Ensure uncompressed layers from c/storage are recorded as uncompresse…

…d on upload

When copying an uncompressed layer without modification, arrange
types.BlobInfo.CompressionOperation to be set to types.Decompress,
so that we edit the manifest to use an uncompressed MIME type
(and not, e.g. a zstd:chunked MIME type which might not be representable
in the destination at all).

This is all still gated by the
> 	if srcInfosUpdated || layerDigestsDiffer(srcInfos, destInfos) {
condition in copyLayers, so we don't do these manifest updates
frivolously.

Signed-off-by: Miloslav Trmač <[email protected]>

copy/compression.go

@@ -250,21 +250,25 @@ func (ic *imageCopier) bpcPreserveOriginal(_ *sourceStream, detected bpDetectCom
 	// so that src.UpdatedImage() doesn’t fail; assume that for such blobs
 	// LayerInfosForCopy() should not be making any changes in the first place.
 	var bpcOp bpcOperation
+	var uploadedOp types.LayerCompression
 	var algorithm *compressiontypes.Algorithm
 	switch {
 	case !layerCompressionChangeSupported:
 		bpcOp = bpcOpPreserveOpaque
+		uploadedOp = types.PreserveOriginal
 		algorithm = nil
 	case detected.isCompressed:
 		bpcOp = bpcOpPreserveCompressed
+		uploadedOp = types.PreserveOriginal
 		algorithm = &detected.format
 	default:
 		bpcOp = bpcOpPreserveUncompressed
+		uploadedOp = types.Decompress
 		algorithm = nil
 	}
 	return &bpCompressionStepData{
 		operation:              bpcOp,
-		uploadedOperation:      types.PreserveOriginal,
+		uploadedOperation:      uploadedOp,
 		uploadedAlgorithm:      algorithm,
 		srcCompressorName:      detected.srcCompressorName,
 		uploadedCompressorName: detected.srcCompressorName,