criuSwrk: prepare for CRIU VERSION RPC

To use the CRIU VERSION RPC the criuSwrk function is adapted to work
with CriuOpts set to 'nil' as CriuOpts is not required for the VERSION
RPC.

Also do not print c.criuVersion if it is '0' as the first RPC call will
always be the VERSION call and only after that the version will be
known.

Signed-off-by: Adrian Reber <areber@redhat.com>

libcontainer/container_linux.go

@@ -1059,7 +1059,14 @@ func (c *linuxContainer) criuSwrk(process *Process, req *criurpc.CriuReq, opts *
 		return err
 	}
 
-	logPath := filepath.Join(opts.WorkDirectory, req.GetOpts().GetLogFile())
+	var logPath string
+	if opts != nil {
+		logPath = filepath.Join(opts.WorkDirectory, req.GetOpts().GetLogFile())
+	} else {
+		// For the VERSION RPC 'opts' is set to 'nil' and therefore
+		// opts.WorkDirectory does not exist. Set logPath to "".
+		logPath = ""
+	}
 	criuClient := os.NewFile(uintptr(fds[0]), "criu-transport-client")
 	criuClientFileCon, err := net.FileConn(criuClient)
 	criuClient.Close()
@@ -1074,7 +1081,11 @@ func (c *linuxContainer) criuSwrk(process *Process, req *criurpc.CriuReq, opts *
 	defer criuServer.Close()
 
 	args := []string{"swrk", "3"}
+	if c.criuVersion != 0 {
+		// If the CRIU Version is still '0' then this is probably
+		// the initial CRIU run to detect the version. Skip it.
 		logrus.Debugf("Using CRIU %d at: %s", c.criuVersion, c.criuPath)
+	}
 	logrus.Debugf("Using CRIU with following args: %s", args)
 	cmd := exec.Command(c.criuPath, args...)
 	if process != nil {