Make `pthread_self()` return a non-zero value in `thread_pthread_stubs.h`

[colesbury]

Currently our stub for pthread_self() (used by WASI) just returns zero:

cpython/Python/thread_pthread_stubs.h

Lines 106 to 109 in 37228bd

	PyAPI_FUNC(pthread_t) pthread_self(void)
	{
	return 0;
	}

I propose we return a non-zero value for better consistency with functional pthread implementations. For example:

 PyAPI_FUNC(pthread_t) pthread_self(void) 
 { 
     return (pthread_t)(uintptr_t)&py_tls_entries; 
 } 

Why?

We occasionally assume that PyThread_get_thread_ident_ex() returns a non-zero value. For example:

• Thread ID assertion in pystate.c failing under WASI #110455
• In _PyRecursiveMutex for the owner field (oops)

We can work around these issues with #ifndef HAVE_PTHREAD_STUBS or by fixing the _PyRecursiveMutex implementation, but I think we might save ourselves a few headaches in the future by just making the pthread_self() stub behave a bit more like actual pthread implementations.

cc @brettcannon @kumaraditya303

Linked PRs

• gh-125161: return non zero value in pthread_self on wasi #125303

[kumaraditya303]

I agree, it's better to fix WASI stubs.

[brettcannon]

Fine by me!