Process.run / Process#wait does not return while UIng.main is running

[kojix2]

Found an issue in a Crystal + uing/libui-ng GUI app where Process.run does not return when called from a background worker while UIng.main is running.

Environment

• Crystal: 1.21.0
• OS: macOS
• GUI: uing / libui-ng

Behavior

Expected:

before Process.run
after Process.run: 0

Actual:

before Process.run

The child process exits, but Process.run / Process#wait never returns.

I confirmed:

• Process.run("true") works outside UIng.main
• LibC.system("true") works in the same GUI worker context
• ps shows no remaining child process
• Calling Crystal::System::SignalChildHandler.call periodically from UIng.timer makes Process.run return

Minimal reproduction

require "uing"

UIng.init

window = UIng::Window.new("probe", 300, 100, menubar: false, margined: true)
box = UIng::Box.new(:vertical, padded: true)
button = UIng::Button.new("Run")
label = UIng::Label.new("Ready")

button.on_clicked do
  label.text = "Running..."

  Fiber::ExecutionContext::Parallel.new("probe", 2).spawn do
    puts "before Process.run"
    status = Process.run("true")
    puts "after Process.run: #{status.exit_code}"

    UIng.queue_main do
      label.text = "Done"
    end
  end
end

box.append(label, false)
box.append(button, false)
window.set_child(box)
window.show

UIng.main
UIng.uninit

Notes

My current workaround is to avoid Crystal Process.run / Process#wait in the GUI path and use POSIX popen/pclose/fgets instead.

It looks like Crystal’s SIGCHLD handling is not being pumped while uiMain owns the main loop. I’m not sure whether this should be handled in uing/libui-ng integration or considered a Crystal runtime limitation with external GUI main loops.