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+# = monitor.rb
+#
+# Copyright (C) 2001  Shugo Maeda <shugo@ruby-lang.org>
+#
+# This library is distributed under the terms of the Ruby license.
+# You can freely distribute/modify this library.
+#
+
+require 'thread'
+
+#
+# In concurrent programming, a monitor is an object or module intended to be
+# used safely by more than one thread.  The defining characteristic of a
+# monitor is that its methods are executed with mutual exclusion.  That is, at
+# each point in time, at most one thread may be executing any of its methods.
+# This mutual exclusion greatly simplifies reasoning about the implementation
+# of monitors compared to reasoning about parallel code that updates a data
+# structure.
+#
+# You can read more about the general principles on the Wikipedia page for
+# Monitors[http://en.wikipedia.org/wiki/Monitor_%28synchronization%29]
+#
+# == Examples
+#
+# === Simple object.extend
+#
+#   require 'monitor.rb'
+#
+#   buf = []
+#   buf.extend(MonitorMixin)
+#   empty_cond = buf.new_cond
+#
+#   # consumer
+#   Thread.start do
+#     loop do
+#       buf.synchronize do
+#         empty_cond.wait_while { buf.empty? }
+#         print buf.shift
+#       end
+#     end
+#   end
+#
+#   # producer
+#   while line = ARGF.gets
+#     buf.synchronize do
+#       buf.push(line)
+#       empty_cond.signal
+#     end
+#   end
+#
+# The consumer thread waits for the producer thread to push a line to buf
+# while <tt>buf.empty?</tt>.  The producer thread (main thread) reads a
+# line from ARGF and pushes it into buf then calls <tt>empty_cond.signal</tt>
+# to notify the consumer thread of new data.
+#
+# === Simple Class include
+#
+#   require 'monitor'
+#
+#   class SynchronizedArray < Array
+#
+#     include MonitorMixin
+#
+#     def initialize(*args)
+#       super(*args)
+#     end
+#
+#     alias :old_shift :shift
+#     alias :old_unshift :unshift
+#
+#     def shift(n=1)
+#       self.synchronize do
+#         self.old_shift(n)
+#       end
+#     end
+#
+#     def unshift(item)
+#       self.synchronize do
+#         self.old_unshift(item)
+#       end
+#     end
+#
+#     # other methods ...
+#   end
+#
+# +SynchronizedArray+ implements an Array with synchronized access to items.
+# This Class is implemented as subclass of Array which includes the
+# MonitorMixin module.
+#
+module MonitorMixin
+  #
+  # FIXME: This isn't documented in Nutshell.
+  #
+  # Since MonitorMixin.new_cond returns a ConditionVariable, and the example
+  # above calls while_wait and signal, this class should be documented.
+  #
+  class ConditionVariable
+    class Timeout < Exception; end
+
+    #
+    # Releases the lock held in the associated monitor and waits; reacquires the lock on wakeup.
+    #
+    # If +timeout+ is given, this method returns after +timeout+ seconds passed,
+    # even if no other thread doesn't signal.
+    #
+    def wait(timeout = nil)
+      @monitor.__send__(:mon_check_owner)
+      count = @monitor.__send__(:mon_exit_for_cond)
+      begin
+        @cond.wait(@monitor.instance_variable_get(:@mon_mutex), timeout)
+        return true
+      ensure
+        @monitor.__send__(:mon_enter_for_cond, count)
+      end
+    end
+
+    #
+    # Calls wait repeatedly while the given block yields a truthy value.
+    #
+    def wait_while
+      while yield
+        wait
+      end
+    end
+
+    #
+    # Calls wait repeatedly until the given block yields a truthy value.
+    #
+    def wait_until
+      until yield
+        wait
+      end
+    end
+
+    #
+    # Wakes up the first thread in line waiting for this lock.
+    #
+    def signal
+      @monitor.__send__(:mon_check_owner)
+      @cond.signal
+    end
+
+    #
+    # Wakes up all threads waiting for this lock.
+    #
+    def broadcast
+      @monitor.__send__(:mon_check_owner)
+      @cond.broadcast
+    end
+
+    private
+
+    def initialize(monitor)
+      @monitor = monitor
+      @cond = ::ConditionVariable.new
+    end
+  end
+
+  def self.extend_object(obj)
+    super(obj)
+    obj.__send__(:mon_initialize)
+  end
+
+  #
+  # Attempts to enter exclusive section.  Returns +false+ if lock fails.
+  #
+  def mon_try_enter
+    if @mon_owner != Thread.current
+      unless @mon_mutex.try_lock
+        return false
+      end
+      @mon_owner = Thread.current
+    end
+    @mon_count += 1
+    return true
+  end
+  # For backward compatibility
+  alias try_mon_enter mon_try_enter
+
+  #
+  # Enters exclusive section.
+  #
+  def mon_enter
+    if @mon_owner != Thread.current
+      @mon_mutex.lock
+      @mon_owner = Thread.current
+    end
+    @mon_count += 1
+  end
+
+  #
+  # Leaves exclusive section.
+  #
+  def mon_exit
+    mon_check_owner
+    @mon_count -=1
+    if @mon_count == 0
+      @mon_owner = nil
+      @mon_mutex.unlock
+    end
+  end
+
+  #
+  # Enters exclusive section and executes the block.  Leaves the exclusive
+  # section automatically when the block exits.  See example under
+  # +MonitorMixin+.
+  #
+  def mon_synchronize
+    mon_enter
+    begin
+      yield
+    ensure
+      mon_exit
+    end
+  end
+  alias synchronize mon_synchronize
+
+  #
+  # Creates a new MonitorMixin::ConditionVariable associated with the
+  # receiver.
+  #
+  def new_cond
+    return ConditionVariable.new(self)
+  end
+
+  private
+
+  # Use <tt>extend MonitorMixin</tt> or <tt>include MonitorMixin</tt> instead
+  # of this constructor.  Have look at the examples above to understand how to
+  # use this module.
+  def initialize(*args)
+    super
+    mon_initialize
+  end
+
+  # Initializes the MonitorMixin after being included in a class or when an
+  # object has been extended with the MonitorMixin
+  def mon_initialize
+    @mon_owner = nil
+    @mon_count = 0
+    @mon_mutex = Mutex.new
+  end
+
+  def mon_check_owner
+    if @mon_owner != Thread.current
+      raise ThreadError, "current thread not owner"
+    end
+  end
+
+  def mon_enter_for_cond(count)
+    @mon_owner = Thread.current
+    @mon_count = count
+  end
+
+  def mon_exit_for_cond
+    count = @mon_count
+    @mon_owner = nil
+    @mon_count = 0
+    return count
+  end
+end
+
+# Use the Monitor class when you want to have a lock object for blocks with
+# mutual exclusion.
+#
+#   require 'monitor'
+#
+#   lock = Monitor.new
+#   lock.synchronize do
+#     # exclusive access
+#   end
+#
+class Monitor
+  include MonitorMixin
+  alias try_enter try_mon_enter
+  alias enter mon_enter
+  alias exit mon_exit
+end
+
+
+# Documentation comments:
+#  - All documentation comes from Nutshell.
+#  - MonitorMixin.new_cond appears in the example, but is not documented in
+#    Nutshell.
+#  - All the internals (internal modules Accessible and Initializable, class
+#    ConditionVariable) appear in RDoc.  It might be good to hide them, by
+#    making them private, or marking them :nodoc:, etc.
+#  - RDoc doesn't recognise aliases, so we have mon_synchronize documented, but
+#    not synchronize.
+#  - mon_owner is in Nutshell, but appears as an accessor in a separate module
+#    here, so is hard/impossible to RDoc.  Some other useful accessors
+#    (mon_count and some queue stuff) are also in this module, and don't appear
+#    directly in the RDoc output.
+#  - in short, it may be worth changing the code layout in this file to make the
+#    documentation easier
+
+# Local variables:
+# mode: Ruby
+# tab-width: 8
+# End: