Compiler warnings on free-threaded build for `_hashopenssl.c`

[picnixz]

Bug report

Bug description:

I get the following warnings since #128886, which was backported to 3.13.

./Modules/_hashopenssl.c:416:69: warning: passing 'const EVP_MD *' (aka 'const struct evp_md_st *') to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  416 |                 other_digest = _Py_atomic_exchange_ptr(&entry->evp, digest);
      |                                                                     ^~~~~~
./Include/cpython/pyatomic_gcc.h:194:42: note: passing argument to parameter 'value' here
  194 | _Py_atomic_exchange_ptr(void *obj, void *value)
      |                                          ^
./Modules/_hashopenssl.c:428:80: warning: passing 'const EVP_MD *' (aka 'const struct evp_md_st *') to parameter of type 'void *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
  428 |                 other_digest = _Py_atomic_exchange_ptr(&entry->evp_nosecurity, digest);
      |                                                                                ^~~~~~
./Include/cpython/pyatomic_gcc.h:194:42: note: passing argument to parameter 'value' here
  194 | _Py_atomic_exchange_ptr(void *obj, void *value)

CPython versions tested on:

CPython main branch

Operating systems tested on:

No response

Linked PRs

• gh-132674: fix _hashopenssl.c warnings on free-threaded build #132675
• [3.13] gh-132674: fix _hashopenssl.c compiler warnings on free-threaded build (GH-132675) #132677

[colesbury]

I think _Py_atomic_exchange_ptr() should take a const void * as the second argument instead of void *.

[picnixz]

I think _Py_atomic_exchange_ptr() should take a const void * as the second argument instead of void *.

I also thought about this, but IIRC, ISO C11 specifies the signature of __builtin_atomic_exchange (I don't remember the exact name) with non-const qualifiers.

[colesbury]

The C11 signature is generic -- it doesn't necessarily require a non-const argument. For example, _Atomic(const void *) x can use an exchange with a const void * pointer.

https://en.cppreference.com/w/c/atomic/atomic_exchange

We can't really support generic functions, so we have to do our best to come up with a reasonable function signature. For example, we lose the distinction between various pointer types as well as const-ness. Since we're not writing through the value pointer, a const void * is more appropriate.

See, for example, _Py_atomic_load_ptr:

cpython/Include/cpython/pyatomic_gcc.h

Lines 298 to 300 in 40ae889

	static inline void *
	_Py_atomic_load_ptr(const void *obj)
	{ return (void *)__atomic_load_n((void * const *)obj, __ATOMIC_SEQ_CST); }