module Crystal::System::FileDescriptor
  def self.pipe(read_blocking, write_blocking)
    # ...

    r = IO::FileDescriptor.new(r_pipe.address, read_blocking)
    w = IO::FileDescriptor.new(w_pipe.address, write_blocking)
    r.read_buffering = false # disable buffering on the read end
    w.sync = true

    {r, w}
  end
end

lib LibC
  fun PeekNamedPipe(hNamedPipe : HANDLE, lpBuffer : Void*, nBufferSize : DWORD, lpBytesRead : DWORD*, lpTotalBytesAvail : DWORD*, lpBytesLeftThisMessage : DWORD*) : BOOL
end

struct Crystal::FiberChannel
  def receive
    # explicitly peek the pipe until there is data available
    while true
      LibC.PeekNamedPipe(LibC::HANDLE.new(@worker_out.fd), nil, 0, nil, out bytes_avail, nil)
      break if bytes_avail >= sizeof(UInt64)
      Fiber.yield
    end

    oid = @worker_out.read_bytes(UInt64)
    Pointer(Fiber).new(oid).as(Fiber)
  end
end

struct Crystal::FiberChannel
  @lock = Crystal::SpinLock.new

  def initialize
    @queue = Deque(Fiber).new
  end

  def send(fiber : Fiber)
    @lock.sync { @queue << fiber }
  end

  def receive
    while true
      fiber = @lock.sync { @queue.shift? }
      return fiber if fiber
      Fiber.yield
    end
  end
end

class Crystal::Scheduler
  {% flag?(:preview_mt) %}
    def run_loop
      spawn_stack_pool_collector

      loop do
        @lock.lock

        if runnable = @runnables.shift?
          @runnables << Fiber.current
          @lock.unlock
          resume(runnable)
        else
          @lock.unlock
          Fiber.yield
        end
      end
    end

    def send_fiber(fiber : Fiber)
      @lock.sync { @runnables << fiber }
    end
  {% end %}
end