Fix finding pthread_*name_np on vanilla musl libc

[mgorny]

Fix the pthread_getname_np and pthread_setname_np search logic to support vanilla versions of musl and CPython, e.g. as used on Gentoo musl systems. On such systems, there is no "libpthread.so" (there is only a static library) and the relevant functions are found in "libc.so". Additionally, ctypes.util.find_library("c") does not work because of an old unsolved bug in CPython (linked in the code).

To resolve the problem, add a fallback to trying libc.so if no pthread library can be found. This roughly covers three possibilities:

• a "typical" system with libpthread.so will find that library and use it

• a musl system will fall back to libc.so, load that library and find pthread functions there

• any other system will try to load libc.so, and fail

The code in get_os_thread_name_func() remains fully relaxed, allowing either CDLL construction (i.e. finding the library) to fail, or the library not to contain pthread_setname_np.

The code in test_threads.py was made more relaxed — rather than skipping if libpthread.so does not exist, it tries to load libc.so as a fallback, and skips if that fails.

Originally reported as https://bugs.gentoo.org/923257.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (05c5df2) 99.64% compared to head (c8c1957) 99.64%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2939 +/- ## ======================================= Coverage 99.64% 99.64% ======================================= Files 116 116 Lines 17503 17506 +3 Branches 3148 3148 ======================================= + Hits 17441 17444 +3 Misses 43 43 Partials 19 19 ``` | [Files](https://app.codecov.io/gh/python-trio/trio/pull/2939?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio) | Coverage Δ | | |---|---|---| | [src/trio/\_core/\_thread\_cache.py](https://app.codecov.io/gh/python-trio/trio/pull/2939?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX2NvcmUvX3RocmVhZF9jYWNoZS5weQ==) | `100.00% <100.00%> (ø)` | | | [src/trio/\_tests/test\_threads.py](https://app.codecov.io/gh/python-trio/trio/pull/2939?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=python-trio#diff-c3JjL3RyaW8vX3Rlc3RzL3Rlc3RfdGhyZWFkcy5weQ==) | `100.00% <100.00%> (ø)` | |

[A5rocks]

I ran into this for https://github.com/python-trio/trio/pull/2933 and ended up just making that require musl-dev instead of figuring this out. Thanks for putting in the work!

[mgorny]

Fixed and added a newsfragment.

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[mgorny]

Thanks!

[jakkdl]

Thank you for the contribution! Can also see that it works in CI over in #2933 https://github.com/python-trio/trio/actions/runs/7712303460/job/21019556518?pr=2933