Fix possibly incorrect media type default on import

estesp · estesp · commit 4aff7431feaa · 2022-01-25T14:11:20.000-05:00
As reported, running import twice without using the compress import
option means that the content store will have existing layers during the
second import and the existing code hardcodes existing layer media type
to compressed. This fixes the issue by actually reading the header bytes
from the store and setting the media type appropriately.

Signed-off-by: Phil Estes &lt;estesp@amazon.com&gt;
diff --git a/images/archive/importer.go b/images/archive/importer.go
@@ -269,9 +269,13 @@ func resolveLayers(ctx context.Context, store content.Store, layerFiles []string
 		if ok {
 			desc := descs[digest.Digest(dgst)]
 			if desc != nil {
-				desc.MediaType = images.MediaTypeDockerSchema2LayerGzip
 				desc.Digest = info.Digest
 				desc.Size = info.Size
+				mediaType, err := detectLayerMediaType(ctx, store, *desc)
+				if err != nil {
+					return fmt.Errorf("failed to detect media type of layer: %w", err)
+				}
+				desc.MediaType = mediaType
 			}
 		}
 		return nil
@@ -381,3 +385,29 @@ func writeManifest(ctx context.Context, cs content.Ingester, manifest interface{
 
 	return desc, nil
 }
+
+func detectLayerMediaType(ctx context.Context, store content.Store, desc ocispec.Descriptor) (string, error) {
+	var mediaType string
+	// need to parse existing blob to use the proper media type
+	bytes := make([]byte, 10)
+	ra, err := store.ReaderAt(ctx, desc)
+	if err != nil {
+		return "", fmt.Errorf("failed to read content store to detect layer media type: %w", err)
+	}
+	defer ra.Close()
+	_, err = ra.ReadAt(bytes, 0)
+	if err != nil && err != io.EOF {
+		return "", fmt.Errorf("failed to read header bytes from layer to detect media type: %w", err)
+	}
+	if err == io.EOF {
+		// in the case of an empty layer then the media type should be uncompressed
+		return images.MediaTypeDockerSchema2Layer, nil
+	}
+	switch c := compression.DetectCompression(bytes); c {
+	case compression.Uncompressed:
+		mediaType = images.MediaTypeDockerSchema2Layer
+	default:
+		mediaType = images.MediaTypeDockerSchema2LayerGzip
+	}
+	return mediaType, nil
+}
