bug#38100: ‘--with-input’ causes unintended rebuilds

[Ludovic Courtès]

Hi,

Ludovic Courtès <address@hidden> skribis:

> Indeed, evaluating:
>
>   (bag-transitive-inputs
>    (package->bag ((package-input-rewriting '()) glib)))
>
> shows that we have two “python” packages there that are not ‘eq?’.

The problem is that ‘glib’ depends on ‘python-libxml2’, which uses
‘python-build-system’ and thus has ‘python’ as an implicit input.

‘package-input-rewriting’ doesn’t touch implicit inputs so it leaves
that implicit ‘python’ untouched.

Since ‘transitive-inputs’ (used by ‘bag-transitive-inputs’) uses pointer
equality, we end up with two “python” packages that are not ‘eq?’ but
are functionally equivalent: the one produced by
‘package-input-rewriting’, and the implicit dependency of
‘python-libxml2’.  QED.

(This is essentially the same as <https://bugs.gnu.org/30155>.)

I’m not sure how to address it.

Ludo’.