Class: Philiprehberger::Semaphore::Counter

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

Overview

Counting semaphore for concurrent access control with timeouts

Examples:

sem = Philiprehberger::Semaphore::Counter.new(permits: 3)
sem.acquire { do_work }

Instance Method Summary collapse

Constructor Details

#initialize(permits:, fair: false) ⇒ Counter

Create a new counting semaphore

Parameters:

  • permits (Integer) —

    the number of permits available

  • fair (Boolean) (defaults to: false) —

    when true, guarantee FIFO ordering for waiters

Raises:



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# File 'lib/philiprehberger/semaphore.rb', line 20

def initialize(permits:, fair: false)
  raise Error, 'permits must be a positive integer' unless permits.is_a?(Integer) && permits.positive?

  @permits = permits
  @available = permits
  @mutex = Mutex.new
  @fair = fair
  @draining = false
  @drain_condition = ConditionVariable.new

  if @fair
    @queue = []
  else
    @condition = ConditionVariable.new
  end
end

Instance Method Details

#acquire(weight: 1) { ... } ⇒ Object

Acquire one or more permits, blocking until available

Parameters:

  • weight (Integer) (defaults to: 1) —

    number of permits to acquire (default: 1)

Yields:

  • executes the block while holding the permit(s)

Returns:

  • (Object) —

    the block’s return value, or true if no block given



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# File 'lib/philiprehberger/semaphore.rb', line 102

def acquire(weight: 1)
  validate_weight!(weight)

  @mutex.synchronize do
    raise Error, 'semaphore is draining' if @draining

    if @fair
      cv = ConditionVariable.new
      @queue.push(cv)
      loop do
        raise_if_draining_fair!(cv)
        break if @queue.first == cv && @available >= weight

        cv.wait(@mutex)
      end
      @queue.shift
    else
      loop do
        raise Error, 'semaphore is draining' if @draining
        break if @available >= weight

        @condition.wait(@mutex)
      end
    end
    @available -= weight
  end

  if block_given?
    begin
      yield
    ensure
      release(weight: weight)
    end
  else
    true
  end
end

#acquired_count ⇒ Integer

Return the number of permits currently held by acquirers

Equivalent to ‘permits - available` but computed under the internal lock for a consistent snapshot. Useful for observability and metrics.

Returns:

  • (Integer) 


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# File 'lib/philiprehberger/semaphore.rb', line 57

def acquired_count
  @mutex.synchronize { @permits - @available }
end

#available ⇒ Integer

Return the number of currently available permits

Returns:

  • (Integer) 


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# File 'lib/philiprehberger/semaphore.rb', line 47

def available
  @mutex.synchronize { @available }
end

#drain ⇒ void

This method returns an undefined value.

Drain the semaphore: reject new acquisitions and block until all permits are returned

Once draining begins, #acquire raises Error and #try_acquire returns false. This method blocks the calling thread until every outstanding permit has been released. After drain completes the semaphore remains in the draining state.



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

def drain
  @mutex.synchronize do
    @draining = true

    # Wake all waiting acquirers so they can observe the draining state and exit
    if @fair
      @queue.each(&:signal)
    else
      @condition.broadcast
    end

    @drain_condition.wait(@mutex) while @available < @permits
  end
end

#draining? ⇒ Boolean

Return whether this semaphore is currently draining

Returns:

  • (Boolean) 


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# File 'lib/philiprehberger/semaphore.rb', line 71

def draining?
  @mutex.synchronize { @draining }
end

#fair? ⇒ Boolean

Return whether this semaphore uses FIFO fairness

Returns:

  • (Boolean) 


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# File 'lib/philiprehberger/semaphore.rb', line 64

def fair?
  @fair
end

#permits ⇒ Integer

Return the total number of permits

Returns:

  • (Integer) 


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

def permits
  @mutex.synchronize { @permits }
end

#release(weight: 1) ⇒ void

This method returns an undefined value.

Release one or more permits back to the semaphore

Parameters:

  • weight (Integer) (defaults to: 1) —

    number of permits to release (default: 1)

Raises:

  • (Error) —

    if more permits are released than acquired



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# File 'lib/philiprehberger/semaphore.rb', line 215

def release(weight: 1)
  validate_weight!(weight)

  @mutex.synchronize do
    raise Error, 'cannot release more permits than total' if @available + weight > @permits

    @available += weight

    if @draining
      @drain_condition.signal if @available >= @permits
    elsif @fair
      @queue.each(&:signal)
    else
      weight.times { @condition.signal }
    end
  end
end

#resize(new_permits) ⇒ void

This method returns an undefined value.

Resize the semaphore to a new total permit count

Parameters:

  • new_permits (Integer) —

    the new total number of permits

Raises:

  • (Error) —

    if new_permits is not a positive integer



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# File 'lib/philiprehberger/semaphore.rb', line 238

def resize(new_permits)
  raise Error, 'permits must be a positive integer' unless new_permits.is_a?(Integer) && new_permits.positive?

  @mutex.synchronize do
    diff = new_permits - @permits
    @permits = new_permits
    @available += diff

    # If permits increased, wake waiters that may now be able to acquire
    if diff.positive?
      if @fair
        @queue.each(&:signal)
      else
        diff.times { @condition.signal }
      end
    end
  end
end

#try_acquire(timeout:, weight: 1) { ... } ⇒ Object, false

Try to acquire one or more permits within the given timeout

Parameters:

  • timeout (Numeric) —

    maximum seconds to wait

  • weight (Integer) (defaults to: 1) —

    number of permits to acquire (default: 1)

Yields:

  • executes the block while holding the permit(s)

Returns:

  • (Object, false) —

    the block’s return value, or false if timeout expired



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# File 'lib/philiprehberger/semaphore.rb', line 146

def try_acquire(timeout:, weight: 1)
  validate_weight!(weight)
  deadline = Time.now + timeout
  acquired = false

  @mutex.synchronize do
    return false if @draining

    if @fair
      cv = ConditionVariable.new
      @queue.push(cv)

      loop do
        if @draining
          @queue.delete(cv)
          break
        end

        if @queue.first == cv && @available >= weight
          @queue.shift
          acquired = true
          break
        end

        remaining = deadline - Time.now
        if remaining <= 0
          @queue.delete(cv)
          break
        end

        cv.wait(@mutex, remaining)
      end
    else
      loop do
        break if @draining

        if @available >= weight
          acquired = true
          break
        end

        remaining = deadline - Time.now
        break if remaining <= 0

        @condition.wait(@mutex, remaining)
      end
    end

    @available -= weight if acquired
  end

  return false unless acquired

  if block_given?
    begin
      yield
    ensure
      release(weight: weight)
    end
  else
    true
  end
end