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

Returns a new instance of 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

#count ⇒ Object (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

#limit ⇒ Object (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

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 67

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

#async(*args) ⇒ 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 47

def async(*args)
  parent = Task.current
  
  wait
  
  parent.async do |task|
    @count += 1
    
    begin
      yield task, *args
    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 42

def blocking?
  @count >= @limit
end

#empty? ⇒ Boolean

Is the semaphore currently acquired?

Returns:

  • (Boolean) 


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

def empty?
  @count.zero?
end

#release ⇒ Object

Release the semaphore. Must match up with a corresponding call to acquire.



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# File 'lib/async/semaphore.rb', line 82

def release
  @count -= 1
  
  available = @waiting.pop(@limit - @count)
  
  available.each do |fiber|
    fiber.resume if fiber.alive?
  end
end