On a bit of further investigation, the NFS files have an xattr `system.nfs4_acl`. This can be read, but attempting to write it fails with EINVAL. Attempting to copy from NFS to non-NFS fails with ENOTSUP, which is caught and ignored, but copying from NFS to NFS raises EINVAL, which raises.

Adding `EINVAL` to the ignored errnos would fix the problem, but might hide real failures (I'm not sure about the real failures, but it seems logical).

Since the `copy_function` is customizable to switch between `copy` and `copy2`, making copystat optional on files, perhaps the `copystat` should be optional on directories, as well.