Add migrations directory for temporary helpers

hosts/Rupert.nix

@@ -13,8 +13,6 @@ in
         ./transcode.nix
         ./development.nix
         ./bspwm.nix
-
-        # ./open-pgsql.nix
     ];
 
     nixpkgs.overlays = let

migrations/pgsql_upgrade.nix

@@ -0,0 +1,33 @@
+{ config, lib, pkgs, ... }:
+{
+    environment.systemPackages = [(
+        let
+            # XXX specify the postgresql package you'd like to upgrade to.
+            # Do not forget to list the extensions you need.
+            newPostgres = pkgs.postgresql_16.withPackages (pp: [
+                # pp.plv8
+            ]);
+            cfg = config.services.postgresql;
+        in
+        pkgs.writeScriptBin "upgrade-pg-cluster" ''
+            set -eux
+            # XXX it's perhaps advisable to stop all services that depend on postgresql
+            systemctl stop postgresql
+
+            export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"
+            export NEWBIN="${newPostgres}/bin"
+
+            export OLDDATA="${cfg.dataDir}"
+            export OLDBIN="${cfg.finalPackage}/bin"
+
+            install -d -m 0700 -o postgres -g postgres "$NEWDATA"
+            cd "$NEWDATA"
+            sudo -u postgres "$NEWBIN/initdb" -D "$NEWDATA" ${lib.escapeShellArgs cfg.initdbArgs}
+
+            sudo -u postgres "$NEWBIN/pg_upgrade" \
+                --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
+                --old-bindir "$OLDBIN" --new-bindir "$NEWBIN" \
+                "$@"
+        ''
+    )];
+}