Use acirenderer for squashing

This commit leverages acirenderer from the spec repository to implement
squashing. This has the benefit of reusing the same code rocket uses for
resolving ACI dependencies.

We create a conversion store that implements the ACIRegistry
interface so we can use the rendering functions.

Resolves appc#10.

lib/conversion_store.go

@@ -0,0 +1,99 @@
+package docker2aci
+
+import (
+	"fmt"
+	"hash"
+	"io"
+	"io/ioutil"
+	"os"
+
+	"github.com/appc/spec/aci"
+	"github.com/appc/spec/schema"
+	"github.com/appc/spec/schema/types"
+)
+
+const (
+	hashPrefix = "sha512-"
+)
+
+type aciInfo struct {
+	path          string
+	key           string
+	ImageManifest *schema.ImageManifest
+}
+
+// ConversionStore is an simple implementation of the acirenderer.ACIRegistry
+// interface. It stores the Docker layers converted to ACI so we can take
+// advantage of acirenderer to generate a squashed ACI Image.
+type ConversionStore struct {
+	acis map[string]*aciInfo
+}
+
+func NewConversionStore() *ConversionStore {
+	return &ConversionStore{acis: make(map[string]*aciInfo)}
+}
+
+func (ms *ConversionStore) WriteACI(path string) (string, error) {
+	data, err := ioutil.ReadFile(path)
+	if err != nil {
+		return "", err
+	}
+
+	f, err := os.Open(path)
+	if err != nil {
+		return "", err
+	}
+	defer f.Close()
+
+	imageID := types.NewHashSHA512(data)
+
+	im, err := aci.ManifestFromImage(f)
+	if err != nil {
+		return "", err
+	}
+
+	key := imageID.String()
+	ms.acis[key] = &aciInfo{path: path, key: key, ImageManifest: im}
+	return key, nil
+}
+
+func (ms *ConversionStore) GetImageManifest(key string) (*schema.ImageManifest, error) {
+	aci, ok := ms.acis[key]
+	if !ok {
+		return nil, fmt.Errorf("aci with key: %s not found", key)
+	}
+	return aci.ImageManifest, nil
+}
+
+func (ms *ConversionStore) GetACI(name types.ACName, labels types.Labels) (string, error) {
+	for _, aci := range ms.acis {
+		// we implement this function to comply with the interface so don't
+		// bother implementing a proper label check
+		if aci.ImageManifest.Name.String() == name.String() {
+			return aci.key, nil
+		}
+	}
+	return "", fmt.Errorf("aci not found")
+}
+
+func (ms *ConversionStore) ReadStream(key string) (io.ReadCloser, error) {
+	aci, ok := ms.acis[key]
+	if !ok {
+		return nil, fmt.Errorf("stream for key: %s not found", key)
+	}
+	f, err := os.Open(aci.path)
+	if err != nil {
+		return nil, fmt.Errorf("error opening aci: %s", aci.path)
+	}
+
+	return f, nil
+}
+
+func (ms *ConversionStore) ResolveKey(key string) (string, error) {
+	return key, nil
+}
+
+func (ms *ConversionStore) HashToKey(h hash.Hash) string {
+	s := h.Sum(nil)
+	return fmt.Sprintf("%s%x", hashPrefix, s)
+}