# = monitor.rb

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#

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# Copyright (C) 2001 Shugo Maeda <shugo@ruby-lang.org>

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#

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# This library is distributed under the terms of the Ruby license.

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# You can freely distribute/modify this library.

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#

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require 'thread'

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#

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# In concurrent programming, a monitor is an object or module intended to be

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# used safely by more than one thread. The defining characteristic of a

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# monitor is that its methods are executed with mutual exclusion. That is, at

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# each point in time, at most one thread may be executing any of its methods.

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# This mutual exclusion greatly simplifies reasoning about the implementation

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# of monitors compared to reasoning about parallel code that updates a data

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# structure.

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#

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# You can read more about the general principles on the Wikipedia page for

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# Monitors[http://en.wikipedia.org/wiki/Monitor_%28synchronization%29]

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#

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# == Examples

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#

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# === Simple object.extend

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#

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# require 'monitor.rb'

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#

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# buf = []

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# buf.extend(MonitorMixin)

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# empty_cond = buf.new_cond

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#

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# # consumer

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# Thread.start do

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# loop do

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# buf.synchronize do

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# empty_cond.wait_while { buf.empty? }

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# print buf.shift

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# end

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# end

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# end

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#

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# # producer

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# while line = ARGF.gets

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# buf.synchronize do

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# buf.push(line)

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# empty_cond.signal

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# end

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# end

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#

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# The consumer thread waits for the producer thread to push a line to buf

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# while <tt>buf.empty?</tt>. The producer thread (main thread) reads a

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# line from ARGF and pushes it into buf then calls <tt>empty_cond.signal</tt>

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# to notify the consumer thread of new data.

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#

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# === Simple Class include

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#

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# require 'monitor'

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#

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# class SynchronizedArray < Array

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#

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# include MonitorMixin

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#

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# def initialize(*args)

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# super(*args)

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# end

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#

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# alias :old_shift :shift

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# alias :old_unshift :unshift

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#

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# def shift(n=1)

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# self.synchronize do

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# self.old_shift(n)

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# end

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# end

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#

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# def unshift(item)

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# self.synchronize do

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# self.old_unshift(item)

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# end

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# end

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#

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# # other methods ...

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# end

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#

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# +SynchronizedArray+ implements an Array with synchronized access to items.

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# This Class is implemented as subclass of Array which includes the

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# MonitorMixin module.

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#

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module MonitorMixin

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#

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# FIXME: This isn't documented in Nutshell.

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#

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# Since MonitorMixin.new_cond returns a ConditionVariable, and the example

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# above calls while_wait and signal, this class should be documented.

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#

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class ConditionVariable

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class Timeout < Exception; end

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#

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# Releases the lock held in the associated monitor and waits; reacquires the lock on wakeup.

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#

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# If +timeout+ is given, this method returns after +timeout+ seconds passed,

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# even if no other thread doesn't signal.

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#

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def wait(timeout = nil)

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@monitor.__send__(:mon_check_owner)

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count = @monitor.__send__(:mon_exit_for_cond)

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begin

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@cond.wait(@monitor.instance_variable_get(:@mon_mutex), timeout)

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return true

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ensure

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@monitor.__send__(:mon_enter_for_cond, count)

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end

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end

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#

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# Calls wait repeatedly while the given block yields a truthy value.

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#

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def wait_while

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while yield

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wait

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end

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end

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#

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# Calls wait repeatedly until the given block yields a truthy value.

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#

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def wait_until

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until yield

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wait

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end

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end

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#

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# Wakes up the first thread in line waiting for this lock.

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#

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def signal

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@monitor.__send__(:mon_check_owner)

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@cond.signal

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end

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#

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# Wakes up all threads waiting for this lock.

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#

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def broadcast

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@monitor.__send__(:mon_check_owner)

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@cond.broadcast

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end

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private

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def initialize(monitor)

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@monitor = monitor

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@cond = ::ConditionVariable.new

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end

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end

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def self.extend_object(obj)

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super(obj)

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obj.__send__(:mon_initialize)

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end

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#

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# Attempts to enter exclusive section. Returns +false+ if lock fails.

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#

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def mon_try_enter

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if @mon_owner != Thread.current

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unless @mon_mutex.try_lock

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return false

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end

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@mon_owner = Thread.current

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end

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@mon_count += 1

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return true

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end

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# For backward compatibility

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alias try_mon_enter mon_try_enter

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#

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# Enters exclusive section.

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#

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def mon_enter

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if @mon_owner != Thread.current

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@mon_mutex.lock

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@mon_owner = Thread.current

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end

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@mon_count += 1

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end

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#

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# Leaves exclusive section.

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#

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def mon_exit

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mon_check_owner

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@mon_count -=1

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if @mon_count == 0

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@mon_owner = nil

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@mon_mutex.unlock

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end

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end

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#

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# Enters exclusive section and executes the block. Leaves the exclusive

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# section automatically when the block exits. See example under

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# +MonitorMixin+.

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#

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def mon_synchronize

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mon_enter

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begin

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yield

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ensure

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mon_exit

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end

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end

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alias synchronize mon_synchronize

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#

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# Creates a new MonitorMixin::ConditionVariable associated with the

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# receiver.

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#

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def new_cond

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return ConditionVariable.new(self)

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end

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private

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# Use <tt>extend MonitorMixin</tt> or <tt>include MonitorMixin</tt> instead

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# of this constructor. Have look at the examples above to understand how to

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# use this module.

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def initialize(*args)

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super

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mon_initialize

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end

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# Initializes the MonitorMixin after being included in a class or when an

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# object has been extended with the MonitorMixin

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def mon_initialize

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@mon_owner = nil

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@mon_count = 0

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@mon_mutex = Mutex.new

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end

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def mon_check_owner

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if @mon_owner != Thread.current

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raise ThreadError, "current thread not owner"

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end

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end

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def mon_enter_for_cond(count)

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@mon_owner = Thread.current

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@mon_count = count

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end

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def mon_exit_for_cond

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count = @mon_count

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@mon_owner = nil

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@mon_count = 0

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return count

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end

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end

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# Use the Monitor class when you want to have a lock object for blocks with

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# mutual exclusion.

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#

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# require 'monitor'

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#

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# lock = Monitor.new

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# lock.synchronize do

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# # exclusive access

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# end

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#

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class Monitor

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include MonitorMixin

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alias try_enter try_mon_enter

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alias enter mon_enter

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alias exit mon_exit

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end

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# Documentation comments:

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# - All documentation comes from Nutshell.

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# - MonitorMixin.new_cond appears in the example, but is not documented in

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# Nutshell.

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# - All the internals (internal modules Accessible and Initializable, class

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# ConditionVariable) appear in RDoc. It might be good to hide them, by

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# making them private, or marking them :nodoc:, etc.

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# - RDoc doesn't recognise aliases, so we have mon_synchronize documented, but

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# not synchronize.

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# - mon_owner is in Nutshell, but appears as an accessor in a separate module

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# here, so is hard/impossible to RDoc. Some other useful accessors

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# (mon_count and some queue stuff) are also in this module, and don't appear

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# directly in the RDoc output.

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# - in short, it may be worth changing the code layout in this file to make the

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# documentation easier

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# Local variables:

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# mode: Ruby

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# tab-width: 8

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# End:

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