# frozen_string_literal: true
# Released under the MIT License.
# Copyright, 2017-2024, by Samuel Williams.
# Copyright, 2017, by Kent Gruber.
# Copyright, 2017, by Devin Christensen.
# Copyright, 2020, by Patrik Wenger.
# Copyright, 2023, by Math Ieu.
require 'fiber'
require_relative 'node'
require_relative 'condition'
module Async
# Raised when a task is explicitly stopped.
class Stop < Exception
class Later
def initialize(task)
@task = task
end
def alive?
true
end
def transfer
@task.stop
end
end
end
# Raised if a timeout occurs on a specific Fiber. Handled gracefully by `Task`.
# @public Since `stable-v1`.
class TimeoutError < StandardError
def initialize(message = "execution expired")
super
end
end
# @public Since `stable-v1`.
class Task < Node
class FinishedError < RuntimeError
def initialize(message = "Cannot create child task within a task that has finished execution!")
super
end
end
# @deprecated With no replacement.
def self.yield
Fiber.scheduler.transfer
end
# Create a new task.
# @parameter reactor [Reactor] the reactor this task will run within.
# @parameter parent [Task] the parent task.
def initialize(parent = Task.current?, finished: nil, **options, &block)
super(parent, **options)
# These instance variables are critical to the state of the task.
# In the initialized state, the @block should be set, but the @fiber should be nil.
# In the running state, the @fiber should be set.
# In a finished state, the @block should be nil, and the @fiber should be nil.
@block = block
@fiber = nil
@status = :initialized
@result = nil
@finished = finished
end
def reactor
self.root
end
def backtrace(*arguments)
@fiber&.backtrace(*arguments)
end
def annotate(annotation, &block)
if @fiber
@fiber.annotate(annotation, &block)
else
super
end
end
def annotation
if @fiber
@fiber.annotation
else
super
end
end
def to_s
"\#<#{self.description} (#{@status})>"
end
# @deprecated Prefer {Kernel#sleep} except when compatibility with `stable-v1` is required.
def sleep(duration = nil)
super
end
# Execute the given block of code, raising the specified exception if it exceeds the given duration during a non-blocking operation.
def with_timeout(duration, exception = TimeoutError, message = "execution expired", &block)
Fiber.scheduler.with_timeout(duration, exception, message, &block)
end
# Yield back to the reactor and allow other fibers to execute.
def yield
Fiber.scheduler.yield
end
# @attr fiber [Fiber] The fiber which is being used for the execution of this task.
attr :fiber
# Whether the internal fiber is alive, i.e. it
def alive?
@fiber&.alive?
end
# Whether we can remove this node from the reactor graph.
# @returns [Boolean]
def finished?
# If the block is nil and the fiber is nil, it means the task has finished execution. This becomes true after `finish!` is called.
super && @block.nil? && @fiber.nil?
end
# Whether the task is running.
# @returns [Boolean]
def running?
@status == :running
end
def failed?
@status == :failed
end
# The task has been stopped
def stopped?
@status == :stopped
end
# The task has completed execution and generated a result.
def completed?
@status == :completed
end
alias complete? completed?
# @attr status [Symbol] The status of the execution of the fiber, one of `:initialized`, `:running`, `:complete`, `:stopped` or `:failed`.
attr :status
# Begin the execution of the task.
def run(*arguments)
if @status == :initialized
@status = :running
schedule do
@block.call(self, *arguments)
end
else
raise RuntimeError, "Task already running!"
end
end
# Run an asynchronous task as a child of the current task.
def async(*arguments, **options, &block)
raise FinishedError if self.finished?
task = Task.new(self, **options, &block)
task.run(*arguments)
return task
end
# Retrieve the current result of the task. Will cause the caller to wait until result is available. If the task resulted in an unhandled error (derived from `StandardError`), this will be raised. If the task was stopped, this will return `nil`.
#
# Conceptually speaking, waiting on a task should return a result, and if it throws an exception, this is certainly an exceptional case that should represent a failure in your program, not an expected outcome. In other words, you should not design your programs to expect exceptions from `#wait` as a normal flow control, and prefer to catch known exceptions within the task itself and return a result that captures the intention of the failure, e.g. a `TimeoutError` might simply return `nil` or `false` to indicate that the operation did not generate a valid result (as a timeout was an expected outcome of the internal operation in this case).
#
# @raises [RuntimeError] If the task's fiber is the current fiber.
# @returns [Object] The final expression/result of the task's block.
def wait
raise "Cannot wait on own fiber!" if Fiber.current.equal?(@fiber)
# `finish!` will set both of these to nil before signaling the condition:
if @block || @fiber
@finished ||= Condition.new
@finished.wait
end
if @status == :failed
raise @result
else
return @result
end
end
# Access the result of the task without waiting. May be nil if the task is not completed. Does not raise exceptions.
attr :result
# Stop the task and all of its children.
#
# If `later` is false, it means that `stop` has been invoked directly. When `later` is true, it means that `stop` is invoked by `stop_children` or some other indirect mechanism. In that case, if we encounter the "current" fiber, we can't stop it right away, as it's currently performing `#stop`. Stopping it immediately would interrupt the current stop traversal, so we need to schedule the stop to occur later.
#
# @parameter later [Boolean] Whether to stop the task later, or immediately.
def stop(later = false)
if self.stopped?
# If we already stopped this task... don't try to stop it again:
return stopped!
end
# If the fiber is alive, we need to stop it:
if @fiber&.alive?
if self.current?
# If the fiber is current, and later is `true`, we need to schedule the fiber to be stopped later, as it's currently invoking `stop`:
if later
# If the fiber is the current fiber and we want to stop it later, schedule it:
Fiber.scheduler.push(Stop::Later.new(self))
else
# Otherwise, raise the exception directly:
raise Stop, "Stopping current task!"
end
else
# If the fiber is not curent, we can raise the exception directly:
begin
# There is a chance that this will stop the fiber that originally called stop. If that happens, the exception handling in `#stopped` will rescue the exception and re-raise it later.
Fiber.scheduler.raise(@fiber, Stop)
rescue FiberError
# In some cases, this can cause a FiberError (it might be resumed already), so we schedule it to be stopped later:
Fiber.scheduler.push(Stop::Later.new(self))
end
end
else
# We are not running, but children might be, so transition directly into stopped state:
stop!
end
end
# Lookup the {Task} for the current fiber. Raise `RuntimeError` if none is available.
# @returns [Task]
# @raises[RuntimeError] If task was not {set!} for the current fiber.
def self.current
Thread.current[:async_task] or raise RuntimeError, "No async task available!"
end
# Check if there is a task defined for the current fiber.
# @returns [Task | Nil]
def self.current?
Thread.current[:async_task]
end
def current?
Fiber.current.equal?(@fiber)
end
private
# Finish the current task, moving any children to the parent.
def finish!
# Don't hold references to the fiber or block after the task has finished:
@fiber = nil
@block = nil # If some how we went directly from initialized to finished.
# Attempt to remove this node from the task tree.
consume
# If this task was being used as a future, signal completion here:
if @finished
@finished.signal(self)
@finished = nil
end
end
# State transition into the completed state.
def completed!(result)
@result = result
@status = :completed
end
# This is a very tricky aspect of tasks to get right. I've modelled it after `Thread` but it's slightly different in that the exception can propagate back up through the reactor. If the user writes code which raises an exception, that exception should always be visible, i.e. cause a failure. If it's not visible, such code fails silently and can be very difficult to debug.
def failed!(exception = false, propagate = true)
@result = exception
@status = :failed
if exception
if propagate
raise exception
elsif @finished.nil?
# If no one has called wait, we log this as a warning:
Console.logger.warn(self, "Task may have ended with unhandled exception.", exception)
else
Console.logger.debug(self, exception)
end
end
end
def stopped!
# Console.logger.info(self, status:) {"Task #{self} was stopped with #{@children&.size.inspect} children!"}
@status = :stopped
stopped = false
begin
# We are not running, but children might be so we should stop them:
stop_children(true)
rescue Stop
stopped = true
# If we are stopping children, and one of them tries to stop the current task, we should ignore it. We will be stopped later.
retry
end
if stopped
raise Stop, "Stopping current task!"
end
end
def stop!
stopped!
finish!
end
def schedule(&block)
@fiber = Fiber.new(annotation: self.annotation) do
set!
begin
completed!(yield)
# Console.logger.debug(self) {"Task was completed with #{@children.size} children!"}
rescue Stop
stopped!
rescue StandardError => error
failed!(error, false)
rescue Exception => exception
failed!(exception, true)
ensure
# Console.logger.info(self) {"Task ensure $! = #{$!} with #{@children&.size.inspect} children!"}
finish!
end
end
self.root.resume(@fiber)
end
# Set the current fiber's `:async_task` to this task.
def set!
# This is actually fiber-local:
Thread.current[:async_task] = self
end
end
end