Add a GetPtyMaster() method to container to retrieve the pty from an other package.

We could also have put ptyMaster public, but then we need to ignore it in json
otherwise the Marshalling fails. I think it is cleaner that way.

container.go

@@ -24,6 +24,11 @@ import (
 	"time"
 )
 
+var (
+	ErrNotATTY = errors.New("The PTY is not a file")
+	ErrNoTTY   = errors.New("No PTY found")
+)
+
 type Container struct {
 	sync.Mutex
 	root   string // Path to the "home" of the container, including metadata.
@@ -1405,3 +1410,13 @@ func (container *Container) Exposes(p Port) bool {
 	_, exists := container.Config.ExposedPorts[p]
 	return exists
 }
+
+func (container *Container) GetPtyMaster() (*os.File, error) {
+	if container.ptyMaster == nil {
+		return nil, ErrNoTTY
+	}
+	if pty, ok := container.ptyMaster.(*os.File); ok {
+		return pty, nil
+	}
+	return nil, ErrNotATTY
+}