The project I was working on in the last weeks was moved under the github.com/containers umbrella.
With Linux 4.18 it will be possible to mount a FUSE file system in an user namespace. fuse-overlayfs is an implementation in user space of the overlay file system already present in the Linux kernel, but that can be mounted only by the root user. Union file systems were around for a long time, allowing multiple layers to be stacked on top of each other where usually the last one is the only writeable.
Overlay is an union file system widely used for mounting OCI image. Each OCI image is made up of different layers, each layer can be used by different images. A list of layers, stacked on each other gives the final image that is used by a container. The last level, that is writeable, is specific for the container. This model enables different containers to use the same image that is accessible as read-only from the lower layers of the overlay file system.
The current implementation of the overlay file system is done directly in the kernel, at a very low level, allowing non privileged users to use it directly poses some security risks. In the longer term, once the security aspect is resolved, non privileged users will probably be able to mount directly an overlay file system.
For now, given the new feature in Linux 5.18, having an implementation of the overlay union in user space will enable rootless containers to use the same storage as containers running as root.
On Fedora Rawhide, where Linux 4.18 is available, it is already possible to take a taste of it with:
podman --storage-opt overlay2.fuse_program=/usr/bin/fuse-overlayfs run ...
The previous command tells podman to mount an overlay file system using the specified FUSE helper instead of mounting it directly through the kernel.