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.
Upstream-commit: fbebe20bc69648c046e3818ca744ae246092a782
Component: engine

components/engine/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
+}