update TODOs, add note about machine-id

TODO.md

@@ -1,33 +1,24 @@
-* migration from impermanence works fine, but needs some specific documentation
-
 * add support for tmpfiles' modifiers?
+* maybe add some recommended config that users can opt-in to use in addition
+  to preservation?
+* with the recent upstream changes the handling of /etc/machine-id changed.
+  That requires another change upstream, i.e. the file needs to be created with content
+  `uninitialized\n` and on a setup with preservation something like the following needs
+  to be done to properly preserve the machine-id across reboots.
+  ```nix
+  {
+    preservation.preserveAt."/state".files = [
+      { file = "/etc/machine-id"; inInitrd = true; how = "symlink"; }
+    ];
 
-* custom systemd-tmpfiles service in stage-2?
-
-  Currently the two upstream services are used, one in initrd to prepare
-  files before switch-root, and the second one in stage-2 after switch-root.
-
-  Right now there is no way to provision files with the same systemd-tmpfiles
-  configuration inside a  directory structure that is also persisted by
-  Preservation, unless the preservation happens in stage-1 and the rest
-  in stage-2. This is because Preservation uses systemd-tmpfiles to create
-  files and directories before creating the corresponding bindmounts between them.
-
-  Another issue occurs with the systemd-journald-flush.service, which is ordered
-  to run before tmpfiles-setup, while having a dependency on var-log.mount.
-  Should /var/log be persisted with Preservation, the var-log.mount unit
-  will be generated by Preservation as a bindmount from the volatile to
-  the persistent volume and because the underlying directories need to
-  exist before the mount is created, var-log.mount will be ordered after
-  systemd-tmpfiles-setup, thus causing a dependency loop.
-
-  At this moment a simple solution to this problem is to configure the
-  preservation of /var/log with `inInitrd = true` so that everything is
-  already set up in stage-1, which avoids the dependency loop in stage-2
-  altogether.
-
-  One alternative to this would be to create a custom systemd-tmpfiles
-  service specifically for Preservation, that is ordered before the
-  default systemd-tmpfiles-setup.service. To make this feasible, it
-  would be nice to have access to the upstream NixOS option definitions
-  for `systemd.tmpfiles.settings` via the systemd-lib.
+    systemd.services.systemd-machine-id-commit = {
+      unitConfig.ConditionPathIsMountPoint = [
+        "" "/state/etc/machine-id"
+      ];
+      serviceConfig.ExecStart = [
+        "" "systemd-machine-id-setup --commit --root /state"
+      ];
+    };
+  }
+  ```
+  see https://github.com/NixOS/nixpkgs/pull/351151#issuecomment-2440122776