add upgrade-pg-cluster

nixos/modules/profiles/base.nix

@@ -4,6 +4,10 @@ let
   inputs = config.dadada.inputs;
 in
 {
+  imports = [
+    ./upgrade-pg-cluster.nix
+  ];
+
   i18n.defaultLocale = mkDefault "en_US.UTF-8";
   console = mkDefault {
     font = "Lat2-Terminus16";

nixos/modules/profiles/upgrade-pg-cluster.nix

@@ -0,0 +1,32 @@
+{ config, pkgs, lib, ... }:
+{
+  environment.systemPackages = lib.mkIf config.services.postgresql.enable [
+    (
+      let
+        # Do not forget to list the extensions you need.
+        newPostgres = pkgs.postgresql_15.withPackages (pp: [ ]);
+      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="${config.services.postgresql.dataDir}"
+        export OLDBIN="${config.services.postgresql.package}/bin"
+
+        install -d -m 0700 -o postgres -g postgres "$NEWDATA"
+        cd "$NEWDATA"
+        sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"
+
+        sudo -u postgres $NEWBIN/pg_upgrade \
+          --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
+          --old-bindir $OLDBIN --new-bindir $NEWBIN \
+          "$@"
+      ''
+    )
+  ];
+}