Function dupAppStdStream

pkg/app/sensor/monitor/composite.go:351–371

Using simple io.MultiWriter(os.Stdout, os.File) would make cmd.Wait() block until either the cmd's stdout is closed or the multi-writer is closed. However, both are impossible. We need the Wait() to return much earlier than the process termination (see pkg/monitors/ptrace logic), and multi-writer ca

(artifactsDir string, w io.Writer, kind string)

// than the process termination (see pkg/monitors/ptrace logic), and multi-writer
// cannot be closed at all. Hence, the pipe trick.
func dupAppStdStream(artifactsDir string, w io.Writer, kind string) (*os.File, *os.File, error) {
	filename := filepath.Join(artifactsDir, "app_"+kind+".log")

	f, err := os.OpenFile(filename, os.O_CREATE|os.O_WRONLY, 0o644)
	if err != nil {
		return nil, nil, fmt.Errorf("cannot open file %q to duplicate app's %s stream: %w", filename, kind, err)
	}

	pr, pw, err := os.Pipe()
	if err != nil {
		f.Close()
		return nil, nil, fmt.Errorf("cannot create pipe for app %s stream: %w", kind, err)
	}

	go func() {
		n, err := io.Copy(io.MultiWriter(w, f), pr)
		log.Debugf("dupAppStdStream: io.Copy() finished; written=%d error=%v", n, err)
	}()

	return pw, f, nil
}

func closeAll(cs []io.Closer) {
	for _, c := range cs {