chunked: define error for partial pulls not available

define a new error type so that the caller can determine whether it is
safe to ignore the error and retrieve the resource fully.

Closes: containers#2115

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

pkg/chunked/internal/errors.go

@@ -0,0 +1,8 @@
+package internal
+
+import (
+	"errors"
+)
+
+// ErrPartialContentNotAvailable is returned when the requested content does not support partial content retrieval
+var ErrPartialContentNotAvailable = errors.New("partial content not available")

pkg/chunked/storage_linux.go

@@ -146,40 +146,48 @@ func (c *chunkedDiffer) convertTarToZstdChunked(destDirectory string, payload *o
 func GetDiffer(ctx context.Context, store storage.Store, blobDigest digest.Digest, blobSize int64, annotations map[string]string, iss ImageSourceSeekable) (graphdriver.Differ, error) {
 	pullOptions := store.PullOptions()
 
+	convertImages := parseBooleanPullOption(pullOptions, "convert_images", false)
+
 	if !parseBooleanPullOption(pullOptions, "enable_partial_images", true) {
-		return nil, errors.New("enable_partial_images not configured")
+		return nil, internal.ErrPartialContentNotAvailable
 	}
 
 	zstdChunkedTOCDigestString, hasZstdChunkedTOC := annotations[internal.ManifestChecksumKey]
 	estargzTOCDigestString, hasEstargzTOC := annotations[estargz.TOCJSONDigestAnnotation]
 
 	if hasZstdChunkedTOC && hasEstargzTOC {
-		return nil, errors.New("both zstd:chunked and eStargz TOC found")
+		if !convertImages {
+			return nil, fmt.Errorf("both zstd:chunked and eStargz TOC found: %w", internal.ErrPartialContentNotAvailable)
+		}
+		// If a conversion is required, ignore both TOCs and retrieve the entire layer.
+		hasZstdChunkedTOC, hasEstargzTOC = false, false
 	}
 
 	if hasZstdChunkedTOC {
 		zstdChunkedTOCDigest, err := digest.Parse(zstdChunkedTOCDigestString)
-		if err != nil {
-			return nil, fmt.Errorf("parsing zstd:chunked TOC digest %q: %w", zstdChunkedTOCDigestString, err)
+		if err == nil {
+			return makeZstdChunkedDiffer(store, blobSize, zstdChunkedTOCDigest, annotations, iss, pullOptions)
+		}
+		if !convertImages {
+			return nil, fmt.Errorf("parsing zstd:chunked TOC digest %q: %w", internal.ErrPartialContentNotAvailable)
 		}
-		return makeZstdChunkedDiffer(store, blobSize, zstdChunkedTOCDigest, annotations, iss, pullOptions)
 	}
 	if hasEstargzTOC {
 		estargzTOCDigest, err := digest.Parse(estargzTOCDigestString)
-		if err != nil {
-			return nil, fmt.Errorf("parsing estargz TOC digest %q: %w", estargzTOCDigestString, err)
+		if err == nil {
+			return makeEstargzChunkedDiffer(store, blobSize, estargzTOCDigest, iss, pullOptions)
+		}
+		if !convertImages {
+			return nil, fmt.Errorf("parsing eStargz TOC digest %q: %w", internal.ErrPartialContentNotAvailable)
 		}
-		return makeEstargzChunkedDiffer(store, blobSize, estargzTOCDigest, iss, pullOptions)
 	}
-
+	if !convertImages {
+		return nil, fmt.Errorf("convert_images not configured: %w", internal.ErrPartialContentNotAvailable)
+	}
 	return makeConvertFromRawDiffer(store, blobDigest, blobSize, iss, pullOptions)
 }
 
 func makeConvertFromRawDiffer(store storage.Store, blobDigest digest.Digest, blobSize int64, iss ImageSourceSeekable, pullOptions map[string]string) (*chunkedDiffer, error) {
-	if !parseBooleanPullOption(pullOptions, "convert_images", false) {
-		return nil, errors.New("convert_images not configured")
-	}
-
 	layersCache, err := getLayersCache(store)
 	if err != nil {
 		return nil, err

pkg/chunked/toc/toc.go

@@ -39,3 +39,10 @@ func GetTOCDigest(annotations map[string]string) (*digest.Digest, error) {
 		return nil, nil
 	}
 }
+
+// IsPartialContentNotAvailableError returns true if the error means that the requested content cannot
+// be fetched partially.  The caller can use this function to determine if the error can be ignored and
+// the content should be fetched fully.
+func IsPartialContentNotAvailableError(err error) bool {
+	return errors.Is(err, internal.ErrPartialContentNotAvailable)
+}