Refactor checking for compressed diff type

Signed-off-by: Darren Stahl <darst@microsoft.com>

images/image.go

@@ -3,6 +3,7 @@ package images
 import (
 	"context"
 	"encoding/json"
+	"strings"
 	"time"
 
 	"github.com/containerd/containerd/content"
@@ -359,3 +360,22 @@ func RootFS(ctx context.Context, provider content.Provider, configDesc ocispec.D
 	}
 	return config.RootFS.DiffIDs, nil
 }
+
+// IsCompressedDiff returns true if mediaType is a known compressed diff media type.
+// It returns false if the media type is a diff, but not compressed. If the media type
+// is not a known diff type, it returns errdefs.ErrNotImplemented
+func IsCompressedDiff(ctx context.Context, mediaType string) (bool, error) {
+	switch mediaType {
+	case ocispec.MediaTypeImageLayer, MediaTypeDockerSchema2Layer:
+	case ocispec.MediaTypeImageLayerGzip, MediaTypeDockerSchema2LayerGzip:
+		return true, nil
+	default:
+		// Still apply all generic media types *.tar[.+]gzip and *.tar
+		if strings.HasSuffix(mediaType, ".tar.gzip") || strings.HasSuffix(mediaType, ".tar+gzip") {
+			return true, nil
+		} else if !strings.HasSuffix(mediaType, ".tar") {
+			return false, errdefs.ErrNotImplemented
+		}
+	}
+	return false, nil
+}