proxy: Add GetLayerInfo API

Extract the LayerInfos of cached image
used for exposing diffIDs of Blobs

Signed-off-by: RishabhSaini <rsaini@redhat.com>

cmd/skopeo/proxy.go

@@ -92,7 +92,8 @@ import (
 // 0.2.2: Added support for fetching image configuration as OCI
 // 0.2.3: Added GetFullConfig
 // 0.2.4: Added OpenImageOptional
-const protocolVersion = "0.2.4"
+// 0.2.5: Added LayerInfoJSON
+const protocolVersion = "0.2.5"
 
 // maxMsgSize is the current limit on a packet size.
 // Note that all non-metadata (i.e. payload data) is sent over a pipe.
@@ -173,6 +174,14 @@ type proxyHandler struct {
 	activePipes map[uint32]*activePipe
 }
 
+// convertedLayerInfo is the reduced form of the OCI type BlobInfo
+// Used in the return value of GetLayerInfo
+type convertedLayerInfo struct {
+	Digest    digest.Digest `json:"digest"`
+	Size      int64         `json:"size"`
+	MediaType string        `json:"media_type"`
+}
+
 // Initialize performs one-time initialization, and returns the protocol version
 func (h *proxyHandler) Initialize(args []interface{}) (replyBuf, error) {
 	h.lock.Lock()
@@ -614,6 +623,56 @@ func (h *proxyHandler) GetBlob(args []interface{}) (replyBuf, error) {
 	return ret, nil
 }
 
+// GetLayerInfo returns data about the layers of an image, useful for reading the layer contents.
+//
+// This needs to be called since the data returned by GetManifest() does not allow to correctly
+// calling GetBlob() for the containers-storage: transport (which doesn’t store the original compressed
+// representations referenced in the manifest).
+func (h *proxyHandler) GetLayerInfo(args []interface{}) (replyBuf, error) {
+	h.lock.Lock()
+	defer h.lock.Unlock()
+
+	var ret replyBuf
+
+	if h.sysctx == nil {
+		return ret, fmt.Errorf("client error: must invoke Initialize")
+	}
+
+	if len(args) != 1 {
+		return ret, fmt.Errorf("found %d args, expecting (imgid)", len(args))
+	}
+
+	imgref, err := h.parseImageFromID(args[0])
+	if err != nil {
+		return ret, err
+	}
+
+	ctx := context.TODO()
+
+	err = h.cacheTargetManifest(imgref)
+	if err != nil {
+		return ret, err
+	}
+	img := imgref.cachedimg
+
+	layerInfos, err := img.LayerInfosForCopy(ctx)
+	if err != nil {
+		return ret, err
+	}
+
+	if layerInfos == nil {
+		layerInfos = img.LayerInfos()
+	}
+
+	var layers []convertedLayerInfo
+	for _, layer := range layerInfos {
+		layers = append(layers, convertedLayerInfo{layer.Digest, layer.Size, layer.MediaType})
+	}
+
+	ret.value = layers
+	return ret, nil
+}
+
 // FinishPipe waits for the worker goroutine to finish, and closes the write side of the pipe.
 func (h *proxyHandler) FinishPipe(args []interface{}) (replyBuf, error) {
 	h.lock.Lock()
@@ -736,6 +795,8 @@ func (h *proxyHandler) processRequest(readBytes []byte) (rb replyBuf, terminate
 		rb, err = h.GetFullConfig(req.Args)
 	case "GetBlob":
 		rb, err = h.GetBlob(req.Args)
+	case "GetLayerInfo":
+		rb, err = h.GetLayerInfo(req.Args)
 	case "FinishPipe":
 		rb, err = h.FinishPipe(req.Args)
 	case "Shutdown":