Class: Async::Task

Inherits:
Node
  • Object
show all
Defined in:
lib/async/task.rb

Overview

Represents a sequential unit of work, defined by a block, which is executed concurrently with other tasks. A task can be in one of the following states: initialized, running, completed, failed, cancelled or stopped.

“‘mermaid stateDiagram-v2

*

–> Initialized

Initialized –> Running : Run

Running –> Completed : Return Value Running –> Failed : Exception

Completed –> [*] Failed –> [*]

Running –> Stopped : Stop Stopped –> [*] Completed –> Stopped : Stop Failed –> Stopped : Stop Initialized –> Stopped : Stop “‘

Examples:

Creating a task that sleeps for 1 second.

require "async"
Async do |task|
	sleep(1)
end

Defined Under Namespace

Classes: FinishedError

Instance Attribute Summary collapse

Attributes inherited from Node

#A useful identifier for the current node., #Optional list of children., #children, #head, #parent, #tail

Class Method Summary collapse

Instance Method Summary collapse

Methods inherited from Node

#The parent node.=, #children?, #consume, #description, #print_hierarchy, #root, #terminate, #transient=, #transient?, #traverse

Constructor Details

#initialize(parent = Task.current?, finished: nil, **options, &block) ⇒ Task

Create a new task.



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# File 'lib/async/task.rb', line 89

def initialize(parent = Task.current?, finished: nil, **options, &block)
	# 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, and @block should be nil.
	# In a finished state, the @block should be nil, and the @fiber should be nil.
	@block = block
	@fiber = nil
	
	@promise = Promise.new
	
	# Handle finished: parameter for backward compatibility:
	case finished
	when false
		# `finished: false` suppresses warnings for expected task failures:
		@promise.suppress_warnings!
	when nil
		# `finished: nil` is the default, no special handling:
	else
		# All other `finished:` values are deprecated:
		warn("finished: argument with non-false value is deprecated and will be removed.", uplevel: 1, category: :deprecated) if $VERBOSE
	end
	
	@defer_stop = nil
	
	# Call this after all state is initialized, as it may call `add_child` which will set the parent and make it visible to the scheduler.
	super(parent, **options)
end

Instance Attribute Details

#fiber ⇒ Object (readonly)

Returns the value of attribute fiber.



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# File 'lib/async/task.rb', line 172

def fiber
  @fiber
end

#The fiber which is being used for the execution of this task.(fiberwhichisbeingused) ⇒ Object (readonly) 



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# File 'lib/async/task.rb', line 172

attr :fiber

Class Method Details

.current ⇒ Object

Lookup the Async::Task for the current fiber. Raise RuntimeError if none is available. @raises If task was not set! for the current fiber.



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# File 'lib/async/task.rb', line 438

def self.current
	Fiber.current.async_task or raise RuntimeError, "No async task available!"
end

.current? ⇒ Boolean

Check if there is a task defined for the current fiber.

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 444

def self.current?
	Fiber.current.async_task
end

.run(scheduler, *arguments, **options, &block) ⇒ Object

Run the given block of code in a task, asynchronously, in the given scheduler.



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# File 'lib/async/task.rb', line 80

def self.run(scheduler, *arguments, **options, &block)
	self.new(scheduler, **options, &block).tap do |task|
		task.run(*arguments)
	end
end

.yield ⇒ Object

Deprecated.

With no replacement.



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# File 'lib/async/task.rb', line 73

def self.yield
	warn("`Async::Task.yield` is deprecated with no replacement.", uplevel: 1, category: :deprecated) if $VERBOSE
	
	Fiber.scheduler.transfer
end

Instance Method Details

#alive? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 175

def alive?
	@fiber&.alive?
end

#annotate(annotation, &block) ⇒ Object

Annotate the task with a description.

This will internally try to annotate the fiber if it is running, otherwise it will annotate the task itself.



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# File 'lib/async/task.rb', line 132

def annotate(annotation, &block)
	if @fiber
		@fiber.annotate(annotation, &block)
	else
		super
	end
end

#annotation ⇒ Object 



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# File 'lib/async/task.rb', line 141

def annotation
	if @fiber
		@fiber.annotation
	else
		super
	end
end

#async(*arguments, **options, &block) ⇒ Object

Run an asynchronous task as a child of the current task.

Raises:



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# File 'lib/async/task.rb', line 256

def async(*arguments, **options, &block)
	raise FinishedError if self.finished?
	
	task = Task.new(self, **options, &block)
	
	# When calling an async block, we deterministically execute it until the first blocking operation. We don't *have* to do this - we could schedule it for later execution, but it's useful to:
	#
	# - Fail at the point of the method call where possible.
	# - Execute determinstically where possible.
	# - Avoid scheduler overhead if no blocking operation is performed.
	#
	# There are different strategies (greedy vs non-greedy). We are currently using a greedy strategy.
	task.run(*arguments)
	
	return task
end

#backtrace(*arguments) ⇒ Object 



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# File 'lib/async/task.rb', line 123

def backtrace(*arguments)
	@fiber&.backtrace(*arguments)
end

#complete? ⇒ Boolean

Alias for #completed?.

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 207

def complete?
	self.completed?
end

#completed? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 202

def completed?
	@promise.completed?
end

#current? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 449

def current?
	Fiber.current.equal?(@fiber)
end

#defer_stop ⇒ Object

Defer the handling of stop. During the execution of the given block, if a stop is requested, it will be deferred until the block exits. This is useful for ensuring graceful shutdown of servers and other long-running tasks. You should wrap the response handling code in a defer_stop block to ensure that the task is stopped when the response is complete but not before.

You can nest calls to defer_stop, but the stop will only be deferred until the outermost block exits.

If stop is invoked a second time, it will be immediately executed.



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# File 'lib/async/task.rb', line 398

def defer_stop
	# Tri-state variable for controlling stop:
	# - nil: defer_stop has not been called.
	# - false: defer_stop has been called and we are not stopping.
	# - true: defer_stop has been called and we will stop when exiting the block.
	if @defer_stop.nil?
		begin
			# If we are not deferring stop already, we can defer it now:
			@defer_stop = false
			
			yield
		rescue Stop
			# If we are exiting due to a stop, we shouldn't try to invoke stop again:
			@defer_stop = nil
			raise
		ensure
			defer_stop = @defer_stop
			
			# We need to ensure the state is reset before we exit the block:
			@defer_stop = nil
			
			# If we were asked to stop, we should do so now:
			if defer_stop
				raise Stop, "Stopping current task (was deferred)!", cause: defer_stop
			end
		end
	else
		# If we are deferring stop already, entering it again is a no-op.
		yield
	end
end

#failed? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 192

def failed?
	@promise.failed?
end

#finished? ⇒ Boolean

Whether we can remove this node from the reactor graph.

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 181

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

#reactor ⇒ Object 



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# File 'lib/async/task.rb', line 118

def reactor
	self.root
end

#result ⇒ Object

Access the result of the task without waiting. May be nil if the task is not completed. Does not raise exceptions.



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# File 'lib/async/task.rb', line 325

def result
	value = @promise.value
	
	# For backward compatibility, return nil for stopped tasks:
	if @promise.cancelled?
		nil
	else
		value
	end
end

#run(*arguments) ⇒ Object

Begin the execution of the task.



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# File 'lib/async/task.rb', line 228

def run(*arguments)
	# Move from initialized to running by clearing @block
	if block = @block
		@block = nil
		
		schedule do
			block.call(self, *arguments)
		rescue => error
			# I'm not completely happy with this overhead, but the alternative is to not log anything which makes debugging extremely difficult. Maybe we can introduce a debug wrapper which adds extra logging.
			unless @promise.waiting?
				warn(self, "Task may have ended with unhandled exception.", exception: error)
			end
			
			raise
		end
	else
		raise RuntimeError, "Task already running!"
	end
end

#running? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 187

def running?
	self.alive?
end

#sleep(duration = nil) ⇒ Object

Deprecated.

Prefer Kernel#sleep except when compatibility with stable-v1 is required.



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# File 'lib/async/task.rb', line 155

def sleep(duration = nil)
	Kernel.warn("`Async::Task#sleep` is deprecated, use `Kernel#sleep` instead.", uplevel: 1, category: :deprecated) if $VERBOSE
	
	super
end

#status ⇒ Object 



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# File 'lib/async/task.rb', line 212

def status
	case @promise.resolved
	when :cancelled
		:stopped
	when :failed
		:failed
	when :completed
		:completed
	when nil
		self.running? ? :running : :initialized
	end
end

#stop(later = false, cause: $!) ⇒ Object

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.



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# File 'lib/async/task.rb', line 342

def stop(later = false, cause: $!)
	# If no cause is given, we generate one from the current call stack:
	unless cause
		cause = Stop::Cause.for("Stopping task!")
	end
	
	if self.stopped?
		# If the task is already stopped, a `stop` state transition re-enters the same state which is a no-op. However, we will also attempt to stop any running children too. This can happen if the children did not stop correctly the first time around. Doing this should probably be considered a bug, but it's better to be safe than sorry.
		return stopped!
	end
	
	# If the fiber is alive, we need to stop it:
	if @fiber&.alive?
		# As the task is now exiting, we want to ensure the event loop continues to execute until the task finishes.
		self.transient = false
		
		# If we are deferring stop...
		if @defer_stop == false
			# Don't stop now... but update the state so we know we need to stop later.
			@defer_stop = cause
			return false
		end
		
		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, cause))
			else
				# Otherwise, raise the exception directly:
				raise Stop, "Stopping current task!", cause: cause
			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, cause: cause)
			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, cause))
			end
		end
	else
		# We are not running, but children might be, so transition directly into stopped state:
		stop!
	end
end

#stop_deferred? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 431

def stop_deferred?
	!!@defer_stop
end

#stopped? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 197

def stopped?
	@promise.cancelled?
end

#The status of the execution of the task, one of `:initialized`, `:running`, `:complete`, `:stopped` or `:failed`.=(statusoftheexecutionofthetask, oneof`: initialized`, `: running`, `: complete`, `: stopped`) ⇒ Object 



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# File 'lib/async/task.rb', line 212

def status
	case @promise.resolved
	when :cancelled
		:stopped
	when :failed
		:failed
	when :completed
		:completed
	when nil
		self.running? ? :running : :initialized
	end
end

#to_s ⇒ Object 



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# File 'lib/async/task.rb', line 150

def to_s
	"\#<#{self.description} (#{self.status})>"
end

#wait ⇒ Object Also known as: join

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).



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# File 'lib/async/task.rb', line 280

def wait
	raise "Cannot wait on own fiber!" if Fiber.current.equal?(@fiber)
	
	# Wait for the task to complete - Promise handles all the complexity:
	begin
		@promise.wait
	rescue Promise::Cancel
		# For backward compatibility, stopped tasks return nil:
		return nil
	end
end

#wait_all ⇒ Object

Wait on all non-transient children to complete, recursively, then wait on the task itself, if it is not the current task.

If any child task fails with an exception, that exception will be raised immediately, and remaining children may not be waited on.

Examples:

Waiting on all children.

Async do |task|
	child = task.async do
		sleep(0.01)
	end
	task.wait_all # Will wait on the child task.
end


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# File 'lib/async/task.rb', line 310

def wait_all
	@children&.each do |child|
		# Skip transient tasks
		next if child.transient?
		
		child.wait_all
	end
	
	# Only wait on the task if we're not waiting on ourselves:
	unless self.current?
		return self.wait
	end
end

#with_timeout(duration, exception = TimeoutError, message = "execution expired", &block) ⇒ Object

Execute the given block of code, raising the specified exception if it exceeds the given duration during a non-blocking operation.



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# File 'lib/async/task.rb', line 162

def with_timeout(duration, exception = TimeoutError, message = "execution expired", &block)
	Fiber.scheduler.with_timeout(duration, exception, message, &block)
end

#yield ⇒ Object

Yield back to the reactor and allow other fibers to execute.



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# File 'lib/async/task.rb', line 167

def yield
	Fiber.scheduler.yield
end