module RSpec
  module Core
    # Allows a thread to lock out other threads from a critical section of code,
    # while allowing the thread with the lock to reenter that section.
    #
    # Based on Monitor as of 2.2 - https://github.com/ruby/ruby/blob/eb7ddaa3a47bf48045d26c72eb0f263a53524ebc/lib/monitor.rb#L9
    #
    # Depends on Mutex, but Mutex is only available as part of core since 1.9.1:
    #   exists - http://ruby-doc.org/core-1.9.1/Mutex.html
    #   dne    - http://ruby-doc.org/core-1.9.0/Mutex.html
    #
    # @private
    class ReentrantMutex
      def initialize
        @owner = nil
        @count = 0
        @mutex = Mutex.new
      end

      def synchronize
        enter
        yield
      ensure
        exit
      end

    private

      def enter
        @mutex.lock if @owner != Thread.current
        @owner = Thread.current
        @count += 1
      end

      def exit
        @count -= 1
        return unless @count == 0
        @owner = nil
        @mutex.unlock
      end
    end

    if defined? ::Mutex
      # On 1.9 and up, this is in core, so we just use the real one
      Mutex = ::Mutex
    else # For 1.8.7
      # :nocov:
      RSpec::Support.require_rspec_core "mutex"
      # :nocov:
    end
  end
end