require 'concurrent/configuration'
require 'concurrent/ivar'
require 'concurrent/synchronization/lockable_object'
module Concurrent
# A mixin module that provides simple asynchronous behavior to a class,
# turning it into a simple actor. Loosely based on Erlang's
# [gen_server](http://www.erlang.org/doc/man/gen_server.html), but without
# supervision or linking.
#
# A more feature-rich {Concurrent::Actor} is also available when the
# capabilities of `Async` are too limited.
#
# ```cucumber
# Feature:
# As a stateful, plain old Ruby class
# I want safe, asynchronous behavior
# So my long-running methods don't block the main thread
# ```
#
# The `Async` module is a way to mix simple yet powerful asynchronous
# capabilities into any plain old Ruby object or class, turning each object
# into a simple Actor. Method calls are processed on a background thread. The
# caller is free to perform other actions while processing occurs in the
# background.
#
# Method calls to the asynchronous object are made via two proxy methods:
# `async` (alias `cast`) and `await` (alias `call`). These proxy methods post
# the method call to the object's background thread and return a "future"
# which will eventually contain the result of the method call.
#
# This behavior is loosely patterned after Erlang's `gen_server` behavior.
# When an Erlang module implements the `gen_server` behavior it becomes
# inherently asynchronous. The `start` or `start_link` function spawns a
# process (similar to a thread but much more lightweight and efficient) and
# returns the ID of the process. Using the process ID, other processes can
# send messages to the `gen_server` via the `cast` and `call` methods. Unlike
# Erlang's `gen_server`, however, `Async` classes do not support linking or
# supervision trees.
#
# ## Basic Usage
#
# When this module is mixed into a class, objects of the class become inherently
# asynchronous. Each object gets its own background thread on which to post
# asynchronous method calls. Asynchronous method calls are executed in the
# background one at a time in the order they are received.
#
# To create an asynchronous class, simply mix in the `Concurrent::Async` module:
#
# ```
# class Hello
# include Concurrent::Async
#
# def hello(name)
# "Hello, #{name}!"
# end
# end
# ```
#
# Mixing this module into a class provides each object two proxy methods:
# `async` and `await`. These methods are thread safe with respect to the
# enclosing object. The former proxy allows methods to be called
# asynchronously by posting to the object's internal thread. The latter proxy
# allows a method to be called synchronously but does so safely with respect
# to any pending asynchronous method calls and ensures proper ordering. Both
# methods return a {Concurrent::IVar} which can be inspected for the result
# of the proxied method call. Calling a method with `async` will return a
# `:pending` `IVar` whereas `await` will return a `:complete` `IVar`.
#
# ```
# class Echo
# include Concurrent::Async
#
# def echo(msg)
# print "#{msg}\n"
# end
# end
#
# horn = Echo.new
# horn.echo('zero') # synchronous, not thread-safe
# # returns the actual return value of the method
#
# horn.async.echo('one') # asynchronous, non-blocking, thread-safe
# # returns an IVar in the :pending state
#
# horn.await.echo('two') # synchronous, blocking, thread-safe
# # returns an IVar in the :complete state
# ```
#
# ## Let It Fail
#
# The `async` and `await` proxy methods have built-in error protection based
# on Erlang's famous "let it fail" philosophy. Instance methods should not be
# programmed defensively. When an exception is raised by a delegated method
# the proxy will rescue the exception, expose it to the caller as the `reason`
# attribute of the returned future, then process the next method call.
#
# ## Calling Methods Internally
#
# External method calls should *always* use the `async` and `await` proxy
# methods. When one method calls another method, the `async` proxy should
# rarely be used and the `await` proxy should *never* be used.
#
# When an object calls one of its own methods using the `await` proxy the
# second call will be enqueued *behind* the currently running method call.
# Any attempt to wait on the result will fail as the second call will never
# run until after the current call completes.
#
# Calling a method using the `await` proxy from within a method that was
# itself called using `async` or `await` will irreversibly deadlock the
# object. Do *not* do this, ever.
#
# ## Instance Variables and Attribute Accessors
#
# Instance variables do not need to be thread-safe so long as they are private.
# Asynchronous method calls are processed in the order they are received and
# are processed one at a time. Therefore private instance variables can only
# be accessed by one thread at a time. This is inherently thread-safe.
#
# When using private instance variables within asynchronous methods, the best
# practice is to read the instance variable into a local variable at the start
# of the method then update the instance variable at the *end* of the method.
# This way, should an exception be raised during method execution the internal
# state of the object will not have been changed.
#
# ### Reader Attributes
#
# The use of `attr_reader` is discouraged. Internal state exposed externally,
# when necessary, should be done through accessor methods. The instance
# variables exposed by these methods *must* be thread-safe, or they must be
# called using the `async` and `await` proxy methods. These two approaches are
# subtly different.
#
# When internal state is accessed via the `async` and `await` proxy methods,
# the returned value represents the object's state *at the time the call is
# processed*, which may *not* be the state of the object at the time the call
# is made.
#
# To get the state *at the current* time, irrespective of an enqueued method
# calls, a reader method must be called directly. This is inherently unsafe
# unless the instance variable is itself thread-safe, preferably using one
# of the thread-safe classes within this library. Because the thread-safe
# classes within this library are internally-locking or non-locking, they can
# be safely used from within asynchronous methods without causing deadlocks.
#
# Generally speaking, the best practice is to *not* expose internal state via
# reader methods. The best practice is to simply use the method's return value.
#
# ### Writer Attributes
#
# Writer attributes should never be used with asynchronous classes. Changing
# the state externally, even when done in the thread-safe way, is not logically
# consistent. Changes to state need to be timed with respect to all asynchronous
# method calls which my be in-process or enqueued. The only safe practice is to
# pass all necessary data to each method as arguments and let the method update
# the internal state as necessary.
#
# ## Class Constants, Variables, and Methods
#
# ### Class Constants
#
# Class constants do not need to be thread-safe. Since they are read-only and
# immutable they may be safely read both externally and from within
# asynchronous methods.
#
# ### Class Variables
#
# Class variables should be avoided. Class variables represent shared state.
# Shared state is anathema to concurrency. Should there be a need to share
# state using class variables they *must* be thread-safe, preferably
# using the thread-safe classes within this library. When updating class
# variables, never assign a new value/object to the variable itself. Assignment
# is not thread-safe in Ruby. Instead, use the thread-safe update functions
# of the variable itself to change the value.
#
# The best practice is to *never* use class variables with `Async` classes.
#
# ### Class Methods
#
# Class methods which are pure functions are safe. Class methods which modify
# class variables should be avoided, for all the reasons listed above.
#
# ## An Important Note About Thread Safe Guarantees
#
# > Thread safe guarantees can only be made when asynchronous method calls
# > are not mixed with direct method calls. Use only direct method calls
# > when the object is used exclusively on a single thread. Use only
# > `async` and `await` when the object is shared between threads. Once you
# > call a method using `async` or `await`, you should no longer call methods
# > directly on the object. Use `async` and `await` exclusively from then on.
#
# @example
#
# class Echo
# include Concurrent::Async
#
# def echo(msg)
# print "#{msg}\n"
# end
# end
#
# horn = Echo.new
# horn.echo('zero') # synchronous, not thread-safe
# # returns the actual return value of the method
#
# horn.async.echo('one') # asynchronous, non-blocking, thread-safe
# # returns an IVar in the :pending state
#
# horn.await.echo('two') # synchronous, blocking, thread-safe
# # returns an IVar in the :complete state
#
# @see Concurrent::Actor
# @see https://en.wikipedia.org/wiki/Actor_model "Actor Model" at Wikipedia
# @see http://www.erlang.org/doc/man/gen_server.html Erlang gen_server
# @see http://c2.com/cgi/wiki?LetItCrash "Let It Crash" at http://c2.com/
module Async
# @!method self.new(*args, &block)
#
# Instanciate a new object and ensure proper initialization of the
# synchronization mechanisms.
#
# @param [Array<Object>] args Zero or more arguments to be passed to the
# object's initializer.
# @param [Proc] block Optional block to pass to the object's initializer.
# @return [Object] A properly initialized object of the asynchronous class.
# Check for the presence of a method on an object and determine if a given
# set of arguments matches the required arity.
#
# @param [Object] obj the object to check against
# @param [Symbol] method the method to check the object for
# @param [Array] args zero or more arguments for the arity check
#
# @raise [NameError] the object does not respond to `method` method
# @raise [ArgumentError] the given `args` do not match the arity of `method`
#
# @note This check is imperfect because of the way Ruby reports the arity of
# methods with a variable number of arguments. It is possible to determine
# if too few arguments are given but impossible to determine if too many
# arguments are given. This check may also fail to recognize dynamic behavior
# of the object, such as methods simulated with `method_missing`.
#
# @see http://www.ruby-doc.org/core-2.1.1/Method.html#method-i-arity Method#arity
# @see http://ruby-doc.org/core-2.1.0/Object.html#method-i-respond_to-3F Object#respond_to?
# @see http://www.ruby-doc.org/core-2.1.0/BasicObject.html#method-i-method_missing BasicObject#method_missing
#
# @!visibility private
def self.validate_argc(obj, method, *args)
argc = args.length
arity = obj.method(method).arity
if arity >= 0 && argc != arity
raise ArgumentError.new("wrong number of arguments (#{argc} for #{arity})")
elsif arity < 0 && (arity = (arity + 1).abs) > argc
raise ArgumentError.new("wrong number of arguments (#{argc} for #{arity}..*)")
end
end
# @!visibility private
def self.included(base)
base.singleton_class.send(:alias_method, :original_new, :new)
base.extend(ClassMethods)
super(base)
end
# @!visibility private
module ClassMethods
def new(*args, &block)
obj = original_new(*args, &block)
obj.send(:init_synchronization)
obj
end
ruby2_keywords :new if respond_to?(:ruby2_keywords, true)
end
private_constant :ClassMethods
# Delegates asynchronous, thread-safe method calls to the wrapped object.
#
# @!visibility private
class AsyncDelegator < Synchronization::LockableObject
safe_initialization!
# Create a new delegator object wrapping the given delegate.
#
# @param [Object] delegate the object to wrap and delegate method calls to
def initialize(delegate)
super()
@delegate = delegate
@queue = []
@executor = Concurrent.global_io_executor
@ruby_pid = $$
end
# Delegates method calls to the wrapped object.
#
# @param [Symbol] method the method being called
# @param [Array] args zero or more arguments to the method
#
# @return [IVar] the result of the method call
#
# @raise [NameError] the object does not respond to `method` method
# @raise [ArgumentError] the given `args` do not match the arity of `method`
def method_missing(method, *args, &block)
super unless @delegate.respond_to?(method)
Async::validate_argc(@delegate, method, *args)
ivar = Concurrent::IVar.new
synchronize do
reset_if_forked
@queue.push [ivar, method, args, block]
@executor.post { perform } if @queue.length == 1
end
ivar
end
# Check whether the method is responsive
#
# @param [Symbol] method the method being called
def respond_to_missing?(method, include_private = false)
@delegate.respond_to?(method) || super
end
# Perform all enqueued tasks.
#
# This method must be called from within the executor. It must not be
# called while already running. It will loop until the queue is empty.
def perform
loop do
ivar, method, args, block = synchronize { @queue.first }
break unless ivar # queue is empty
begin
ivar.set(@delegate.send(method, *args, &block))
rescue => error
ivar.fail(error)
end
synchronize do
@queue.shift
return if @queue.empty?
end
end
end
def reset_if_forked
if $$ != @ruby_pid
@queue.clear
@ruby_pid = $$
end
end
end
private_constant :AsyncDelegator
# Delegates synchronous, thread-safe method calls to the wrapped object.
#
# @!visibility private
class AwaitDelegator
# Create a new delegator object wrapping the given delegate.
#
# @param [AsyncDelegator] delegate the object to wrap and delegate method calls to
def initialize(delegate)
@delegate = delegate
end
# Delegates method calls to the wrapped object.
#
# @param [Symbol] method the method being called
# @param [Array] args zero or more arguments to the method
#
# @return [IVar] the result of the method call
#
# @raise [NameError] the object does not respond to `method` method
# @raise [ArgumentError] the given `args` do not match the arity of `method`
def method_missing(method, *args, &block)
ivar = @delegate.send(method, *args, &block)
ivar.wait
ivar
end
# Check whether the method is responsive
#
# @param [Symbol] method the method being called
def respond_to_missing?(method, include_private = false)
@delegate.respond_to?(method) || super
end
end
private_constant :AwaitDelegator
# Causes the chained method call to be performed asynchronously on the
# object's thread. The delegated method will return a future in the
# `:pending` state and the method call will have been scheduled on the
# object's thread. The final disposition of the method call can be obtained
# by inspecting the returned future.
#
# @!macro async_thread_safety_warning
# @note The method call is guaranteed to be thread safe with respect to
# all other method calls against the same object that are called with
# either `async` or `await`. The mutable nature of Ruby references
# (and object orientation in general) prevent any other thread safety
# guarantees. Do NOT mix direct method calls with delegated method calls.
# Use *only* delegated method calls when sharing the object between threads.
#
# @return [Concurrent::IVar] the pending result of the asynchronous operation
#
# @raise [NameError] the object does not respond to the requested method
# @raise [ArgumentError] the given `args` do not match the arity of
# the requested method
def async
@__async_delegator__
end
alias_method :cast, :async
# Causes the chained method call to be performed synchronously on the
# current thread. The delegated will return a future in either the
# `:fulfilled` or `:rejected` state and the delegated method will have
# completed. The final disposition of the delegated method can be obtained
# by inspecting the returned future.
#
# @!macro async_thread_safety_warning
#
# @return [Concurrent::IVar] the completed result of the synchronous operation
#
# @raise [NameError] the object does not respond to the requested method
# @raise [ArgumentError] the given `args` do not match the arity of the
# requested method
def await
@__await_delegator__
end
alias_method :call, :await
# Initialize the internal serializer and other stnchronization mechanisms.
#
# @note This method *must* be called immediately upon object construction.
# This is the only way thread-safe initialization can be guaranteed.
#
# @!visibility private
def init_synchronization
return self if defined?(@__async_initialized__) && @__async_initialized__
@__async_initialized__ = true
@__async_delegator__ = AsyncDelegator.new(self)
@__await_delegator__ = AwaitDelegator.new(@__async_delegator__)
self
end
end
end