bug#74532: with-extensions does not add native extensions to the load path

[Tomas Volf <~@wolfsden.cz>]

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Ludovic Courtès <ludo@gnu.org> writes:

> Hi,
>
> Tomas Volf <~@wolfsden.cz> skribis:
>
>> The documentation for `with-extensions' says:
>>
>>> In the same vein, sometimes you want to import not just pure-Scheme
>>> modules, but also “extensions” such as Guile bindings to C libraries
>>> or other “full-blown” packages.
>
> It’s not just bindings but also pure Scheme libraries like Guile-JSON.

True, and for those it works well.  However the text documentation
explicitly mentions "bindings to C libraries" as one of the use
cases. :)

>
>> However it does not actually add those C libraries into
>> (guile-extensions-path), which means that trying to actually use a
>> library imported this way will lead to an error.
>
> [...]
>
>>    190:25  1 (load-foreign-library _ #:extensions _ #:search-ltdl-library-path? _ # _ # _ # …)
>> In unknown file:
>>            0 (dlopen "libguile-yamlpp.so" 1)
>>
>> ERROR: In procedure dlopen:
>> In procedure dlopen: file "libguile-yamlpp.so", message "libguile-yamlpp.so: cannot open shared object file: No such file or directory"
>
> Usually, packages like these (guile-gnutls, guile-git, guile-ssh, etc.)
> have their .so absolute file name hard-coded, which sidesteps this
> problem entirely.  I recommend doing that for guile-yamlpp as well.

How it that usually done?  I cannot do that *before* build, because then
tests would not run (the library is not yet installed into the absolute
path), and I cannot do it after, because I would need to rebuild the .go
files after patching the source code.

Assuming I have a guile library that creates a new .so library during
the build, and the .so library must be loadable by the other modules in
the library during the build (and for the tests), how should I approach
that?

I am pretty sure I cannot just patch the source code, since the library
would not be installed into the absolute path yet during the build.

Honestly, setting GUILE_EXTENSIONS_PATH via pre-inst-env seemed like
fairly elegant solution.

>
> That said, it would probably make sense to arrange for ‘with-extensions’
> to set GUILE_EXTENSIONS_PATH.

That would be great.  My current work-around is

  (with-extensions (list guile-wolfsden)
    (program-file
     "audio-cycle-sinks"
     #~(begin
         ;; Bug 74532: Native extensions are not added to the load path.
         (eval-when (expand load eval)
           (let ((ext-path (@ (system foreign-library) guile-extensions-path)))
             ;; Just a temporary hack, we can live with duplicates in the path.
             (ext-path (cons #$(file-append guile-wolfsden
                                            "/lib/guile/3.0/extensions")
                             (ext-path)))))
         ...)))

which is anything but elegant.

>
> Ludo’.
>
> PS: Your MUA sets “Mail-Followup-To: bug-guix@gnu.org”, which is kinda
>     annoying because that’s the wrong address when replying to a bug.
>     :-)

Thanks for letting me know, I was not aware of that.  After reading
(message)Mailing Lists bit more carefully, adjusting the Posting Styles
and sending a bug fix to the Emacs' bug tracker, I believe it should not
happen anymore.  Sorry for the annoyance.

-- 
There are only two hard things in Computer Science:
cache invalidation, naming things and off-by-one errors.

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