lib/concurrent-ruby/concurrent/exchanger.rb
require 'concurrent/constants'
require 'concurrent/errors'
require 'concurrent/maybe'
require 'concurrent/atomic/atomic_reference'
require 'concurrent/atomic/count_down_latch'
require 'concurrent/utility/engine'
require 'concurrent/utility/monotonic_time'
module Concurrent
# @!macro exchanger
#
# A synchronization point at which threads can pair and swap elements within
# pairs. Each thread presents some object on entry to the exchange method,
# matches with a partner thread, and receives its partner's object on return.
#
# @!macro thread_safe_variable_comparison
#
# This implementation is very simple, using only a single slot for each
# exchanger (unlike more advanced implementations which use an "arena").
# This approach will work perfectly fine when there are only a few threads
# accessing a single `Exchanger`. Beyond a handful of threads the performance
# will degrade rapidly due to contention on the single slot, but the algorithm
# will remain correct.
#
# @see http://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Exchanger.html java.util.concurrent.Exchanger
# @example
#
# exchanger = Concurrent::Exchanger.new
#
# threads = [
# Thread.new { puts "first: " << exchanger.exchange('foo', 1) }, #=> "first: bar"
# Thread.new { puts "second: " << exchanger.exchange('bar', 1) } #=> "second: foo"
# ]
# threads.each {|t| t.join(2) }
# @!visibility private
class AbstractExchanger < Synchronization::Object
# @!visibility private
CANCEL = ::Object.new
private_constant :CANCEL
def initialize
super
end
# @!macro exchanger_method_do_exchange
#
# Waits for another thread to arrive at this exchange point (unless the
# current thread is interrupted), and then transfers the given object to
# it, receiving its object in return. The timeout value indicates the
# approximate number of seconds the method should block while waiting
# for the exchange. When the timeout value is `nil` the method will
# block indefinitely.
#
# @param [Object] value the value to exchange with another thread
# @param [Numeric, nil] timeout in seconds, `nil` blocks indefinitely
#
# @!macro exchanger_method_exchange
#
# In some edge cases when a `timeout` is given a return value of `nil` may be
# ambiguous. Specifically, if `nil` is a valid value in the exchange it will
# be impossible to tell whether `nil` is the actual return value or if it
# signifies timeout. When `nil` is a valid value in the exchange consider
# using {#exchange!} or {#try_exchange} instead.
#
# @return [Object] the value exchanged by the other thread or `nil` on timeout
def exchange(value, timeout = nil)
(value = do_exchange(value, timeout)) == CANCEL ? nil : value
end
# @!macro exchanger_method_do_exchange
# @!macro exchanger_method_exchange_bang
#
# On timeout a {Concurrent::TimeoutError} exception will be raised.
#
# @return [Object] the value exchanged by the other thread
# @raise [Concurrent::TimeoutError] on timeout
def exchange!(value, timeout = nil)
if (value = do_exchange(value, timeout)) == CANCEL
raise Concurrent::TimeoutError
else
value
end
end
# @!macro exchanger_method_do_exchange
# @!macro exchanger_method_try_exchange
#
# The return value will be a {Concurrent::Maybe} set to `Just` on success or
# `Nothing` on timeout.
#
# @return [Concurrent::Maybe] on success a `Just` maybe will be returned with
# the item exchanged by the other thread as `#value`; on timeout a
# `Nothing` maybe will be returned with {Concurrent::TimeoutError} as `#reason`
#
# @example
#
# exchanger = Concurrent::Exchanger.new
#
# result = exchanger.exchange(:foo, 0.5)
#
# if result.just?
# puts result.value #=> :bar
# else
# puts 'timeout'
# end
def try_exchange(value, timeout = nil)
if (value = do_exchange(value, timeout)) == CANCEL
Concurrent::Maybe.nothing(Concurrent::TimeoutError)
else
Concurrent::Maybe.just(value)
end
end
private
# @!macro exchanger_method_do_exchange
#
# @return [Object, CANCEL] the value exchanged by the other thread; {CANCEL} on timeout
def do_exchange(value, timeout)
raise NotImplementedError
end
end
# @!macro internal_implementation_note
# @!visibility private
class RubyExchanger < AbstractExchanger
# A simplified version of java.util.concurrent.Exchanger written by
# Doug Lea, Bill Scherer, and Michael Scott with assistance from members
# of JCP JSR-166 Expert Group and released to the public domain. It does
# not include the arena or the multi-processor spin loops.
# http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/6-b14/java/util/concurrent/Exchanger.java
safe_initialization!
class Node < Concurrent::Synchronization::Object
attr_atomic :value
safe_initialization!
def initialize(item)
super()
@Item = item
@Latch = Concurrent::CountDownLatch.new
self.value = nil
end
def latch
@Latch
end
def item
@Item
end
end
private_constant :Node
def initialize
super
end
private
attr_atomic(:slot)
# @!macro exchanger_method_do_exchange
#
# @return [Object, CANCEL] the value exchanged by the other thread; {CANCEL} on timeout
def do_exchange(value, timeout)
# ALGORITHM
#
# From the original Java version:
#
# > The basic idea is to maintain a "slot", which is a reference to
# > a Node containing both an Item to offer and a "hole" waiting to
# > get filled in. If an incoming "occupying" thread sees that the
# > slot is null, it CAS'es (compareAndSets) a Node there and waits
# > for another to invoke exchange. That second "fulfilling" thread
# > sees that the slot is non-null, and so CASes it back to null,
# > also exchanging items by CASing the hole, plus waking up the
# > occupying thread if it is blocked. In each case CAS'es may
# > fail because a slot at first appears non-null but is null upon
# > CAS, or vice-versa. So threads may need to retry these
# > actions.
#
# This version:
#
# An exchange occurs between an "occupier" thread and a "fulfiller" thread.
# The "slot" is used to setup this interaction. The first thread in the
# exchange puts itself into the slot (occupies) and waits for a fulfiller.
# The second thread removes the occupier from the slot and attempts to
# perform the exchange. Removing the occupier also frees the slot for
# another occupier/fulfiller pair.
#
# Because the occupier and the fulfiller are operating independently and
# because there may be contention with other threads, any failed operation
# indicates contention. Both the occupier and the fulfiller operate within
# spin loops. Any failed actions along the happy path will cause the thread
# to repeat the loop and try again.
#
# When a timeout value is given the thread must be cognizant of time spent
# in the spin loop. The remaining time is checked every loop. When the time
# runs out the thread will exit.
#
# A "node" is the data structure used to perform the exchange. Only the
# occupier's node is necessary. It's the node used for the exchange.
# Each node has an "item," a "hole" (self), and a "latch." The item is the
# node's initial value. It never changes. It's what the fulfiller returns on
# success. The occupier's hole is where the fulfiller put its item. It's the
# item that the occupier returns on success. The latch is used for synchronization.
# Because a thread may act as either an occupier or fulfiller (or possibly
# both in periods of high contention) every thread creates a node when
# the exchange method is first called.
#
# The following steps occur within the spin loop. If any actions fail
# the thread will loop and try again, so long as there is time remaining.
# If time runs out the thread will return CANCEL.
#
# Check the slot for an occupier:
#
# * If the slot is empty try to occupy
# * If the slot is full try to fulfill
#
# Attempt to occupy:
#
# * Attempt to CAS myself into the slot
# * Go to sleep and wait to be woken by a fulfiller
# * If the sleep is successful then the fulfiller completed its happy path
# - Return the value from my hole (the value given by the fulfiller)
# * When the sleep fails (time ran out) attempt to cancel the operation
# - Attempt to CAS myself out of the hole
# - If successful there is no contention
# - Return CANCEL
# - On failure, I am competing with a fulfiller
# - Attempt to CAS my hole to CANCEL
# - On success
# - Let the fulfiller deal with my cancel
# - Return CANCEL
# - On failure the fulfiller has completed its happy path
# - Return th value from my hole (the fulfiller's value)
#
# Attempt to fulfill:
#
# * Attempt to CAS the occupier out of the slot
# - On failure loop again
# * Attempt to CAS my item into the occupier's hole
# - On failure the occupier is trying to cancel
# - Loop again
# - On success we are on the happy path
# - Wake the sleeping occupier
# - Return the occupier's item
value = NULL if value.nil? # The sentinel allows nil to be a valid value
me = Node.new(value) # create my node in case I need to occupy
end_at = Concurrent.monotonic_time + timeout.to_f # The time to give up
result = loop do
other = slot
if other && compare_and_set_slot(other, nil)
# try to fulfill
if other.compare_and_set_value(nil, value)
# happy path
other.latch.count_down
break other.item
end
elsif other.nil? && compare_and_set_slot(nil, me)
# try to occupy
timeout = end_at - Concurrent.monotonic_time if timeout
if me.latch.wait(timeout)
# happy path
break me.value
else
# attempt to remove myself from the slot
if compare_and_set_slot(me, nil)
break CANCEL
elsif !me.compare_and_set_value(nil, CANCEL)
# I've failed to block the fulfiller
break me.value
end
end
end
break CANCEL if timeout && Concurrent.monotonic_time >= end_at
end
result == NULL ? nil : result
end
end
if Concurrent.on_jruby?
require 'concurrent/utility/native_extension_loader'
# @!macro internal_implementation_note
# @!visibility private
class JavaExchanger < AbstractExchanger
def initialize
@exchanger = java.util.concurrent.Exchanger.new
end
private
# @!macro exchanger_method_do_exchange
#
# @return [Object, CANCEL] the value exchanged by the other thread; {CANCEL} on timeout
def do_exchange(value, timeout)
result = nil
if timeout.nil?
Synchronization::JRuby.sleep_interruptibly do
result = @exchanger.exchange(value)
end
else
Synchronization::JRuby.sleep_interruptibly do
result = @exchanger.exchange(value, 1000 * timeout, java.util.concurrent.TimeUnit::MILLISECONDS)
end
end
result
rescue java.util.concurrent.TimeoutException
CANCEL
end
end
end
# @!visibility private
# @!macro internal_implementation_note
ExchangerImplementation = case
when Concurrent.on_jruby?
JavaExchanger
else
RubyExchanger
end
private_constant :ExchangerImplementation
# @!macro exchanger
class Exchanger < ExchangerImplementation
# @!method initialize
# Creates exchanger instance
# @!method exchange(value, timeout = nil)
# @!macro exchanger_method_do_exchange
# @!macro exchanger_method_exchange
# @!method exchange!(value, timeout = nil)
# @!macro exchanger_method_do_exchange
# @!macro exchanger_method_exchange_bang
# @!method try_exchange(value, timeout = nil)
# @!macro exchanger_method_do_exchange
# @!macro exchanger_method_try_exchange
end
end