jungle/m/module/shared-nix-store.nix

{ ... }:
{
  # Don't make the nix store read-only, as this would prevent the overlay FS
  # from being able to mount it.
  boot.readOnlyNixStore = false;

  # The nix-daemon.socket has an unnecessary dependency over the /nix/store
  # mount point. But that mount point won't be provided until the network is
  # ready. However, the network-address-eno1.service, has a dependency over
  # sockets.target, causing a cycle.
  # One solution is to make the nix-daemon.socket depend only on the socket
  # patch (which is already covered by ConditionPathIsReadWrite =
  # /nix/var/nix/daemon-socket), instead on the /nix/store.
  #
  # Using systemd.sockets.nix-daemon.unitConfig.RequiresMountsFor =
  # "/nix/var/nix/daemon-socket" doesn't work, as the the mount options get
  # added by systemd when the override config is merged with the one that Nix
  # provides:
  #
  # owl2% sudo systemctl show nix-daemon.socket | grep RequiresMountsFor
  # RequiresMountsFor=/nix/store /nix/var/nix/daemon-socket/socket /nix/var/nix/daemon-socket
  # 
  # To fix this, the Nix package is patched to only depend on /nix/var instead.
  # See ../../pkgs/overlay.nix for details.

  # Mount the hut nix store via NFS in read-only mode.
  fileSystems."/mnt/hut-nix-store" = {
    device = "hut:/nix/store";
    fsType = "nfs";
    options = [ "ro" ];
  };

  # A workdir is also needed, so setup a permanent dir using tmpfiles.
  systemd.tmpfiles.rules = [
    "d /mnt/nix-work 0700 root root -"
  ];

  # Mount an overlay in /nix/store using as lower layer the NFS store and upper
  # layer the disk nix store. The destination is still the nix store in
  # /nix/store (confusing). We need rw access, as the daemon need to write the
  # lock files to build derivations locally. Use a systemd mount unit directly
  # so we can specify the LazyUmount option and we avoid having it mounted
  # in the stage1 before systemd.
  systemd.mounts = [
    {
      what = "overlay";
      type = "overlay";
      where = "/nix/store";
      # We need the local-fs.target to be ready, so the network interfaces can
      # be configured to the network.target is reached. So make this a netdev
      # mount.
      options = "_netdev,lowerdir=/mnt/hut-nix-store,upperdir=/nix/store,workdir=/mnt/nix-work";
      description = "Overlay /nix/store mount";
      mountConfig = {
        LazyUnmount = true;
      };
      unitConfig = {
        # We need to wait for the NFS mount
        RequiresMountsFor = "/nix/store /mnt/hut-nix-store"; 
        Before = "remote-fs.target";
      };
    }
  ];
}