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

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

[Ludovic Courtès]

Hello,

Consider this example:

--8<---------------cut here---------------start------------->8---
$ guix build glib -nd
/gnu/store/9zz9hvzaz06f40a4cbvhskb183x676w4-glib-2.60.6.drv
$ guix build glib --with-input=inkscape=libreoffice -nd
/gnu/store/15f9jkpakmsaz8i2a0gy4kir1zyk29vi-glib-2.60.6.drv
$ guix describe
Generacio 114	Nov 02 2019 11:32:51	(nuna)
  guix ab1c063
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: ab1c063ab08e069fbe62919828fa634a2e222bbf
--8<---------------cut here---------------end--------------->8---

Since GLib does not depend on Inkscape, the ‘--with-input’ flag should
have no effect: we should get the same glib derivation.  However, we’re
not.

If we diff the ‘glibc-2.60.6-guile-builder’ files of each derivation, we
see that the second one has a duplicate entry:

  (define %build-inputs
    `(…
      ("python" . "/gnu/store/78w7y0lxar70j512iqw8x3nimzj10yga-python-3.7.4")
      ("python" . "/gnu/store/78w7y0lxar70j512iqw8x3nimzj10yga-python-3.7.4")
      …))

whereas the first one doesn’t have this duplicate entry.  IOW, the two
derivations are functionally equivalent but are not bit-identical.

Indeed, evaluating:

  (bag-transitive-inputs
   (package->bag ((package-input-rewriting '()) glib)))

shows that we have two “python” packages there that are not ‘eq?’.

To be continued…

Ludo’.

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

[Ludovic Courtès]

Hi,

Ludovic Courtès <ludovic.courtes@inria.fr> 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’.

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

[Ludovic Courtès]

Hey there!

Ludovic Courtès <ludo@gnu.org> skribis:

> Ludovic Courtès <ludovic.courtes@inria.fr> 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>.)

Good news, this is fixed by 2bf6f962b91123b0474c0f7123cd17efe7f09a66,
which introduces package rewriting including implicit inputs!

Before getting there, this issue did get on my nerves for a while.  Here
are several ways to address this issue that I thought of:

  1. Have ‘package-input-rewriting/spec’ traverse implicit inputs, at
     least optionally.  We wouldn’t end up with an
     equivalent-but-not-eq? ‘python’ in the example above.  It does
     change the semantics though, and it may be nice to keep a “shallow”
     replacement option.  That’s what
     2bf6f962b91123b0474c0f7123cd17efe7f09a66 does.

  2. Do (delete-duplicates input-drvs) in ‘bag->derivation’.  That seems
     wise, but it’s unfortunately impossible on ‘master’ because of
     <https://issues.guix.gnu.org/43508>.

  3. ‘package-input-rewriting/spec’ preserves eq?-ness for packages not
     transformed; in the example above, the transformation result would
     be eq? to ‘glib’ because ‘--with-input=libreoffice=inkscape’ had no
     effect.  Tricky to implement efficiently, perhaps not worth it.

I think #2 might still be worth investigating, but it may have
undesirable implications too.  #3 is hardly doable.

All in all, I’m glad that #1 addresses the issue, because it’s also
something we wanted anyway.

Ludo’.