lib/concurrent-ruby/concurrent/promise.rb
require 'thread'
require 'concurrent/constants'
require 'concurrent/errors'
require 'concurrent/ivar'
require 'concurrent/executor/safe_task_executor'
require 'concurrent/options'
module Concurrent
PromiseExecutionError = Class.new(StandardError)
# Promises are inspired by the JavaScript [Promises/A](http://wiki.commonjs.org/wiki/Promises/A)
# and [Promises/A+](http://promises-aplus.github.io/promises-spec/) specifications.
#
# > A promise represents the eventual value returned from the single
# > completion of an operation.
#
# Promises are similar to futures and share many of the same behaviours.
# Promises are far more robust, however. Promises can be chained in a tree
# structure where each promise may have zero or more children. Promises are
# chained using the `then` method. The result of a call to `then` is always
# another promise. Promises are resolved asynchronously (with respect to the
# main thread) but in a strict order: parents are guaranteed to be resolved
# before their children, children before their younger siblings. The `then`
# method takes two parameters: an optional block to be executed upon parent
# resolution and an optional callable to be executed upon parent failure. The
# result of each promise is passed to each of its children upon resolution.
# When a promise is rejected all its children will be summarily rejected and
# will receive the reason.
#
# Promises have several possible states: *:unscheduled*, *:pending*,
# *:processing*, *:rejected*, or *:fulfilled*. These are also aggregated as
# `#incomplete?` and `#complete?`. When a Promise is created it is set to
# *:unscheduled*. Once the `#execute` method is called the state becomes
# *:pending*. Once a job is pulled from the thread pool's queue and is given
# to a thread for processing (often immediately upon `#post`) the state
# becomes *:processing*. The future will remain in this state until processing
# is complete. A future that is in the *:unscheduled*, *:pending*, or
# *:processing* is considered `#incomplete?`. A `#complete?` Promise is either
# *:rejected*, indicating that an exception was thrown during processing, or
# *:fulfilled*, indicating success. If a Promise is *:fulfilled* its `#value`
# will be updated to reflect the result of the operation. If *:rejected* the
# `reason` will be updated with a reference to the thrown exception. The
# predicate methods `#unscheduled?`, `#pending?`, `#rejected?`, and
# `#fulfilled?` can be called at any time to obtain the state of the Promise,
# as can the `#state` method, which returns a symbol.
#
# Retrieving the value of a promise is done through the `value` (alias:
# `deref`) method. Obtaining the value of a promise is a potentially blocking
# operation. When a promise is *rejected* a call to `value` will return `nil`
# immediately. When a promise is *fulfilled* a call to `value` will
# immediately return the current value. When a promise is *pending* a call to
# `value` will block until the promise is either *rejected* or *fulfilled*. A
# *timeout* value can be passed to `value` to limit how long the call will
# block. If `nil` the call will block indefinitely. If `0` the call will not
# block. Any other integer or float value will indicate the maximum number of
# seconds to block.
#
# Promises run on the global thread pool.
#
# @!macro copy_options
#
# ### Examples
#
# Start by requiring promises
#
# ```ruby
# require 'concurrent/promise'
# ```
#
# Then create one
#
# ```ruby
# p = Concurrent::Promise.execute do
# # do something
# 42
# end
# ```
#
# Promises can be chained using the `then` method. The `then` method accepts a
# block and an executor, to be executed on fulfillment, and a callable argument to be executed
# on rejection. The result of the each promise is passed as the block argument
# to chained promises.
#
# ```ruby
# p = Concurrent::Promise.new{10}.then{|x| x * 2}.then{|result| result - 10 }.execute
# ```
#
# And so on, and so on, and so on...
#
# ```ruby
# p = Concurrent::Promise.fulfill(20).
# then{|result| result - 10 }.
# then{|result| result * 3 }.
# then(executor: different_executor){|result| result % 5 }.execute
# ```
#
# The initial state of a newly created Promise depends on the state of its parent:
# - if parent is *unscheduled* the child will be *unscheduled*
# - if parent is *pending* the child will be *pending*
# - if parent is *fulfilled* the child will be *pending*
# - if parent is *rejected* the child will be *pending* (but will ultimately be *rejected*)
#
# Promises are executed asynchronously from the main thread. By the time a
# child Promise finishes intialization it may be in a different state than its
# parent (by the time a child is created its parent may have completed
# execution and changed state). Despite being asynchronous, however, the order
# of execution of Promise objects in a chain (or tree) is strictly defined.
#
# There are multiple ways to create and execute a new `Promise`. Both ways
# provide identical behavior:
#
# ```ruby
# # create, operate, then execute
# p1 = Concurrent::Promise.new{ "Hello World!" }
# p1.state #=> :unscheduled
# p1.execute
#
# # create and immediately execute
# p2 = Concurrent::Promise.new{ "Hello World!" }.execute
#
# # execute during creation
# p3 = Concurrent::Promise.execute{ "Hello World!" }
# ```
#
# Once the `execute` method is called a `Promise` becomes `pending`:
#
# ```ruby
# p = Concurrent::Promise.execute{ "Hello, world!" }
# p.state #=> :pending
# p.pending? #=> true
# ```
#
# Wait a little bit, and the promise will resolve and provide a value:
#
# ```ruby
# p = Concurrent::Promise.execute{ "Hello, world!" }
# sleep(0.1)
#
# p.state #=> :fulfilled
# p.fulfilled? #=> true
# p.value #=> "Hello, world!"
# ```
#
# If an exception occurs, the promise will be rejected and will provide
# a reason for the rejection:
#
# ```ruby
# p = Concurrent::Promise.execute{ raise StandardError.new("Here comes the Boom!") }
# sleep(0.1)
#
# p.state #=> :rejected
# p.rejected? #=> true
# p.reason #=> "#<StandardError: Here comes the Boom!>"
# ```
#
# #### Rejection
#
# When a promise is rejected all its children will be rejected and will
# receive the rejection `reason` as the rejection callable parameter:
#
# ```ruby
# p = Concurrent::Promise.execute { Thread.pass; raise StandardError }
#
# c1 = p.then(-> reason { 42 })
# c2 = p.then(-> reason { raise 'Boom!' })
#
# c1.wait.state #=> :fulfilled
# c1.value #=> 45
# c2.wait.state #=> :rejected
# c2.reason #=> #<RuntimeError: Boom!>
# ```
#
# Once a promise is rejected it will continue to accept children that will
# receive immediately rejection (they will be executed asynchronously).
#
# #### Aliases
#
# The `then` method is the most generic alias: it accepts a block to be
# executed upon parent fulfillment and a callable to be executed upon parent
# rejection. At least one of them should be passed. The default block is `{
# |result| result }` that fulfills the child with the parent value. The
# default callable is `{ |reason| raise reason }` that rejects the child with
# the parent reason.
#
# - `on_success { |result| ... }` is the same as `then {|result| ... }`
# - `rescue { |reason| ... }` is the same as `then(Proc.new { |reason| ... } )`
# - `rescue` is aliased by `catch` and `on_error`
class Promise < IVar
# Initialize a new Promise with the provided options.
#
# @!macro executor_and_deref_options
#
# @!macro promise_init_options
#
# @option opts [Promise] :parent the parent `Promise` when building a chain/tree
# @option opts [Proc] :on_fulfill fulfillment handler
# @option opts [Proc] :on_reject rejection handler
# @option opts [object, Array] :args zero or more arguments to be passed
# the task block on execution
#
# @yield The block operation to be performed asynchronously.
#
# @raise [ArgumentError] if no block is given
#
# @see http://wiki.commonjs.org/wiki/Promises/A
# @see http://promises-aplus.github.io/promises-spec/
def initialize(opts = {}, &block)
opts.delete_if { |k, v| v.nil? }
super(NULL, opts.merge(__promise_body_from_block__: block), &nil)
end
# Create a new `Promise` and fulfill it immediately.
#
# @!macro executor_and_deref_options
#
# @!macro promise_init_options
#
# @raise [ArgumentError] if no block is given
#
# @return [Promise] the newly created `Promise`
def self.fulfill(value, opts = {})
Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, true, value, nil) }
end
# Create a new `Promise` and reject it immediately.
#
# @!macro executor_and_deref_options
#
# @!macro promise_init_options
#
# @raise [ArgumentError] if no block is given
#
# @return [Promise] the newly created `Promise`
def self.reject(reason, opts = {})
Promise.new(opts).tap { |p| p.send(:synchronized_set_state!, false, nil, reason) }
end
# Execute an `:unscheduled` `Promise`. Immediately sets the state to `:pending` and
# passes the block to a new thread/thread pool for eventual execution.
# Does nothing if the `Promise` is in any state other than `:unscheduled`.
#
# @return [Promise] a reference to `self`
def execute
if root?
if compare_and_set_state(:pending, :unscheduled)
set_pending
realize(@promise_body)
end
else
compare_and_set_state(:pending, :unscheduled)
@parent.execute
end
self
end
# @!macro ivar_set_method
#
# @raise [Concurrent::PromiseExecutionError] if not the root promise
def set(value = NULL, &block)
raise PromiseExecutionError.new('supported only on root promise') unless root?
check_for_block_or_value!(block_given?, value)
synchronize do
if @state != :unscheduled
raise MultipleAssignmentError
else
@promise_body = block || Proc.new { |result| value }
end
end
execute
end
# @!macro ivar_fail_method
#
# @raise [Concurrent::PromiseExecutionError] if not the root promise
def fail(reason = StandardError.new)
set { raise reason }
end
# Create a new `Promise` object with the given block, execute it, and return the
# `:pending` object.
#
# @!macro executor_and_deref_options
#
# @!macro promise_init_options
#
# @return [Promise] the newly created `Promise` in the `:pending` state
#
# @raise [ArgumentError] if no block is given
#
# @example
# promise = Concurrent::Promise.execute{ sleep(1); 42 }
# promise.state #=> :pending
def self.execute(opts = {}, &block)
new(opts, &block).execute
end
# Chain a new promise off the current promise.
#
# @return [Promise] the new promise
# @yield The block operation to be performed asynchronously.
# @overload then(rescuer, executor, &block)
# @param [Proc] rescuer An optional rescue block to be executed if the
# promise is rejected.
# @param [ThreadPool] executor An optional thread pool executor to be used
# in the new Promise
# @overload then(rescuer, executor: executor, &block)
# @param [Proc] rescuer An optional rescue block to be executed if the
# promise is rejected.
# @param [ThreadPool] executor An optional thread pool executor to be used
# in the new Promise
def then(*args, &block)
if args.last.is_a?(::Hash)
executor = args.pop[:executor]
rescuer = args.first
else
rescuer, executor = args
end
executor ||= @executor
raise ArgumentError.new('rescuers and block are both missing') if rescuer.nil? && !block_given?
block = Proc.new { |result| result } unless block_given?
child = Promise.new(
parent: self,
executor: executor,
on_fulfill: block,
on_reject: rescuer
)
synchronize do
child.state = :pending if @state == :pending
child.on_fulfill(apply_deref_options(@value)) if @state == :fulfilled
child.on_reject(@reason) if @state == :rejected
@children << child
end
child
end
# Chain onto this promise an action to be undertaken on success
# (fulfillment).
#
# @yield The block to execute
#
# @return [Promise] self
def on_success(&block)
raise ArgumentError.new('no block given') unless block_given?
self.then(&block)
end
# Chain onto this promise an action to be undertaken on failure
# (rejection).
#
# @yield The block to execute
#
# @return [Promise] self
def rescue(&block)
self.then(block)
end
alias_method :catch, :rescue
alias_method :on_error, :rescue
# Yield the successful result to the block that returns a promise. If that
# promise is also successful the result is the result of the yielded promise.
# If either part fails the whole also fails.
#
# @example
# Promise.execute { 1 }.flat_map { |v| Promise.execute { v + 2 } }.value! #=> 3
#
# @return [Promise]
def flat_map(&block)
child = Promise.new(
parent: self,
executor: ImmediateExecutor.new,
)
on_error { |e| child.on_reject(e) }
on_success do |result1|
begin
inner = block.call(result1)
inner.execute
inner.on_success { |result2| child.on_fulfill(result2) }
inner.on_error { |e| child.on_reject(e) }
rescue => e
child.on_reject(e)
end
end
child
end
# Builds a promise that produces the result of promises in an Array
# and fails if any of them fails.
#
# @overload zip(*promises)
# @param [Array<Promise>] promises
#
# @overload zip(*promises, opts)
# @param [Array<Promise>] promises
# @param [Hash] opts the configuration options
# @option opts [Executor] :executor (ImmediateExecutor.new) when set use the given `Executor` instance.
# @option opts [Boolean] :execute (true) execute promise before returning
#
# @return [Promise<Array>]
def self.zip(*promises)
opts = promises.last.is_a?(::Hash) ? promises.pop.dup : {}
opts[:executor] ||= ImmediateExecutor.new
zero = if !opts.key?(:execute) || opts.delete(:execute)
fulfill([], opts)
else
Promise.new(opts) { [] }
end
promises.reduce(zero) do |p1, p2|
p1.flat_map do |results|
p2.then do |next_result|
results << next_result
end
end
end
end
# Builds a promise that produces the result of self and others in an Array
# and fails if any of them fails.
#
# @overload zip(*promises)
# @param [Array<Promise>] others
#
# @overload zip(*promises, opts)
# @param [Array<Promise>] others
# @param [Hash] opts the configuration options
# @option opts [Executor] :executor (ImmediateExecutor.new) when set use the given `Executor` instance.
# @option opts [Boolean] :execute (true) execute promise before returning
#
# @return [Promise<Array>]
def zip(*others)
self.class.zip(self, *others)
end
# Aggregates a collection of promises and executes the `then` condition
# if all aggregated promises succeed. Executes the `rescue` handler with
# a `Concurrent::PromiseExecutionError` if any of the aggregated promises
# fail. Upon execution will execute any of the aggregate promises that
# were not already executed.
#
# @!macro promise_self_aggregate
#
# The returned promise will not yet have been executed. Additional `#then`
# and `#rescue` handlers may still be provided. Once the returned promise
# is execute the aggregate promises will be also be executed (if they have
# not been executed already). The results of the aggregate promises will
# be checked upon completion. The necessary `#then` and `#rescue` blocks
# on the aggregating promise will then be executed as appropriate. If the
# `#rescue` handlers are executed the raises exception will be
# `Concurrent::PromiseExecutionError`.
#
# @param [Array] promises Zero or more promises to aggregate
# @return [Promise] an unscheduled (not executed) promise that aggregates
# the promises given as arguments
def self.all?(*promises)
aggregate(:all?, *promises)
end
# Aggregates a collection of promises and executes the `then` condition
# if any aggregated promises succeed. Executes the `rescue` handler with
# a `Concurrent::PromiseExecutionError` if any of the aggregated promises
# fail. Upon execution will execute any of the aggregate promises that
# were not already executed.
#
# @!macro promise_self_aggregate
def self.any?(*promises)
aggregate(:any?, *promises)
end
protected
def ns_initialize(value, opts)
super
@executor = Options.executor_from_options(opts) || Concurrent.global_io_executor
@args = get_arguments_from(opts)
@parent = opts.fetch(:parent) { nil }
@on_fulfill = opts.fetch(:on_fulfill) { Proc.new { |result| result } }
@on_reject = opts.fetch(:on_reject) { Proc.new { |reason| raise reason } }
@promise_body = opts[:__promise_body_from_block__] || Proc.new { |result| result }
@state = :unscheduled
@children = []
end
# Aggregate a collection of zero or more promises under a composite promise,
# execute the aggregated promises and collect them into a standard Ruby array,
# call the given Ruby `Ennnumerable` predicate (such as `any?`, `all?`, `none?`,
# or `one?`) on the collection checking for the success or failure of each,
# then executing the composite's `#then` handlers if the predicate returns
# `true` or executing the composite's `#rescue` handlers if the predicate
# returns false.
#
# @!macro promise_self_aggregate
def self.aggregate(method, *promises)
composite = Promise.new do
completed = promises.collect do |promise|
promise.execute if promise.unscheduled?
promise.wait
promise
end
unless completed.empty? || completed.send(method){|promise| promise.fulfilled? }
raise PromiseExecutionError
end
end
composite
end
# @!visibility private
def set_pending
synchronize do
@state = :pending
@children.each { |c| c.set_pending }
end
end
# @!visibility private
def root? # :nodoc:
@parent.nil?
end
# @!visibility private
def on_fulfill(result)
realize Proc.new { @on_fulfill.call(result) }
nil
end
# @!visibility private
def on_reject(reason)
realize Proc.new { @on_reject.call(reason) }
nil
end
# @!visibility private
def notify_child(child)
if_state(:fulfilled) { child.on_fulfill(apply_deref_options(@value)) }
if_state(:rejected) { child.on_reject(@reason) }
end
# @!visibility private
def complete(success, value, reason)
children_to_notify = synchronize do
set_state!(success, value, reason)
@children.dup
end
children_to_notify.each { |child| notify_child(child) }
observers.notify_and_delete_observers{ [Time.now, self.value, reason] }
end
# @!visibility private
def realize(task)
@executor.post do
success, value, reason = SafeTaskExecutor.new(task, rescue_exception: true).execute(*@args)
complete(success, value, reason)
end
end
# @!visibility private
def set_state!(success, value, reason)
set_state(success, value, reason)
event.set
end
# @!visibility private
def synchronized_set_state!(success, value, reason)
synchronize { set_state!(success, value, reason) }
end
end
end