Re: Graft hooks

[Christopher Lemmer Webber <cwebber@dustycloud.org>]

Timothy Sample writes:

> Hi Guix,
>
> I just submitted a patch for <https://bugs.gnu.org/30680>, but now I’m
> wondering if there isn’t a more general way to solve the problem.
>
> The bug has to do with grafting and checksums.  I know three bugs that
> follow this theme: the one above (Racket), <https://bugs.gnu.org/19973>
> (GDB), and <https://bugs.gnu.org/25752> (Go).  The basic problem is that
> these packages store checksums of files during build time.  If they get
> updated due to grafting, the files change, but the checksums do not.
> The checksums become invalid, which causes other problems like trying to
> update files in the store or asserting that debugging information is
> invalid.
>
> The patch I submitted makes Racket assume that files in the store are
> good.  It patches Racket to skip checksum validation if it is checking a
> file in the store.  A similar approach could be taken for GDB and Go.
>
> It occurs to me that if we had some way to run package-specific code
> during grafting we could solve problems like this easily and without
> patching software that is not broken.
>
> The basic idea would be to add a field (or use a property) to the
> package record.  Let’s call it “graft-hook”.  It would be Scheme code
> that gets run after grafting takes place, giving us a chance to patch
> special things like checksums.  The hook would be passed the list of
> files that were been modified during grafting.  Then, in the Racket
> package for example, I could write a graft-hook that updates the SHA-1
> hash of each of the modified source files.

This seems like a really good approach to me and seems also much nicer /
safer in the long run than the solution in #30680 since it wouldn't just
patch out the package in question's checks, it would correct them.  That
seems very good indeed to me.

> Since grafting is done at the derivation level, the hook code would have
> to be propagated down from the package level.  I haven’t looked at all
> the details yet, because maybe this is a bad idea and I shouldn’t waste
> my time!  :)  My first impression is that it is not too tricky.
>
> Are these problems too specialized to deserve a general mechanism like
> this?  Let me know what you think!
>
>
> -- Tim

As said, it seems good to me.  But I would be interested in what Mark
would think, since he is mostly responsible for the grafts design.