Re: [PATCH] load-foreign-library: perform substring match on library files

[Sören Tempel]

Hi,

How do we proceed with this?

I would especially be interested in the "patching" that you do in Guix.
Maybe that would also be a suitable workaround for us on the Alpine side
until this is sorted out properly.

Greetings,
Sören

Sören Tempel <soeren@soeren-tempel.net> wrote:
> Hi,
> 
> Thanks for your feedback, comments below.
> 
> Maxime Devos <maximedevos@telenet.be> wrote:
> > Long term, I think it would be ideal for Guile to decide upon a major 
> > version (and maybe even location, depending on the choices of the 
> > distro) at _compile_ time instead of runtime, not unlike how other 
> > compilers work.
> 
> Sure, that could work too.
> 
> > If the Guile module is being compiled with --rpath, it searches
> > $CROSS_LIBRARY_PATH or $LIBRARY_PATH and encodes the full file name
> > (/usr/lib/.../libguile-... or /gnu/store/.../lib/...) in the .go,
> > which avoids some manual patching we have to do in Guix.
> 
> What kind of manual patching do you do on Guix? Could you refer me to
> the code for that? Maybe that's something we could also do on Alpine in
> the meantime.
> 
> > IIUC, the string-prefix? search is non-deterministic, which can make 
> > debugging complicated when multiple versions are installed. 
> 
> Well, on Linux readdir(3) file name order depends on the file system
> implementation. Since the algorithm returns the first file with a
> substring match, it depends on the readdir order. As long as the same
> filesystem is used, it should be deterministic.
> 
> > I think it would be better to bail out if there are multiple matches
> > instead of a risk of guessing incorrectly.
> 
> Many packages provide multiple .so files with different version
> granularity so bailing out if there are multiple substring matches
> doesn't really work. For example, for libgit2 I have the following files
> installed on my system:
> 
>       $ ls /usr/lib/libgit2*
>       /usr/lib/libgit2.so.1.4
>       /usr/lib/libgit2.so.1.4.4
> 
> Where the former is a symlink to the latter. However, it would be
> possible to collect all substring matches and prioritize them according
> to some algorithm (e.g. alphabetical order). This would also make the
> algorithm independent of the readdir(3) order.
> 
> > The prefixing behaviour is also not documented, so some documentation is 
> > needed in the manual.
> 
> I can add some documentation if there is interest in merging this.
> 
> >   * Does it go scanning the dir even if libfoo.so could be found?
> >     Otherwise, there are some possible performance gains by checking for
> >     libfoo.so first -- consider the case where /usr/lib is huge.
> 
> Yes, a fast path could be added though you probably really need to have
> a lot of files installed in /usr/lib for this to be worth it.
> 
> >   * When doing (load-foreign-library "/gnu/store/.../libfoo.so")
> >     (absolute file name!), would it search for
> >     /gnu/store/.../libfoo.so.N?  If so, that would be surprising,
> >     especially if libfoo.so.N exists.
> 
> Yep, it does. I originally didn't want to modify the handling of
> absolute paths but unfortunately during testing I noticed that Guile
> extensions seem to be loaded with an absolute path and hence don't
> work without the libfoo.so symlink [1].
> 
> >   * If doing libfoo.so.N, will it search for libfoo.so.N.M?
> 
> Yes, since libfoo.so.N is a prefix of libfoo.so.N.M.
> 
> >   * Does it only apply to the system paths, or also to
> >     GUILE_SYSTEM_EXTENSION_PATH, LTDL_LIBRARY_PATH and
> >     GUILE_EXTENSION_PATH? The latter would be surprising to me, as
> >     versioning is more of a system thing.
> 
> If those paths are also searched using the load-foreign-library
> procedure then they are affected by this change. Also, I am not a Guile
> expert but on Alpine, Guile extensions such as guile-reader also ship
> versioned sonames [1].
> 
> >   * To test it, and avoid breaking things later with future changes to
> >     load-foreign-library, could some tests be added?
> 
> Probably, though I am not familiar with the Guile test setup and there
> don't seem to be any existing tests for foreign-library.
> 
> >  * Is this change desirable?  I mean, this is an FFI API, so the ABI of
> >    the library is rather important. If a Guile module links to
> >    libfoo.so, and they had version N in mind, then it's important it
> >    doesn't link to N-1 or N+1 instead, because of ABI
> >    incompatibilities. As such, to me it seems _good_ that you got some
> >    errors, as now you get a reminder to explicitly state which ABI
> >    version is needed. (YMMV, and the mileage of the Guile maintainers
> >    might vary, etc.)
> 
> In my experience, most languages which don't link against shared
> libraries directly but instead load them at run-time don't hardcode ABI
> versions (for example, refer to Python's ctypes.util.find_library).
> Also, the current implementation of load-foreign-library does not force
> you to specify an ABI version but instead loads whatever the libfoo.so
> symlink refers to.
> 
> > Also, this seems like a non-trivial change to me, so a copyright line 
> > might be in order, unless you did the copyright assignment.
> 
> I didn't do any copyright assignment yet but if there is actually any
> interest in merging this then I can do it once we agreed on changes to
> the algorithm.
> 
> Greetings,
> Sören
> 
> [1]: https://gitlab.alpinelinux.org/alpine/aports/-/issues/12783