Console streams are blocking on Windows about crystal

[HertzDevil]

IIRC Win32 console handles do not support overlapped I/O, so any kind of asynchronous capability requires threads; the standard input and output streams are not opened with FILE_FLAG_OVERLAPPED, and additional console handles opened using CreateFile do not respect that flag

[HertzDevil]

This snippet echoes the standard input while printing an asterisk every second in the background, using a dedicated thread for reading from STDIN. It should work with or without -Dpreview_mt, and with both redirected or console input:

require "colorize"

lib LibC
  fun PostQueuedCompletionStatus(completionPort : HANDLE, dwNumberOfBytesTransferred : DWORD, dwCompletionKey : ULONG_PTR, lpOverlapped : OVERLAPPED*) : BOOL
end

module AsyncStdin
  @@read_cv = Thread::ConditionVariable.new
  @@read_mtx = Thread::Mutex.new

  @@iocp : LibC::HANDLE = LibC::INVALID_HANDLE_VALUE
  @@read_key = Crystal::IOCP::CompletionKey.new

  @@done_cv = Thread::ConditionVariable.new
  @@done_mtx = Thread::Mutex.new

  @@buf : String?

  @@stdin_read = Thread.new do
    while true
      @@read_mtx.synchronize do
        until @@read_key.fiber
          @@read_cv.wait(@@read_mtx)
        end
      end

      # this shall be baked into `ConsoleUtils.read_console` instead, which also implies
      # this entire thread shouldn't need to interact with any Crystal scheduler directly
      @@buf = STDIN.gets.not_nil!

      # `JOB_OBJECT_MSG_EXIT_PROCESS` is to reuse the resumption mechanism for `Process.run`
      LibC.PostQueuedCompletionStatus(@@iocp, LibC::JOB_OBJECT_MSG_EXIT_PROCESS, @@read_key.object_id, nil)

      @@done_mtx.synchronize do
        while @@buf
          @@done_cv.wait(@@done_mtx)
        end
      end
    end
  end

  def self.read_stdin
    @@read_mtx.synchronize do
      # TODO: what happens if there are concurrent reads? replace these with a queue?
      @@iocp = Crystal::EventLoop.current.iocp
      @@read_key.fiber = Fiber.current
      @@read_cv.signal
    end

    Fiber.suspend

    @@done_mtx.synchronize do
      buf, @@buf = @@buf, nil
      @@done_cv.signal
      buf
    end
  end

  spawn do
    while true
      print '*'.colorize.magenta
      sleep 1
    end
  end

  while true
    print read_stdin.colorize.green
  end
end

[HertzDevil]

I'm sure STDOUT and STDERR can be made non-blocking in a similar manner, but what differences would that make?