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=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
#
# Adds monitor functionality to an arbitrary object by mixing the module with
# +include+. For 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.
#
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
# Create a new timer with the argument timeout, and add the
# current thread to the list of waiters. Then the thread is
# stopped. It will be resumed when a corresponding #signal
# occurs.
def wait(timeout = nil)
@monitor.instance_eval {mon_check_owner()}
timer = create_timer(timeout)
Thread.critical = true
count = @monitor.instance_eval {mon_exit_for_cond()}
@waiters.push(Thread.current)
begin
Thread.stop
return true
rescue Timeout
return false
ensure
Thread.critical = true
begin
if timer && timer.alive?
Thread.kill(timer)
end
if @waiters.include?(Thread.current) # interrupted?
@waiters.delete(Thread.current)
end
@monitor.instance_eval {mon_enter_for_cond(count)}
ensure
Thread.critical = false
end
end
end
# call #wait while the supplied block returns +true+.
def wait_while
while yield
wait
end
end
# call #wait until the supplied block returns +true+.
def wait_until
until yield
wait
end
end
# Wake up and run the next waiter
def signal
@monitor.instance_eval {mon_check_owner()}
Thread.critical = true
t = @waiters.shift
t.wakeup if t
Thread.critical = false
Thread.pass
end
# Wake up all the waiters.
def broadcast
@monitor.instance_eval {mon_check_owner()}
Thread.critical = true
for t in @waiters
t.wakeup
end
@waiters.clear
Thread.critical = false
Thread.pass
end
def count_waiters
return @waiters.length
end
private
def initialize(monitor)
@monitor = monitor
@waiters = []
end
def create_timer(timeout)
if timeout
waiter = Thread.current
return Thread.start {
Thread.pass
sleep(timeout)
Thread.critical = true
waiter.raise(Timeout.new)
}
else
return nil
end
end
end
def self.extend_object(obj)
super(obj)
obj.instance_eval {mon_initialize()}
end
#
# Attempts to enter exclusive section. Returns +false+ if lock fails.
#
def mon_try_enter
result = false
Thread.critical = true
if @mon_owner.nil?
@mon_owner = Thread.current
end
if @mon_owner == Thread.current
@mon_count += 1
result = true
end
Thread.critical = false
return result
end
# For backward compatibility
alias try_mon_enter mon_try_enter
#
# Enters exclusive section.
#
def mon_enter
Thread.critical = true
mon_acquire(@mon_entering_queue)
@mon_count += 1
ensure
Thread.critical = false
end
#
# Leaves exclusive section.
#
def mon_exit
mon_check_owner
Thread.critical = true
@mon_count -= 1
if @mon_count == 0
mon_release
end
Thread.critical = false
Thread.pass
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
#
# FIXME: This isn't documented in Nutshell.
#
# Create a new condition variable for this monitor.
# This facilitates control of the monitor with #signal and #wait.
#
def new_cond
return ConditionVariable.new(self)
end
private
def initialize(*args)
super
mon_initialize
end
# called by initialize method to set defaults for instance variables.
def mon_initialize
@mon_owner = nil
@mon_count = 0
@mon_entering_queue = []
@mon_waiting_queue = []
end
# Throw a ThreadError exception if the current thread
# does't own the monitor
def mon_check_owner
if @mon_owner != Thread.current
raise ThreadError, "current thread not owner"
end
end
def mon_acquire(queue)
while @mon_owner && @mon_owner != Thread.current
queue.push(Thread.current)
Thread.stop
Thread.critical = true
end
@mon_owner = Thread.current
end
# mon_release requires Thread.critical == true
def mon_release
@mon_owner = nil
while t = @mon_waiting_queue.shift || @mon_entering_queue.shift
if t.alive?
t.wakeup
return
end
end
end
def mon_enter_for_cond(count)
mon_acquire(@mon_waiting_queue)
@mon_count = count
end
def mon_exit_for_cond
count = @mon_count
@mon_count = 0
return count
ensure
mon_release
end
end
# Monitors provide means of mutual exclusion for Thread programming.
# A critical region is created by means of the synchronize method,
# which takes a block.
# The condition variables (created with #new_cond) may be used
# to control the execution of a monitor with #signal and #wait.
#
# the Monitor class wraps MonitorMixin, and provides aliases
# alias try_enter try_mon_enter
# alias enter mon_enter
# alias exit mon_exit
# to access its methods more concisely.
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:
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