gh-115035: Mark ThreadHandles as non-joinable earlier after forking

[colesbury]

This marks dead ThreadHandles as non-joinable earlier in PyOS_AfterFork_Child() before we execute any Python code. The handles are stored in a global linked list in _PyRuntimeState because fork() affects the entire process.

• Issue: Crash in ThreadHandle_dealloc after fork in free-threaded build #115035

[colesbury]

This doesn't call PyThread_update_thread_after_fork(). As far as I can tell, that call did not have any effect: we determine the current thread using PyThread_get_thread_ident_ex() (via get_ident) ¹ so there's no way we could have a thread with a stale identifier. And if we don't find a matching thread, we create a new _MainThread() with the correct current identifier (and no handle).

Footnotes

1. https://github.com/python/cpython/blob/c32bae52904723d99e1f98e2ef54570268d86467/Lib/threading.py#L1725-L1731↩

[pitrou]

As far as I can tell, that call did not have any effect: we determine the current thread using PyThread_get_thread_ident_ex() (via get_ident) 1 so there's no way we could have a thread with a stale identifier.

That's a good point, but then we can probably remove the PyThread_update_thread_after_fork function?

[pitrou]

Neat, thank you @colesbury !

[pitrou]

Note: you're a core developer now ;-)

[gpshead]

nice refactoring, thanks! happy first-ish merge!