docker tarfile: report correct size of compressed layers

Tools like umoci will always compress layers, and in order to work
around some *lovely* issues with DiffIDs, tarfile.GetBlob would always
decompress them. However the BlobInfo returned from tarfile.GetBlob
would incorrectly give the size of the *compressed* layer because the
size caching code didn't actually check the layer size, resulting in
"skopeo copy" failing whenever sourcing umoci images.

As an aside, for some reason the oci: transport doesn't report errors
when the size is wrong...

Signed-off-by: Aleksa Sarai <[email protected]>

(Updated primarily to handle closing the decompressed stream.)
Signed-off-by: Miloslav Trmač <[email protected]>

docker/tarfile/src.go

@@ -307,7 +307,25 @@ func (s *Source) prepareLayerData(tarManifest *ManifestItem, parsedConfig *manif
 			return nil, err
 		}
 		if li, ok := unknownLayerSizes[h.Name]; ok {
-			li.size = h.Size
+			// Since GetBlob will decompress layers that are compressed we need
+			// to do the decompression here as well, otherwise we will
+			// incorrectly report the size. Pretty critical, since tools like
+			// umoci always compress layer blobs. Obviously we only bother with
+			// the slower method of checking if it's compressed.
+			uncompressedStream, isCompressed, err := compression.AutoDecompress(t)
+			if err != nil {
+				return nil, errors.Wrapf(err, "Error auto-decompressing %s to determine its size", h.Name)
+			}
+			defer uncompressedStream.Close()
+
+			uncompressedSize := h.Size
+			if isCompressed {
+				uncompressedSize, err = io.Copy(ioutil.Discard, uncompressedStream)
+				if err != nil {
+					return nil, errors.Wrapf(err, "Error reading %s to find its size", h.Name)
+				}
+			}
+			li.size = uncompressedSize
 			delete(unknownLayerSizes, h.Name)
 		}
 	}