feat: upgrade postgres & add update script

hosts/home-server.nix

@@ -25,6 +25,7 @@ in
     ../virtualisation/podman.nix
     ../modules/inadyn.nix
     ../modules/systemdNotify.nix
+    ../services/postgres
     ../services/mail.nix
     ../services/lldap.nix
     ../services/authelia.nix

services/postgres/default.nix

@@ -0,0 +1,9 @@
+{ pkgs, ... }:
+
+{
+  # Update `newPostgres` version, uncomment & run `upgrade-pg-cluster`. For details,
+  # see https://nixos.org/manual/nixos/stable/#module-services-postgres-upgrading
+  # imports = [ ./upgrade-pg-cluster.nix ];
+
+  services.postgresql.package = pkgs.postgresql_16;
+}

services/postgres/upgrade-pg-cluster.nix

@@ -0,0 +1,41 @@
+{
+  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.package}/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 $NEWBIN \
+          "$@"
+      ''
+    )
+  ];
+}