Automatic rebuild on every push to master

[zupo]

I want the tv device to automatically pull latest changes and run nixos-rebuild && reboot every time tv.nix is updated in GitHub's master.

Ideas:

• use cron to run git pull && nixos-rebuild
• run a small webservice that listens to GitHub webhooks and then runs git pull && nixos-rebuild
• something smarter?

Bonus karma points if it automatically boots into previous configuration in case it cannot boot into the new configuration.

[infinisil]

Unless very low latency updates are needed, I'd think a cron job or systemd timer would work best for this indeed, because then it doesn't depend on having some external stateful setup (e.g. a webhook setup for the GitHub repo), so it's more self-contained. This in turn also means that it's possible to easily use alternate git repositories.

Personally I'd recommend a systemd timer/service as they're more flexible than cron jobs (though a bit more verbose to set up too). A timer/service combo can look like this:

{
  systemd.timers.git-updater = {
    wantedBy = [ "timers.target" ];
    # Wait 60 seconds after the service finished before starting it again
    # This should prevent the service being started again too early if an update is in progress
    timerConfig.OnUnitInactiveSec = 60;
  };
  systemd.services.git-updater = {
    # I'm not entirely sure why this would be needed
    serviceConfig.Type = "oneshot";
    script = ''
      # Update script here
    '';
  };
}

And I already have a working update script for git repos here that can be adapted: https://github.com/Infinisil/nixbot/blob/feefc301bbe44742570bce0974005a2714a950e6/module.nix#L84-L113