find_library broken in alpine image

[rwrnet]

According to http://bugs.alpinelinux.org/issues/5264 its known, that find_library is broken with python3 on Alpine.

However, with python2 it works on pure alpine:3.3,

$ docker run --rm -ti alpine:3.3 sh -c "apk add --no-cache file python && python -c 'from ctypes.util import find_library;print find_library(\"c\")'"
...
libc.musl-x86_64.so.1

but not on python:2-alpine,

$ docker run --rm -ti --entrypoint sh python:2-alpine -c "python -c 'from ctypes.util import find_library;print(find_library(\"c\"))'"
None

so python:2-alpine seems to break something in addition to the general alpine issue, which renders even version 2 broken. Maybe the find_library issue could also be worked around by python image in general.

[ushuz]

find_library relies on ldconfig gcc or objdump according to its document. gcc objdump need to be installed (by apk add gcc) separately as they are not included in the docker image, but won't work for some libraries, like libcairo. I've encountered this myself.

ldconfig seems broken according to the author of this bug, but the maintainer didn't say whether ldconfig is broken, instead he provided a patch addressing the issue.

Alpine image may need to apply the same patch to make find_library functional.

@yosifkit @tianon @ncopa

[ncopa]

@ushuz ldconfig works just fine. it does not cache anything like gnu libc does so there is nothing to print with ldconfig -p. as you understand they (rightfully) don't trust the ldconfig output parsing so the fall back to using gcc or objdump because the likely thought that there is no way there exist any system without those tools installed.

The patch we use in alpine is incomplete. The fundamental problem is when you try to dlopen libc itself, then bad things will happen. upstream musl libc has said they will fix but I don't know what the status is there.

We should probably report this to upstream python.

[ushuz]

@ncopa But why calling ldconfig -p on alpine:latest outputs Illegal option -p?

[danielfaust]

Twisted server: monkey-patch file
Running Twisted (Python 2.7.x) on Alpine Linux 3.7 inside Docker.
https://stackoverflow.com/q/48234723/277267

[tianon]

I'm a little confused here -- what's the use case for find_library('c') in the first place? It seems strange to me to be trying to dlopen libc itself in this way -- aren't most of the functions it provides already available through the Python standard library in some other way?