Prohibit more than 42 layers in the core

We need to do this because we still support aufs
and users on other drivers can push incompat images
to the registory that aufs users cannot use.

image.go

@@ -211,6 +211,25 @@ func (img *Image) getParentsSize(size int64) int64 {
 	return parentImage.getParentsSize(size)
 }
 
+// Depth returns the number of parents for a
+// current image
+func (img *Image) Depth() (int, error) {
+	var (
+		count  = 0
+		parent = img
+		err    error
+	)
+
+	for parent != nil {
+		count++
+		parent, err = parent.GetParent()
+		if err != nil {
+			return -1, err
+		}
+	}
+	return count, nil
+}
+
 // Build an Image object from raw json data
 func NewImgJSON(src []byte) (*Image, error) {
 	ret := &Image{}

runtime.go

@@ -23,6 +23,9 @@ import (
 	"time"
 )
 
+// Set the max depth to the aufs restriction
+const MaxImageDepth = 42
+
 var defaultDns = []string{"8.8.8.8", "8.8.4.4"}
 
 type Capabilities struct {
@@ -366,6 +369,17 @@ func (runtime *Runtime) Create(config *Config, name string) (*Container, []strin
 		return nil, nil, err
 	}
 
+	// We add 2 layers to the depth because the container's rw and
+	// init layer add to the restriction
+	depth, err := img.Depth()
+	if err != nil {
+		return nil, nil, err
+	}
+
+	if depth+2 >= MaxImageDepth {
+		return nil, nil, fmt.Errorf("Cannot create container with more than %d parents", MaxImageDepth)
+	}
+
 	checkDeprecatedExpose := func(config *Config) bool {
 		if config != nil {
 			if config.PortSpecs != nil {