ruby/lib/monitor.rb

=begin

= 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.

== example

This is a simple example.

  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 buf.empty?, and the producer thread (main thread)
reads a line from ARGF and push it to buf, then call
empty_cond.signal.

=end

require 'thread'

#
# Adds monitor functionality to an arbitrary object by mixing the module with
# +include+.  For example:
#
#    require 'monitor'
#
#    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 buf.empty?, and the producer thread (main thread)
# reads a line from ARGF and push it to buf, then call
# empty_cond.signal.
#
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

  def initialize(*args)
    super
    mon_initialize
  end

  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

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.
#  - The entire example from the RD section at the top is replicated in the RDoc
#    comment for MonitorMixin.  Does the RD section need to remain?
#  - 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: