Subject: Re: bug#36876: guix system delete-generations removes custom boot menu entries

From: [email-masked] (Jakob L. Kreuze)
Date: Tue, 06 Aug 2019 09:22:59 -0400

Jesse Gibbons <[email-masked]> writes:

> Alternatively, we could save in the store a derivation for the the grub
> config generated from the bootloader of the configuration. When the
> user calls "guix system delete-generations", the derivation can be run,
> and the remaining system generations (if any) can be appended in a menu
> like when the user calls "guix system reconfigure". (Although it does
> not work for me right now, I'm assuming "guix system delete-generations 
> 2m" as described in the manual will be implemented in the future.)
>
> The immediate downsides I see to my solution:
>
> - It would take space in the store per generation, which can add up if
> the user does not often call "guix system delete-generations" and calls
> "guix system reconfigure" on a healthy basis. The user could just be
> reminded to call "guix system delete-generations" occasionally, and any
>  official service that automatically updates the system via "guix
> system reconfigure" can (and considering how large a generation with a
> lot of updated system packages can get, probably should) also be
> configured to call "guix system delete-generations".
>
> - If someone hand-edits the grub config the changes would be lost. This
> is the case as it is right now, and grub options can be edited in the
> configuration, so I'm not too concerned about this.
>
> -It would be much simpler to identify menu entries generated by guix
> that are no longer in the store and remove them, and remove all empty
> submenus. Parsing would make hand-editing grub.cfg more dangerous than
> a solution that simply scraps the hand-made changes and rebuilds as I
> propose, because the user doing the hand-editing would not necessarily
> be aware what patterns the parser checks. It would also be
> inconsitent: edits to grub.cfg being scrapped when "guix system
> reconfigure" is called, but not when 'guix system delete-generations"
> is called looks to me like a good way to introduce a bug to the more
> adventurous "Murphy's Law"-type users down the road.

I haven't tried it yet, but 'menuentry' seems as though it would be a
fairly simple structure to parse.

> These are just my thoughts. I would love to hear other downsides to my
> solution.

I prefer your suggestion to mine and think that the benefits may
outweigh the costs in this case. This seems to fit into the Guix idea of
purity a bit better than parsing a grub.cfg that may have been
overwritten by another system.

Regards,
Jakob