add snaplink

I know snaplink should be put to the nix store, but currently I have no idea how to do it.

configuration.nix

@@ -85,6 +85,7 @@ let ssh_pubkeys = {
         daily = 7;
         autosnap = true;
         autoprune = true;
+        post_snapshot_script = "/var/src/snaplink";
       };
       datasets."rpool/nixos/home".use_template = [ "prod" ];
       datasets."rpool/nixos/var".use_template = [ "prod" ];

krops.nix

@@ -9,6 +9,9 @@ let
     {
       nixpkgs.symlink = "/root/.nix-defexpr/channels/nixos";
       nixos-config.file = toString ./configuration.nix;
+
+      # TODO: how to make this nix-managed?
+      snaplink.file = toString ./scripts/snaplink;
       secrets.pass = {
         dir = toString ./secrets;
         name = "hel1-a";

scripts/snaplink

@@ -0,0 +1,10 @@
+#!/usr/bin/env bash
+set -euo pipefail
+_fatal() { >&2 echo "ERROR: $*"; exit 1; }
+mountpoint=$(zfs list -H -t filesystem -o mountpoint "$SANOID_TARGET")
+[[ -n "$mountpoint" ]] || _fatal "expected non-empty $SANOID_TARGET"
+snap_path="$mountpoint/.zfs/snapshot/$SANOID_SNAPNAME"
+[[ -d "$snap_path" ]] || _fatal "expected to find a directory in $snap_path"
+link_path="$mountpoint/.snapshot-latest"
+ln -sf "$snap_path" "$link_path"
+>&2 printf "successfully created %s -> %s\n" "$link_path" "$snap_path"