setns: add bootstrap data

add bootstrap data to setns process. If we have any bootstrap data then copy it
to the bootstrap process (i.e. nsexec) using the sync pipe. This will allow us
to eventually replace environment variable usage with more structured data
to setup namespaces, write pid/gid map, setgroup etc.

Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com>

libcontainer/process_linux.go

@@ -41,13 +41,14 @@ type parentProcess interface {
 }
 
 type setnsProcess struct {
-	cmd         *exec.Cmd
-	parentPipe  *os.File
-	childPipe   *os.File
-	cgroupPaths map[string]string
-	config      *initConfig
-	fds         []string
-	process     *Process
+	cmd           *exec.Cmd
+	parentPipe    *os.File
+	childPipe     *os.File
+	cgroupPaths   map[string]string
+	config        *initConfig
+	fds           []string
+	process       *Process
+	bootstrapData io.Reader
 }
 
 func (p *setnsProcess) startTime() (string, error) {
@@ -64,6 +65,16 @@ func (p *setnsProcess) signal(sig os.Signal) error {
 
 func (p *setnsProcess) start() (err error) {
 	defer p.parentPipe.Close()
+	err = p.cmd.Start()
+	p.childPipe.Close()
+	if err != nil {
+		return newSystemError(err)
+	}
+	if p.bootstrapData != nil {
+		if _, err := io.Copy(p.parentPipe, p.bootstrapData); err != nil {
+			return newSystemError(err)
+		}
+	}
 	if err = p.execSetns(); err != nil {
 		return newSystemError(err)
 	}
@@ -96,11 +107,6 @@ func (p *setnsProcess) start() (err error) {
 // before the go runtime boots, we wait on the process to die and receive the child's pid
 // over the provided pipe.
 func (p *setnsProcess) execSetns() error {
-	err := p.cmd.Start()
-	p.childPipe.Close()
-	if err != nil {
-		return newSystemError(err)
-	}
 	status, err := p.cmd.Process.Wait()
 	if err != nil {
 		p.cmd.Wait()