class ActiveSupport::Concurrency::ShareLock

def start_sharing
  synchronize do
    if @sharing[Thread.current] > 0 || @exclusive_thread == Thread.current
      # We already hold a lock; nothing to wait for
    elsif @waiting[Thread.current]
      # We're nested inside a +yield_shares+ call: we'll resume as
      # soon as there isn't an exclusive lock in our way
      @cv.wait_while { @exclusive_thread }
    else
      # This is an initial / outermost share call: any outstanding
      # requests for an exclusive lock get to go first
      @cv.wait_while { busy_for_sharing?(false) }
    end
    @sharing[Thread.current] += 1
  end
end