}()
	closer := func() {
		cmd.Process.Kill()
		<-errChan
	}
	// Dial localhost:3222 to ensure the proxy is ready.
	delay := time.Second / 4
	for attempt := 0; attempt < 5; attempt++ {
		conn, err := net.DialTimeout("tcp", "localhost:3222", 5*time.Second)
		if err == nil {
			conn.Close()
			return closer, nil
		}
		select {
		case <-time.After(delay):
			delay *= 2
		case err := <-errChan:
			return nil, err