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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+// 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 "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_