[cache] Fix descriptor duplication in getAvailableBlobs

The walkBlob function was called with a function having a side effect: appending a descriptor to a list. However, the function could be called multiple times for the same descriptor (once in `walkBlob` and once in `walkBlobVariantsOnly` in that case) leading to duplicate entries in the final list.
Instead, we will check each time we want to add an element, that it's not already in the list using slice.ContainsFunc.
This should be fine from a performance perspective as the list is small (at most 2 compressions in the vast majority of cases).

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

Author: Fricounet

cache/remote.go

@@ -114,7 +114,16 @@ func getAvailableBlobs(ctx context.Context, cs content.Store, chain *solver.Remo
 	}
 	var descs []ocispecs.Descriptor
 	if err := walkBlob(ctx, cs, target, func(desc ocispecs.Descriptor) bool {
-		descs = append(descs, desc)
+		// Nothing prevents this function from being called multiple times for the same descriptor.
+		// 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
+		if !slices.ContainsFunc(descs, func(d ocispecs.Descriptor) bool {
+			return d.Digest == desc.Digest
+		}) {
+			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.