copy: make using partial blobs configurable

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

copy/copy.go

@@ -111,6 +111,7 @@ type copier struct {
 	compressionLevel  *int
 	ociDecryptConfig  *encconfig.DecryptConfig
 	ociEncryptConfig  *encconfig.EncryptConfig
+	fetchPartialBlobs bool
 }
 
 // imageCopier tracks state specific to a single image (possibly an item of a manifest list)
@@ -182,6 +183,8 @@ type Options struct {
 	// OciDecryptConfig contains the config that can be used to decrypt an image if it is
 	// encrypted if non-nil. If nil, it does not attempt to decrypt an image.
 	OciDecryptConfig *encconfig.DecryptConfig
+
+	FetchPartialBlobs bool // attempt to fetch the blob partially.  Experimental.
 }
 
 // validateImageListSelection returns an error if the passed-in value is not one that we recognize as a valid ImageListSelection value
@@ -257,9 +260,10 @@ func Image(ctx context.Context, policyContext *signature.PolicyContext, destRef,
 		// FIXME? The cache is used for sources and destinations equally, but we only have a SourceCtx and DestinationCtx.
 		// For now, use DestinationCtx (because blob reuse changes the behavior of the destination side more); eventually
 		// we might want to add a separate CommonCtx — or would that be too confusing?
-		blobInfoCache:    blobinfocache.DefaultCache(options.DestinationCtx),
-		ociDecryptConfig: options.OciDecryptConfig,
-		ociEncryptConfig: options.OciEncryptConfig,
+		blobInfoCache:     blobinfocache.DefaultCache(options.DestinationCtx),
+		ociDecryptConfig:  options.OciDecryptConfig,
+		ociEncryptConfig:  options.OciEncryptConfig,
+		fetchPartialBlobs: options.FetchPartialBlobs,
 	}
 	// Default to using gzip compression unless specified otherwise.
 	if options.DestinationCtx == nil || options.DestinationCtx.CompressionFormat == nil {
@@ -1135,7 +1139,7 @@ func (ic *imageCopier) copyLayer(ctx context.Context, srcInfo types.BlobInfo, to
 
 	imgSource, okSource := ic.c.rawSource.(types.ImageSourceSeekable)
 	imgDest, okDest := ic.c.dest.(types.ImageDestinationPartial)
-	if okSource && okDest && !diffIDIsNeeded {
+	if ic.c.fetchPartialBlobs && okSource && okDest && !diffIDIsNeeded {
 		bar := ic.c.createProgressBar(pool, true, srcInfo, "blob", "done")
 
 		progress := make(chan int64)