Add syncpipe for passing context

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

libcontainer/nsinit/sync_pipe.go

@@ -0,0 +1,73 @@
+package nsinit
+
+import (
+	"encoding/json"
+	"fmt"
+	"github.com/dotcloud/docker/pkg/libcontainer"
+	"github.com/dotcloud/docker/pkg/system"
+	"io/ioutil"
+	"os"
+)
+
+// SyncPipe allows communication to and from the child processes
+// to it's parent and allows the two independent processes to
+// syncronize their state.
+type SyncPipe struct {
+	parent, child *os.File
+}
+
+func NewSyncPipe() (s *SyncPipe, err error) {
+	s = &SyncPipe{}
+	s.child, s.parent, err = os.Pipe()
+	if err != nil {
+		return nil, err
+	}
+	system.UsetCloseOnExec(s.child.Fd())
+	return s, nil
+}
+
+func NewSyncPipeFromFd(parendFd, childFd uintptr) (*SyncPipe, error) {
+	s := &SyncPipe{}
+	if parendFd > 0 {
+		s.parent = os.NewFile(parendFd, "parendPipe")
+	} else if childFd > 0 {
+		s.child = os.NewFile(childFd, "childPipe")
+	} else {
+		return nil, fmt.Errorf("no valid sync pipe fd specified")
+	}
+	return s, nil
+}
+
+func (s *SyncPipe) SendToChild(context libcontainer.Context) error {
+	data, err := json.Marshal(context)
+	if err != nil {
+		return err
+	}
+	s.parent.Write(data)
+	return nil
+}
+
+func (s *SyncPipe) ReadFromParent() (libcontainer.Context, error) {
+	data, err := ioutil.ReadAll(s.child)
+	if err != nil {
+		return nil, fmt.Errorf("error reading from sync pipe %s", err)
+	}
+	var context libcontainer.Context
+	if len(data) > 0 {
+		if err := json.Unmarshal(data, &context); err != nil {
+			return nil, err
+		}
+	}
+	return context, nil
+
+}
+
+func (s *SyncPipe) Close() error {
+	if s.parent != nil {
+		s.parent.Close()
+	}
+	if s.child != nil {
+		s.child.Close()
+	}
+	return nil
+}