Class: Async::Semaphore

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

Overview

A semaphore is used to control access to a common resource in a concurrent system. A useful way to think of a semaphore as used in the real-world systems is as a record of how many units of a particular resource are available, coupled with operations to adjust that record safely (i.e. to avoid race conditions) as units are required or become free, and, if necessary, wait until a unit of the resource becomes available.

Instance Attribute Summary collapse

Instance Method Summary collapse

Constructor Details

#initialize(limit = 1, parent: nil) ⇒ Semaphore

Returns a new instance of Semaphore.


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# File 'lib/async/semaphore.rb', line 26

def initialize(limit = 1, parent: nil)
	@count = 0
	@limit = limit
	@waiting = []
	
	@parent = parent
end

Instance Attribute Details

#countObject (readonly)

The current number of tasks that have acquired the semaphore.


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# File 'lib/async/semaphore.rb', line 35

def count
  @count
end

#limitObject (readonly)

The maximum number of tasks that can acquire the semaphore.


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# File 'lib/async/semaphore.rb', line 38

def limit
  @limit
end

#waitingObject (readonly)

The tasks waiting on this semaphore.


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# File 'lib/async/semaphore.rb', line 41

def waiting
  @waiting
end

Instance Method Details

#acquire { ... } ⇒ Object

Acquire the semaphore, block if we are at the limit. If no block is provided, you must call release manually.

Yields:

  • when the semaphore can be acquired

Returns:

  • the result of the block if invoked


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# File 'lib/async/semaphore.rb', line 72

def acquire
	wait
	
	@count += 1
	
	return unless block_given?
	
	begin
		return yield
	ensure
		self.release
	end
end

#async(*arguments, parent: (@parent or Task.current), **options) ⇒ Object

Run an async task. Will wait until the semaphore is ready until spawning and running the task.


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# File 'lib/async/semaphore.rb', line 54

def async(*arguments, parent: (@parent or Task.current), **options)
	wait
	
	parent.async(**options) do |task|
		@count += 1
		
		begin
			yield task, *arguments
		ensure
			self.release
		end
	end
end

#blocking?Boolean

Whether trying to acquire this semaphore would block.

Returns:

  • (Boolean)

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# File 'lib/async/semaphore.rb', line 49

def blocking?
	@count >= @limit
end

#empty?Boolean

Is the semaphore currently acquired?

Returns:

  • (Boolean)

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# File 'lib/async/semaphore.rb', line 44

def empty?
	@count.zero?
end

#releaseObject

Release the semaphore. Must match up with a corresponding call to `acquire`. Will release waiting fibers in FIFO order.


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# File 'lib/async/semaphore.rb', line 87

def release
	@count -= 1
	
	while (@limit - @count) > 0 and fiber = @waiting.shift
		if fiber.alive?
			fiber.resume
		end
	end
end