example.rb

queue = TBQueue.new 5
a = Thread.new do
  loop do
    puts "producing"
    Concurrent.atomically do
      queue.write "hiya"
    end
  end
end

b = Thread.new do
  loop do
    sleep 1
    puts "consuming"
    Concurrent.atomically do
      a = queue.read
    end
  end
end

tbqueue.rb

class TBQueue
  def initialize size
    tvar = Concurrent::TVar

    @r = tvar.new []
    @w = tvar.new []
    @rs = tvar.new 0
    @ws = tvar.new size
  end

  def write(a)
    t = Concurrent::Transaction.current
    w = t.read(@ws)

    if w != 0
      t.write(@ws, w - 1)
    else
      r = t.read(@rs)
      if r != 0
        t.write(@rs, 0)
        t.write(@ws, r - 1)
      else
        Concurrent.abort_transaction
      end
    end

    as = t.read(@w)
    as.push(a)
    t.write(@w, as)
  end

  def read
    t = Concurrent::Transaction.current
    xss = t.read(@r)
    r = t.read(@rs)
    t.write(@rs, r + 1)

    if !xss.empty?
      x, *xs = xss
      t.write(@r, xs)
      return x
    else
      ys = t.read(@w)
      if ys.empty?
        Concurrent.abort_transaction
      else
        z, *zs = ys
        t.write(@w, [])
        t.write(@r, zs)

        return z
      end
    end
  end
end