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) ⇒ Semaphore


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# File 'lib/async/semaphore.rb', line 24

def initialize(limit = 1)
  @count = 0
  @limit = limit
  @waiting = []
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 31

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 34

def limit
  @limit
end

#waitingObject (readonly)

The tasks waiting on this semaphore.


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# File 'lib/async/semaphore.rb', line 37

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


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# File 'lib/async/semaphore.rb', line 68

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

#async(*args, parent: 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 50

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

#blocking?Boolean

Whether trying to acquire this semaphore would block.


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# File 'lib/async/semaphore.rb', line 45

def blocking?
  @count >= @limit
end

#empty?Boolean

Is the semaphore currently acquired?


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# File 'lib/async/semaphore.rb', line 40

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 83

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