home-manager: add backup overwrite option

[saygo-png]

Description

Closes #4199.

When using the backupFileExtension option, if the backup file exists, the activation process fails. This adds an opt in option to instead overwrite the old backup instead of failing.

This feature is highly requested, the issue above is the 6th most liked issue on the tracker.

I have not added a help text mentioning this option on collision to be conservative. This setting removes data, so I don't think suggesting it for any collision is responsible.

There was a previous attempt at implementing this here #4971 but the contributor gave up due to opposition to overwriting data. Since then Hjem (A minimal home-manager alternative) has implemented a very similar feature https://hjem.feel-co.org/options.html#option-hjem-clobberByDefault.

With Hjem setting the precedent, and it being one of the most requested features (judging by reactions) I hope this change can receive more favorable second look.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt deadnix keep-sorted --run treefmt.

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  nix-shell --pure tests -A run.all.
  (It was taking too long due to my slow connection)

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[khaneliman]

I did have pretty much the exact same change earlier this year, too. #6491

[saygo-png]

I did have pretty much the exact same change earlier this year,

I missed that! Looks like the objection was to this overwriting backups. I would be okay with just overwriting the files directly — this is what Hjem does but I'm guessing @teto would object to this too. If we really don't want this I can compromise on generating multiple backups (though i'd rather not) but Teto seems to be against this too... #6243 (comment)

I know about force and it is not an adequate solution. It only allows you to set that per file which still allows for random programs to break home manager activation unless you force every file.

To put this into perspective, many NixOS users wipe most of their $HOME on reboot with impermanence (including me). Overwriting config files that are mostly autogenerated (its unlikely a user causes multiple backups by hand) is really not a big deal. Once again this is opt in with a clear name — anyone turning this option on will be aware of potential data loss.

[khaneliman]

Yeah, I'm leaning towards an opt in feature makes sense. It harms no one unexpectedly since you have to choose to do so.

[rycee]

Fair enough, I think it's worth trying and see if it fulfills peoples' needs.

[saygo-png]

Thank you so much!!!