WebSocket in child process gets stuck on Windows

[HertzDevil]

Compile the following sources:

# hello.cr
require "http"

ws = HTTP::WebSocket.new("ws://127.0.0.1:55000/")
puts "Hello world"

# client.cr
require "http"

ws = HTTP::WebSocket.new("ws://127.0.0.1:55000/")
ws.send("x")

# server.cr
require "http"

ws_handler = HTTP::WebSocketHandler.new do |ws, ctx|
  ws.on_message do |message|
    p message
    begin
      process = Process.new({% if flag?(:win32) %} "hello.exe" {% else %} "./hello" {% end %}, [] of String, output: :inherit)
      p process
      p process.wait
    rescue ex
      p ex
    end
  end
end
server = HTTP::Server.new [ws_handler]
address = server.bind_tcp "127.0.0.1", 55000
puts "Listening on http://#{address}"
server.listen

Run server.exe and then client.exe. On Linux Hello world and Process::Status[0] will be printed on the server. On Windows the client returns, but hello.exe gets stuck and server.exe is also stuck waiting on it; subsequent runs of the client will get stuck.

I believe this is also why the playground doesn't work on Windows.

[HertzDevil]

I believe this is just another consequence of Process#wait being a blocking call, due to LibC.WaitForSingleObject; if the on_message handler blocks the current thread, the HTTP::Server#listen fiber never gets to run, so the child process will never progress.

On POSIX Process#wait is indeed asynchronous and non-blocking, as it relies on SIGCHLD and Channel.