_BROWSER_CHROME_THREAD_H_

#include "base/lock.h"
#include "base/task.h"
#include "base/thread.h"

///////////////////////////////////////////////////////////////////////////////
// ChromeThread
//
// This class represents a thread that is known by a browser-wide name.  For
// example, there is one IO thread for the entire browser process, and various
// pieces of code find it useful to retrieve a pointer to the IO thread's
// Invoke a task by thread ID:
//
//   ChromeThread::PostTask(ChromeThread::IO, FROM_HERE, task);
//
// The return value is false if the task couldn't be posted because the target
// thread doesn't exist.  If this could lead to data loss, you need to check the
// result and restructure the code to ensure it doesn't occur.
//
// This class automatically handles the lifetime of different threads.
// It's always safe to call PostTask on any thread.  If it's not yet created,
// the task is deleted.  There are no race conditions.  If the thread that the
// task is posted to is guaranteed to outlive the current thread, then no locks
// are used.  You should never need to cache pointers to MessageLoops, since
// they're not thread safe.
class ChromeThread : public base::Thread {
 public:
  // An enumeration of the well-known threads.
  // NOTE: threads must be listed in the order of their life-time, with each
  // thread outliving every other thread below it.
  enum ID {
    // The main thread in the browser.
    UI,

    // This is the thread that interacts with the database.
    DB,

    // This is the "main" thread for WebKit within the browser process when
    // NOT in --single-process mode.
    WEBKIT,

    // This is the thread that interacts with the file system.
    FILE,

    // Used to launch and terminate processes.
    PROCESS_LAUNCHER,

    // This is the thread that processes IPC and network messages.
    IO,

#if defined(USE_X11)
    // This thread has a second connection to the X server and is used to
    // process UI requests when routing the request to the UI thread would risk
    // deadlock.
    BACKGROUND_X11,
#endif

    // This identifier does not represent a thread.  Instead it counts the
    // number of well-known threads.  Insert new well-known threads before this
    // identifier.
    ID_COUNT
  };

  // Construct a ChromeThread with the supplied identifier.  It is an error
  // to construct a ChromeThread that already exists.
  explicit ChromeThread(ID identifier);

  // Special constructor for the main (UI) thread and unittests. We use a dummy
  // thread here since the main thread already exists.
  ChromeThread(ID identifier, MessageLoop* message_loop);

  virtual ~ChromeThread();

  // These are the same methods in message_loop.h, but are guaranteed to either
  // get posted to the MessageLoop if it's still alive, or be deleted otherwise.
  // They return true iff the thread existed and the task was posted.  Note that
  // even if the task is posted, there's no guarantee that it will run, since
  // the target thread may already have a Quit message in its queue.
  static bool PostTask(ID identifier,
                       const tracked_objects::Location& from_here,
                       Task* task);
  static bool PostDelayedTask(ID identifier,
                              const tracked_objects::Location& from_here,
                              Task* task,
                              int64 delay_ms);
  static bool PostNonNestableTask(ID identifier,
                                  const tracked_objects::Location& from_here,
                                  Task* task);
  static bool PostNonNestableDelayedTask(
      ID identifier,
      const tracked_objects::Location& from_here,
      Task* task,
      int64 delay_ms);

  template <class T>
  static bool DeleteSoon(ID identifier,
                         const tracked_objects::Location& from_here,
                         T* object) {
    return PostNonNestableTask(
        identifier, from_here, new DeleteTask<T>(object));
  }

  template <class T>
  static bool ReleaseSoon(ID identifier,
                          const tracked_objects::Location& from_here,
                          T* object) {
    return PostNonNestableTask(
        identifier, from_here, new ReleaseTask<T>(object));
  }

  // Callable on any thread.  Returns whether the given ID corresponds to a well
  // known thread.
  static bool IsWellKnownThread(ID identifier);

  // Callable on any thread.  Returns whether you're currently on a particular
  // thread.
  static bool CurrentlyOn(ID identifier);

  // If the current message loop is one of the known threads, returns true and
  // sets identifier to its ID.  Otherwise returns false.
  static bool GetCurrentThreadIdentifier(ID* identifier);

  // Use these templates in conjuction with RefCountedThreadSafe when you want
  // to ensure that an object is deleted on a specific thread.  This is needed
  // when an object can hop between threads (i.e. IO -> FILE -> IO), and thread
  // switching delays can mean that the final IO tasks executes before the FILE
  // task's stack unwinds.  This would lead to the object destructing on the
  // FILE thread, which often is not what you want (i.e. to unregister from
  // NotificationService, to notify other objects on the creating thread etc).
  template<ID thread>
  struct DeleteOnThread {
    template<typename T>
    static void Destruct(T* x) {
      if (CurrentlyOn(thread)) {
        delete x;
      } else {
        DeleteSoon(thread, FROM_HERE, x);
      }
    }
  };

  // Sample usage:
  // class Foo
  //     : public base::RefCountedThreadSafe<
  //           Foo, ChromeThread::DeleteOnIOThread> {
  //
  // ...
  //  private:
  //   friend class ChromeThread;
  //   friend class DeleteTask<Foo>;
  //
  //   ~Foo();
  struct DeleteOnUIThread : public DeleteOnThread<UI> { };
  struct DeleteOnIOThread : public DeleteOnThread<IO> { };
  struct DeleteOnFileThread : public DeleteOnThread<FILE> { };
  struct DeleteOnDBThread : public DeleteOnThread<DB> { };
  struct DeleteOnWebKitThread : public DeleteOnThread<WEBKIT> { };

 private:
  // Common initialization code for the constructors.
  void Initialize();

  static bool PostTaskHelper(
      ID identifier,
      const tracked_objects::Location& from_here,
      Task* task,
      int64 delay_ms,
      bool nestable);

  // The identifier of this thread.  Only one thread can exist with a given
  // identifier at a given time.
  ID identifier_;

  // This lock protects |chrome_threads_|.  Do not read or modify that array
  // without holding this lock.  Do not block while holding this lock.
  static Lock lock_;

  // An array of the ChromeThread objects.  This array is protected by |lock_|.
  // The threads are not owned by this array.  Typically, the threads are owned
  // on the UI thread by the g_browser_process object.  ChromeThreads remove
  // themselves from this array upon destruction.
  static ChromeThread* chrome_threads_[ID_COUNT];
};

#endif  // CHROME_BROWSER_CHROME_THREAD_H_

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