services: postgresql: add upgrade script

services/postgresql.nix

@@ -8,4 +8,32 @@
   config.services.postgresql = {
     package = pkgs.postgresql_16;
   };
+
+  config.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_17;
+      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" \
+        "$@"
+    '')
+  ];
 }