virtiofs: stop sandbox when virtiofsd quits

Commit 89e0dfae11 ("qemu: stop qemu process when virtiofsd quits")
stops sandbox when virtiofsd quits so that virtiofs mount inside guest
won't hang. But commit d5a3d0a61c ("virtiofs: use virtiofsd
--fd=FDNUM") deleted this monitor logic.

Add the Scanner back to monitor virtiofsd's stderr and stop sandbox if
Scanner returns error.

Note that we don't monitor the virtiofsd process itself is because
virtiofsd may be live-upgraded (when available) and the original
process may quit, but virtiofs service is still running.

Fixes: #2315
Signed-off-by: Eryu Guan <eguan@linux.alibaba.com>

virtcontainers/qemu.go

@@ -655,12 +655,28 @@ func (q *qemu) setupVirtiofsd() (err error) {
 	const sockFd = 3 // Cmd.ExtraFiles[] fds are numbered starting from 3
 	cmd := exec.Command(q.config.VirtioFSDaemon, q.virtiofsdArgs(sockFd)...)
 	cmd.ExtraFiles = append(cmd.ExtraFiles, fd)
+	stderr, err := cmd.StderrPipe()
+	if err != nil {
+		return err
+	}
 
 	err = cmd.Start()
 	if err == nil {
 		q.state.VirtiofsdPid = cmd.Process.Pid
 	}
 	fd.Close()
+
+	// Monitor virtiofsd's stderr and stop sandbox if virtiofsd quits
+	go func() {
+		scanner := bufio.NewScanner(stderr)
+		for scanner.Scan() {
+			q.Logger().WithField("source", "virtiofsd").Info(scanner.Text())
+		}
+		q.Logger().Info("virtiofsd quits")
+		// Wait to release resources of virtiofsd process
+		cmd.Process.Wait()
+		q.stopSandbox()
+	}()
 	return err
 }