Missing labels when `Mount` is called on custom remote snapshotter

[aochagavia]

I'm trying out the "Make your remote snapshotter" feature, mentioned in the overview, but I can't get it working.

Context

Here's what I've done:

1. Create a type that implements the FileSystem interface (with Mount, Check and Unmount functions) and expose it through a gRPC server on unix:///dev/shm/test-snapshotter.sock.

2. Configure containerd's config.toml, registering the snapshotter under the name mysnap:

   [proxy_plugins]
     [proxy_plugins.mysnap]
       type = "snapshot"
       address = "/dev/shm/test-snapshotter.sock"

3. Run nerdctl pull debian --snapshotter mysnap

Problem

During nerdctl pull, the snapshotter receives a call to Mount. However, the only provided label is containerd.io/snapshot.ref, which is not enough information to retrieve the remote layer. Note that, if I pull using crictl pull, I do receive additional labels such as containerd.io/snapshot/cri.layer-digest.

Is there anything I need to configure to get this working? If it helps, I can create a repository that reproduces the issue.