Add Postgres upgrade script

overlays/default.nix

@@ -60,6 +60,7 @@ in rec {
 	setup-ssh = prev.callPackage ./setup-ssh {
 		pkgs = final.pkgs;
 	};
+	upgrade-pg-cluster = prev.callPackage ./upgrade-pg-cluster.nix {};
 
 	# Overrides of packages
 	brew = prev.callPackage ./homebrew.nix {};

overlays/upgrade-pg-cluster.nix

@@ -0,0 +1,24 @@
+{ postgresql_15, postgresql_14, writeScriptBin, ... }:
+
+let
+	newPostgres = postgresql_15;
+	oldPostgres = postgresql_14;
+in writeScriptBin "upgrade-pg-cluster" ''
+	set -eux
+	systemctl stop postgresql
+
+	export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"
+	export NEWBIN="${newPostgres}/bin"
+
+	export OLDDATA="/var/lib/postgresql/${oldPostgres.psqlSchema}"
+	export OLDBIN="${oldPostgres}/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 "$NEWBIN" \
+	    "$@"
+''