checkpoint: let criu set work-dir defaultly

Dump/restrore logs are very useful when process fails.
New runc puts these files in c.root, which will be deleted
when container exits. So if checkpinting/restoring failed,
we can not get these logs and analyze why.

Signed-off-by: Liu Hua <weldonliu@tencent.com>

libcontainer/container_linux.go

@@ -974,20 +974,6 @@ func (c *linuxContainer) Checkpoint(criuOpts *CriuOpts) error {
 		return err
 	}
 
-	if criuOpts.WorkDirectory == "" {
-		criuOpts.WorkDirectory = filepath.Join(c.root, "criu.work")
-	}
-
-	if err := os.Mkdir(criuOpts.WorkDirectory, 0700); err != nil && !os.IsExist(err) {
-		return err
-	}
-
-	workDir, err := os.Open(criuOpts.WorkDirectory)
-	if err != nil {
-		return err
-	}
-	defer workDir.Close()
-
 	imageDir, err := os.Open(criuOpts.ImagesDirectory)
 	if err != nil {
 		return err
@@ -996,7 +982,6 @@ func (c *linuxContainer) Checkpoint(criuOpts *CriuOpts) error {
 
 	rpcOpts := criurpc.CriuOpts{
 		ImagesDirFd:     proto.Int32(int32(imageDir.Fd())),
-		WorkDirFd:       proto.Int32(int32(workDir.Fd())),
 		LogLevel:        proto.Int32(4),
 		LogFile:         proto.String("dump.log"),
 		Root:            proto.String(c.config.Rootfs),
@@ -1014,6 +999,19 @@ func (c *linuxContainer) Checkpoint(criuOpts *CriuOpts) error {
 		LazyPages:       proto.Bool(criuOpts.LazyPages),
 	}
 
+	// if criuOpts.WorkDirectory is not set. let criu set it.
+	if criuOpts.WorkDirectory != "" {
+		if err := os.Mkdir(criuOpts.WorkDirectory, 0700); err != nil && !os.IsExist(err) {
+			return err
+		}
+		workDir, err := os.Open(criuOpts.WorkDirectory)
+		if err != nil {
+			return err
+		}
+		defer workDir.Close()
+		rpcOpts.WorkDirFd = proto.Int32(int32(workDir.Fd()))
+	}
+
 	c.handleCriuConfigurationFile(&rpcOpts)
 
 	// If the container is running in a network namespace and has
@@ -1307,7 +1305,7 @@ func (c *linuxContainer) Restore(process *Process, criuOpts *CriuOpts) error {
 		return err
 	}
 	if criuOpts.WorkDirectory == "" {
-		criuOpts.WorkDirectory = filepath.Join(c.root, "criu.work")
+		criuOpts.WorkDirectory = criuOpts.ImagesDirectory
 	}
 	// Since a container can be C/R'ed multiple times,
 	// the work directory may already exist.