threadpool_info() before os.fork() causes SIGABRT on child exit (Python 3.13+)

## `threadpool_info()` before `os.fork()` causes SIGABRT on child exit (Python 3.13+)

**Versions:** threadpoolctl 3.6.0, Python 3.14.5, libffi 3.4.x (Linux)

Hi all, 
I have updated my code library from Python3.9 to 3.14. I have noticed that stopping my Python service produced core dumps. My service has a parent which forks multiple children which use scikit-learn.

I could reproduce a minimal running code with a coredump. Sorry, I am not sure, whether it is threadpoolctl or CPython issue. Thus, I open the issue with CPython as well. 

Below you will find the structured output summarized by our glorious AI overlords.

Thanks! 



### Problem

Calling `threadpoolctl.threadpool_info()` in a parent process before
`os.fork()`, then letting the forked child exit via `sys.exit()`, reliably
crashes the child with `SIGABRT` inside libffi's allocator:

```
abort
dlfree.cold (libffi.so.8)
CThunkObject_dealloc (_ctypes)
... (Py_Finalize / module teardown) ...
```

### Cause

`threadpoolctl` imports `ctypes.util.find_library`. On Python 3.13+,
`ctypes.util` implements Linux lookups via `dl_iterate_phdr()` using a
module-level `ctypes.CFUNCTYPE` closure (`ctypes.util._info_callback`) that
lives for the process lifetime. If this closure is created in a parent
*before* `fork()`, the forked child inherits it via copy-on-write. libffi's
private closure allocator keeps global free-list state that diverges
between parent and child after fork; when the child later frees the
inherited closure at interpreter shutdown, the allocator aborts on
corrupted metadata.

### Minimal repro

```python
import os, sys, signal, time
import threadpoolctl

threadpoolctl.threadpool_info()  # creates ctypes.util._info_callback

def child():
    signal.signal(signal.SIGTERM, lambda s, f: sys.exit(0))
    threadpoolctl.threadpool_info()
    time.sleep(30)

pid = os.fork()
if pid == 0:
    child()
else:
    time.sleep(3)
    os.kill(pid, signal.SIGTERM)
    os.waitpid(pid, 0)
```

`python3.14 repro.py` → `Aborted (core dumped)`.

Does **not** crash if `threadpoolctl.threadpool_info()` is only called
after the fork (i.e., not in the parent).

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threadpool_info() before os.fork() causes SIGABRT on child exit (Python 3.13+) #225