Impermanence issues with SSH when provisioning with nixos-anywhere + disko + flakes

[visualphoenix]

I encountered an issue when trying to use impermanence to mount SSH host keys in NixOS while provisioning a new host with nixos-anywhere + flakes + disko.

I believe the problem is because the SSH host keys are generated during at boot time. The nixos-anywhere provisioning process fails with the following output:

### Installing NixOS ###
Pseudo-terminal will not be allocated because stdin is not a terminal.
Warning: Permanently added 'xxx.xxx.xxx.xxx' (ED25519) to the list of known hosts.
installing the boot loader...
...
...
During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/nix/store/1lksf0kkffcnw5l8ryq5imai8pdlpy13-bza6dmx1w5c0xrvs1m7704ijnzqcrsfi-systemd-boot", line 394, in <module>
    main()
  File "/nix/store/1lksf0kkffcnw5l8ryq5imai8pdlpy13-bza6dmx1w5c0xrvs1m7704ijnzqcrsfi-systemd-boot", line 377, in main
    install_bootloader(args)
  File "/nix/store/1lksf0kkffcnw5l8ryq5imai8pdlpy13-bza6dmx1w5c0xrvs1m7704ijnzqcrsfi-systemd-boot", line 267, in install_bootloader
    machine_id = subprocess.run(
                 ^^^^^^^^^^^^^^^
  File "/nix/store/7hnr99nxrd2aw6lghybqdmkckq60j6l9-python3-3.11.9/lib/python3.11/subprocess.py", line 571, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['/nix/store/gv0jmdv734pdxg6ilb4kq2np2fxxkr39-systemd-255.6/bin/systemd-machine-id-setup', '--print']' returned non-zero exit status 1.
installation finished!
umount: /mnt/boot unmounted
umount: /mnt/nix unmounted
umount: /mnt unmounted
### Waiting for the machine to become reachable again ###
ssh: connect to host xxx.xxx.xxx.xxx port 22: Connection refused
### Done! ###

As a workaround, I tried making the entire /etc/ssh directory persistent. This allows nixos-anywhere to provision the system successfully. However, on the first boot, the sshd_config file from the nix-store is not present in the /etc/ssh persistent directory.

A reproducible MVP is here: https://github.com/visualphoenix/nixos-anywhere-disko-impermanence-mvp

Expected behavior:

• Impermanence should handle the mounting of SSH host keys if the individual host files are persisted.
• The sshd_config file should be available in the persistent directory on the first boot if persisting the whole /etc/ssh directory
• nixos-anywhere should be able to provision a new host successfully when using impermanence in both cases

Actual behavior:

• Provisioning with nixos-anywhere fails when using impermanence to mount SSH host keys.
• Making /etc/ssh a persistent directory allows provisioning to succeed, but the sshd_config file is missing from the persistent directory on the first boot.

Please let me know if you need any further information or clarification regarding this issue.