[cache] Revert to using a list for stable ordering

Do deduplication by simply iterating over the list which is fine due to the small number of available compressions (1 or 2 in the vast majority of cases)

Signed-off-by: Baptiste Girard-Carrabin <[email protected]>

Author: Fricounet

cache/remote.go

@@ -112,11 +112,19 @@ func getAvailableBlobs(ctx context.Context, cs content.Store, chain *solver.Remo
 	if err != nil {
 		return nil, err
 	}
-	var descs = map[digest.Digest]ocispecs.Descriptor{}
+	var descs []ocispecs.Descriptor
 	if err := walkBlob(ctx, cs, target, func(desc ocispecs.Descriptor) bool {
 		// Nothing prevents this function from being called multiple times for the same descriptor.
-		// Using a map will prevent duplicates in the result.
-		descs[desc.Digest] = desc
+		// So we need to make sure we don't add the same descriptor again.
+		// Looping over the list is preferable:
+		// 1. to avoid using a map, which don't preserve the order of descriptors,
+		// 2. descs will have a length the number of compression variants for a blob, which is usually very small
+		for _, d := range descs {
+			if d.Digest == desc.Digest {
+				return true // already seen this descriptor
+			}
+		}
+		descs = append(descs, desc)
 		return true
 	}); err != nil {
 		bklog.G(ctx).WithError(err).Warn("failed to walk variant blob") // is not a critical error at this moment.