modules: services: postgres: add migration script

The process to upgrade is:

* Make sure the version number of the script is one major version over
  the service version.

* Activate the script, rebuild configuration.

* Run `upgrade-pg-cluster` as `root`. One can give arguments like
  `--link` or `--jobs 4` to speedup the process. See documentation for
  some details.

* Change package to new version once the upgrade is finished, rebuild
  configuration.

* Optionally, `ANALYZE` the new database.

modules/services/postgresql.nix

@@ -5,6 +5,9 @@ in
 {
   options.my.services.postgresql = with lib; {
     enable = my.mkDisableOption "postgres configuration";
+
+    # Transient option to be enabled for migrations
+    upgradeScript = mkEnableOption "postgres upgrade script";
   };
 
   config = lib.mkMerge [
@@ -14,5 +17,45 @@ in
         package = pkgs.postgresql_12;
       };
     })
+
+    # Taken from the manual
+    (lib.mkIf cfg.upgradeScript {
+      containers.temp-pg.config.services.postgresql = {
+        enable = true;
+        package = pkgs.postgresql_13;
+      };
+
+      environment.systemPackages =
+        let
+          newpg = config.containers.temp-pg.config.services.postgresql;
+        in
+        [
+          (pkgs.writeScriptBin "upgrade-pg-cluster" ''
+            #!/usr/bin/env bash
+
+            set -x
+            export OLDDATA="${config.services.postgresql.dataDir}"
+            export NEWDATA="${newpg.dataDir}"
+            export OLDBIN="${config.services.postgresql.package}/bin"
+            export NEWBIN="${newpg.package}/bin"
+
+            if [ "$OLDDATA" -ef "$NEWDATA" ]; then
+              echo "Cannot migrate to same data directory" >&2
+              exit 1
+            fi
+
+            install -d -m 0700 -o postgres -g postgres "$NEWDATA"
+            cd "$NEWDATA"
+            sudo -u postgres $NEWBIN/initdb -D "$NEWDATA"
+
+            systemctl stop postgresql    # old one
+
+            sudo -u postgres $NEWBIN/pg_upgrade \
+              --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
+              --old-bindir $OLDBIN --new-bindir $NEWBIN \
+              "$@"
+          '')
+        ];
+    })
   ];
 }