Lookup ld.so.cache instead of hardcoding search paths

[pslacerda]

Fixes #3244

Changes proposed in this pull request:

• Search library dirs in /etc/ld.so.cache instead of hardcoded assumptions;
• The _find_strings_in_binary() parses the cache and extract directories.

It worked on my Fedora 28 workstation, but with little or even no modification it can works on FreeBSD, NetBSD and Android. Darwin architecture also has the dyld command that is similar to ldconfig on Linux.

[pslacerda]

The build fails only at Alpine Linux docker image because Alpline's ldconfig (http://www.musl-libc.org/) don't has a cache. Related to docker-library/python#111.

Note that ctypes.util.find_library() first try ldconfig then gcc and ld if the former fails.

[pslacerda]

Sorry for the noise on the last comment.

If ctypes.util returned a list of DLLs instead of picking only the required, it would be the more general solution to find where DLLs are located.

If Pillow has it dependencies installed then it can be build, right? So worth the effort to adapt ctypes.util and pack it in setup.py?

https://github.com/python/cpython/blob/master/Lib/ctypes/util.py

[pslacerda]

I just searched a bit about ldconfig and its cache.

The cache always has the same filename, and I don't expect it on a directory other than /etc, except probably /var like BSD (but it don't use glibc). And the cache format is pretty stable, apparently there is currently two formats and probably the old one nobody cares anymore.

[pslacerda]

Any thoughts on this? I had a hard time trying to improve it.

[hugovk]

I'm not so familiar with this area so would prefer someone else to review it.

[pslacerda]

Let me know if you need some information to review.

[pslacerda]

Very cool that upstream developers can commit into pull requests.

Happy New year!