Library in system-wide base libraries not found by executable from python venv

[hcsch]

I was hoping to try out nix-ld to fix my issue of the numexpr 2.8.4 python package not finding libstdc++.so.6 (imported by tables 3.7.0, which is imported by a flask 1.1.4 app), but it seems nix-ld can't fix that issue either at the moment. NIX_LD_LIBRARY_PATH is set correctly to /run/current-system/sw/share/nix-ld/lib, and that also contains libstdc++.so.6, but I guess the relevant program is not using the correct linker or something else is going wrong.

LD_DEBUG=libs gets me:

     -- more ld_debug=libs find/search/init stuff --
     16974: find library=libstdc++.so.6 [0]; searching
     16974:  search cache=/nix/store/9xfad3b5z4y00mzmk2wnn4900q0qmxns-glibc-2.35-224/etc/ld.so.cache
     16974:  search path=/nix/store/9xfad3b5z4y00mzmk2wnn4900q0qmxns-glibc-2.35-224/lib     (system search path)
     16974:   trying file=/nix/store/9xfad3b5z4y00mzmk2wnn4900q0qmxns-glibc-2.35-224/lib/libstdc++.so.6
     -- python backtrace to the import of numexpr.interpreter --
     ImportError: libstdc++.so.6: cannot open shared object file: No such file or directory
     -- more ld_debug=libs fini stuff --

Since /run/current-system/sw/share/nix-ld/lib is nowhere to be found in those logs, this makes me believe that the paths in NIX_LD_LIBRARY_PATH are just being ignored, possibly due to the wrong linker being used.

I'm running flask run in a nix shell nixpkgs#python39 with a venv setup with that python, with dependencies installed via pip.

I'll try to fumble together a small example to reproduce this in the coming days, but maybe if this is an obvious mistake on my part, someone could already point it out to me.

Edit: I almost forgot to mention that the program runs just fine if I set LD_LIBRARY_PATH=/run/current-system/sw/share/nix-ld/lib, except that this breaks other stuff like fish and what not, so I can only set it as I start flask run

[hcsch]

Thanks for adding some explanatory docs for this. I searched around rather unsuccessfully before opening the issue and this will probably be a good resource now. ^^