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condition_variable.h

// Copyright (c) 2006-2008 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.

// ConditionVariable wraps pthreads condition variable synchronization or, on
// Windows, simulates it.  This functionality is very helpful for having
// several threads wait for an event, as is common with a thread pool managed
// by a master.  The meaning of such an event in the (worker) thread pool
// scenario is that additional tasks are now available for processing.  It is
// used in Chrome in the DNS prefetching system to notify worker threads that
// a queue now has items (tasks) which need to be tended to.  A related use
// would have a pool manager waiting on a ConditionVariable, waiting for a
// thread in the pool to announce (signal) that there is now more room in a
// (bounded size) communications queue for the manager to deposit tasks, or,
// as a second example, that the queue of tasks is completely empty and all
// workers are waiting.
//
// USAGE NOTE 1: spurious signal events are possible with this and
// most implementations of condition variables.  As a result, be
// *sure* to retest your condition before proceeding.  The following
// is a good example of doing this correctly:
//
// while (!work_to_be_done()) Wait(...);
//
// In contrast do NOT do the following:
//
// if (!work_to_be_done()) Wait(...);  // Don't do this.
//
// Especially avoid the above if you are relying on some other thread only
// issuing a signal up *if* there is work-to-do.  There can/will
// be spurious signals.  Recheck state on waiting thread before
// assuming the signal was intentional. Caveat caller ;-).
//
// USAGE NOTE 2: Broadcast() frees up all waiting threads at once,
// which leads to contention for the locks they all held when they
// called Wait().  This results in POOR performance.  A much better
// approach to getting a lot of threads out of Wait() is to have each
// thread (upon exiting Wait()) call Signal() to free up another
// Wait'ing thread.  Look at condition_variable_unittest.cc for
// both examples.
//
// Broadcast() can be used nicely during teardown, as it gets the job
// done, and leaves no sleeping threads... and performance is less
// critical at that point.
//
// The semantics of Broadcast() are carefully crafted so that *all*
// threads that were waiting when the request was made will indeed
// get signaled.  Some implementations mess up, and don't signal them
// all, while others allow the wait to be effectively turned off (for
// a while while waiting threads come around).  This implementation
// appears correct, as it will not "lose" any signals, and will guarantee
// that all threads get signaled by Broadcast().
//
// This implementation offers support for "performance" in its selection of
// which thread to revive.  Performance, in direct contrast with "fairness,"
// assures that the thread that most recently began to Wait() is selected by
// Signal to revive.  Fairness would (if publicly supported) assure that the
// thread that has Wait()ed the longest is selected. The default policy
// may improve performance, as the selected thread may have a greater chance of
// having some of its stack data in various CPU caches.
//
// For a discussion of the many very subtle implementation details, see the FAQ
// at the end of condition_variable_win.cc.

#ifndef BASE_CONDITION_VARIABLE_H_
#define BASE_CONDITION_VARIABLE_H_

#include "build/build_config.h"

#if defined(OS_WIN)
#include <windows.h>
#elif defined(OS_POSIX)
#include <pthread.h>
#endif

#include "base/basictypes.h"
#include "base/lock.h"

namespace base {
  class TimeDelta;
}

class ConditionVariable {
 public:
  // Construct a cv for use with ONLY one user lock.
  explicit ConditionVariable(Lock* user_lock);

  ~ConditionVariable();

  // Wait() releases the caller's critical section atomically as it starts to
  // sleep, and the reacquires it when it is signaled.
  void Wait();
  void TimedWait(const base::TimeDelta& max_time);

  // Broadcast() revives all waiting threads.
  void Broadcast();
  // Signal() revives one waiting thread.
  void Signal();

 private:

#if defined(OS_WIN)

  // Define Event class that is used to form circularly linked lists.
  // The list container is an element with NULL as its handle_ value.
  // The actual list elements have a non-zero handle_ value.
  // All calls to methods MUST be done under protection of a lock so that links
  // can be validated.  Without the lock, some links might asynchronously
  // change, and the assertions would fail (as would list change operations).
  class Event {
   public:
    // Default constructor with no arguments creates a list container.
    Event();
    ~Event();

    // InitListElement transitions an instance from a container, to an element.
    void InitListElement();

    // Methods for use on lists.
    bool IsEmpty() const;
    void PushBack(Event* other);
    Event* PopFront();
    Event* PopBack();

    // Methods for use on list elements.
    // Accessor method.
    HANDLE handle() const;
    // Pull an element from a list (if it's in one).
    Event* Extract();

    // Method for use on a list element or on a list.
    bool IsSingleton() const;

   private:
    // Provide pre/post conditions to validate correct manipulations.
    bool ValidateAsDistinct(Event* other) const;
    bool ValidateAsItem() const;
    bool ValidateAsList() const;
    bool ValidateLinks() const;

    HANDLE handle_;
    Event* next_;
    Event* prev_;
    DISALLOW_COPY_AND_ASSIGN(Event);
  };

  // Note that RUNNING is an unlikely number to have in RAM by accident.
  // This helps with defensive destructor coding in the face of user error.
  enum RunState { SHUTDOWN = 0, RUNNING = 64213 };

  // Internal implementation methods supporting Wait().
  Event* GetEventForWaiting();
  void RecycleEvent(Event* used_event);

  RunState run_state_;

  // Private critical section for access to member data.
  Lock internal_lock_;

  // Lock that is acquired before calling Wait().
  Lock& user_lock_;

  // Events that threads are blocked on.
  Event waiting_list_;

  // Free list for old events.
  Event recycling_list_;
  int recycling_list_size_;

  // The number of allocated, but not yet deleted events.
  int allocation_counter_;

#elif defined(OS_POSIX)

  pthread_cond_t condition_;
  pthread_mutex_t* user_mutex_;

#endif

  DISALLOW_COPY_AND_ASSIGN(ConditionVariable);
};

#endif  // BASE_CONDITION_VARIABLE_H_

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