Exploratory: Add bootstrap script hook

[exFalso]

Hi, this PR is more of an exploratory PR, to see whether this idea is useful for the general public. Internally at our company we're using nixops to deploy to non-NixOS systemd distributions. One of the issues we used to have is that nixops makes certain assumptions about the target machine, namely that it has a working multi-user nix installation, and that it has a keys group defined for secrets. For a while we've handled this explicitly outside nixops, we basically made the machine look like NixOS for the purposes of nixops.

However, with a small addition to nixops, we now tackle this issue much more elegantly. Namely, this PR adds a nullable config option deployment.bootstrapScript, which (if set) is run on the master SSH connection before any nix-specific actions are taken, but after the resource closure has been built. This allows us to do the nix installation and certain early setup (like mounting of /nix from a separate hard disk), independent of the /nix/store.

The actual mechanism I added is a bit broken as it only handles None type targets, but the control flow of the resource configuration is a bit too tricky and I couldn't figure out how to do it for a general resource type.

Question is, is this a useful enough feature to be included upstream? If so, can I get any pointers to how to expose the option to a more generic resource type? Also, the changes are on top of the 1.7 tag (the PR is submitted against release-1.7, which seems to have diverged from the tag(??)), would this feature go into 2.0 only? I tried upgrading to 2.0 but got into a bunch of unrelated issues with deployment.

Thank you!

[pasqui23]

Suggested change

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