Class: Async::Task

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

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 63

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, 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
end

Instance Attribute Details

#fiber ⇒ Object (readonly)

Returns the value of attribute fiber.



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# File 'lib/async/task.rb', line 145

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 145

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 378

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 384

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 54

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 47

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 148

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 105

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 114

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 229

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 96

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

#complete? ⇒ Boolean

Alias for #completed?.

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 180

def complete?
	self.completed?
end

#completed? ⇒ Boolean

Returns:

  • (Boolean) 


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

def completed?
	@promise.completed?
end

#current? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 389

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 338

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 165

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 154

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 91

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 266

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 201

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 160

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 128

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 185

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 282

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 371

def stop_deferred?
	!!@defer_stop
end

#stopped? ⇒ Boolean

Returns:

  • (Boolean) 


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# File 'lib/async/task.rb', line 170

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 185

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 123

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

#wait ⇒ Object

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 253

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

#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 135

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 140

def yield
	Fiber.scheduler.yield
end