services: postgres: add upgrade script

services/postgresql.nix

@@ -1,10 +1,40 @@
 {
   config,
   pkgs,
+  lib,
   ...
 }: {
   # set postgresql version so we don't get any bad surprise
   config.services.postgresql = {
     package = pkgs.postgresql_15;
   };
+
+  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;
+      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 $NEWBIN \
+        "$@"
+    '')
+  ];
 }