conformance tests: don't break on trailing zeroes in layer blobs

When analyzing a layer blob's contents, don't break if the blob has more
zeroes padding it out even after the tar reader thinks it's hit the end
of the archive.

Add more detail to the diagnostic error we print when there's a digest
or length mismatch, too, in case it's triggered by something other than
zero padding.

Don't ignore errors which might be encountered when we try to use skopeo
to copy an image to a directory.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com>

tests/blobcache.bats

@@ -61,8 +61,7 @@ function _check_matches() {
 	run_buildah login --tls-verify=false --authfile ${TEST_SCRATCH_DIR}/test.auth --username testuser --password testpassword localhost:${REGISTRY_PORT}
 	outputdir=${TEST_SCRATCH_DIR}/outputdir
 	mkdir -p ${outputdir}
-	run podman run --rm --mount type=bind,src=${TEST_SCRATCH_DIR}/test.auth,target=/test.auth,Z --mount type=bind,src=${outputdir},target=/output,Z --net host quay.io/skopeo/stable copy --preserve-digests --authfile=/test.auth --tls-verify=false docker://registry.fedoraproject.org/fedora-minimal dir:/output
-
+	podman run --rm --mount type=bind,src=${TEST_SCRATCH_DIR}/test.auth,target=/test.auth,Z --mount type=bind,src=${outputdir},target=/output,Z --net host quay.io/skopeo/stable copy --preserve-digests --authfile=/test.auth --tls-verify=false docker://registry.fedoraproject.org/fedora-minimal dir:/output
 	run_buildah rmi --all -f
 	run_buildah pull dir:${outputdir}
 	run_buildah images -a --format '{{.ID}}'

tests/conformance/conformance_test.go

@@ -39,6 +39,7 @@ import (
 	"github.com/containers/storage"
 	"github.com/containers/storage/pkg/archive"
 	"github.com/containers/storage/pkg/idtools"
+	"github.com/containers/storage/pkg/ioutils"
 	"github.com/containers/storage/pkg/reexec"
 	dockertypes "github.com/docker/docker/api/types"
 	dockerdockerclient "github.com/docker/docker/client"
@@ -934,11 +935,15 @@ func saveReport(ctx context.Context, t *testing.T, ref types.ImageReference, dir
 // summarizeLayer reads a blob and returns a summary of the parts of its contents that we care about
 func summarizeLayer(t *testing.T, imageName string, blobInfo types.BlobInfo, reader io.Reader) (layer Layer) {
 	compressedDigest := digest.Canonical.Digester()
-	uncompressedBlob, _, err := compression.AutoDecompress(io.TeeReader(reader, compressedDigest.Hash()))
+	counter := ioutils.NewWriteCounter(compressedDigest.Hash())
+	compressionAlgorithm, _, reader, err := compression.DetectCompressionFormat(reader)
+	require.NoErrorf(t, err, "error checking if blob %+v for image %q is compressed", blobInfo, imageName)
+	uncompressedBlob, wasCompressed, err := compression.AutoDecompress(io.TeeReader(reader, counter))
 	require.NoErrorf(t, err, "error decompressing blob %+v for image %q", blobInfo, imageName)
 	defer uncompressedBlob.Close()
 	uncompressedDigest := digest.Canonical.Digester()
-	tr := tar.NewReader(io.TeeReader(uncompressedBlob, uncompressedDigest.Hash()))
+	tarToRead := io.TeeReader(uncompressedBlob, uncompressedDigest.Hash())
+	tr := tar.NewReader(tarToRead)
 	hdr, err := tr.Next()
 	for err == nil {
 		header := fsHeaderForEntry(hdr)
@@ -953,8 +958,18 @@ func summarizeLayer(t *testing.T, imageName string, blobInfo types.BlobInfo, rea
 		hdr, err = tr.Next()
 	}
 	require.Equal(t, io.EOF, err, "unexpected error reading layer contents %+v for image %q", blobInfo, imageName)
+	_, err = io.Copy(io.Discard, tarToRead)
+	require.NoError(t, err, "reading out any not-usually-present zero padding at the end")
 	layer.CompressedDigest = compressedDigest.Digest()
-	require.Equal(t, blobInfo.Digest, layer.CompressedDigest, "calculated digest of compressed blob didn't match expected digest")
+	blobFormatDescription := "uncompressed"
+	if wasCompressed {
+		if compressionAlgorithm.Name() != "" {
+			blobFormatDescription = "compressed with " + compressionAlgorithm.Name()
+		} else {
+			blobFormatDescription = "compressed (?)"
+		}
+	}
+	require.Equalf(t, blobInfo.Digest, layer.CompressedDigest, "calculated digest of %s blob didn't match expected digest (expected length %d, actual length %d)", blobFormatDescription, blobInfo.Size, counter.Count)
 	layer.UncompressedDigest = uncompressedDigest.Digest()
 	return layer
 }