Enable free-threaded builds of Python 3.14 on musl

[zanieb]

Extends #568

[zanieb]

cpython-3.14> checking for --with-mimalloc...
cpython-3.14> configure: error: mimalloc requires stdatomic.h, use --without-mimalloc to disable mimalloc.

It's not clear why this fails yet. I presumed this was a problem with Clang 14, but this should be available there. We should reproduce and look at a config.log.

I was kindly linked to https://gist.github.com/corona10/09459355b64d395aba14eba5be6f6f63 which showed a compiler-rt issue as the culprit (not using our clang version or build system!):

configure:18535: checking for stdatomic.h
configure:18558: clang-19 -o conftest   -fsanitize=thread  conftest.c  >&5
/usr/bin/ld: cannot find /usr/local/lib/clang/19/lib/x86_64-unknown-linux-gnu/libclang_rt.tsan.a: No such file or directory
clang-19: error: linker command failed with exit code 1 (use -v to see invocation)

I'll setup a local build so I can see if the same thing is happening to us.

cc @indygreg in case there's something obvious here?

resolved in 5786594

[indygreg]

Why is thread sanitizer not working a fatal configure error? That seems like the underlying issue here.

[zanieb]

I'm not sure. It might not be relevant — that build includes --with-thread-sanitizer and ours does not. I'll get logs for our setup.

[zanieb]

Yeah that looks irrelevant

configure:18506: checking for stdatomic.h
configure:18529: musl-clang -o conftest  -fPIC -I/tools/deps/include -I/tools/deps/include/ncursesw  -fPIC -I/tools/deps/include -I/tools/deps/include/ncursesw  -L/tools/deps/lib -Wl,--exclude-libs,ALL -LModules/_hacl conf
test.c -ldl  -lpthread >&5
conftest.c:158:14: fatal error: 'stdatomic.h' file not found
    #include <stdatomic.h>
             ^~~~~~~~~~~~~
1 error generated.
configure:18529: $? = 1
configure: failed program was:

...

| 
| 
|     #include <stdatomic.h>
|     atomic_int int_var;
|     atomic_uintptr_t uintptr_var;
|     int main() {
|       atomic_store_explicit(&int_var, 5, memory_order_relaxed);
|       atomic_store_explicit(&uintptr_var, 0, memory_order_relaxed);
|       int loaded_value = atomic_load_explicit(&int_var, memory_order_seq_cst);
|       return 0;
|     }
| 
| 
configure:18541: result: no

[zanieb]

This file is present in Clang 14 (as expected)

root@5e42d8cf2e23:~/Python-3.14.0a6# find $(musl-clang -print-file-name=include) -name stdatomic.h
/tools/llvm/lib/clang/14.0.3/include/stdatomic.h

[zanieb]

Trying d335fbf as we do for the intrinsic headers for blake2 (edit: this worked)

[indygreg]

It's possible musl-clang's implicit compiler options are not using clang's normal includes search paths. That musl-clang wrapper does weird things.

There's an arg to tell clang to dump all its arguments upon invocation. Can't recall offhand. Add that to see what's actually going on.

[Gankra]

is this my bash scripting knowledge failing me or are you bailing on the entire build here if the include file exists?

[zanieb]

uhh I guess so, I copied this from above though. It seems fine to bail because it's a wrong assumption in the build script.