In one of patches I'm building, (yet another attempt at caching LOAD_GLOBALS)[1], I'm using the private APIs from PEP 523 to store an array with every code object. I'm calling _PyEval_RequestCodeExtraIndex with PyMem_Free for the freefunc argument. While running  the cpython testsuite, I found that test_embed case crashes with a segfault. The gdb backtrace[2] seems to indicate that PyInterpreterState::co_extra_freefuncs is uninitialized, while it should be a pointer to the PyMem_Free function. 

One way to work around this is to set the array as a member on the PyCodeObject struct and use it directly. And I've verified that it works. Am I using the PEP 523 private api correctly? Also, on Linux, this consistently crashes while on OSX, it occasionally doesn't crash which makes me wonder if it's some kind of a race condition involving Sub-interpreters. The attached gist[2] has steps for repro.

[1]: https://github.com/python/cpython/compare/master...jeethu:py3.7_load_global_cache
[2]: https://gist.github.com/jeethu/6d92185ca97dd692e7fadcd105e0ef70

co_extra_freefuncs is an array of pointers, so there's no way for it to be uninitialized unless you didn't initialize the interpreter state (https://github.com/python/cpython/blob/b193fa996a746111252156f11fb14c12fd6267e6/Include/pystate.h#L155). And looking at your gdb session it shows the memory wipeout pattern which means you're accessing an interpreter state that has already been freed itself. IOW I don't think this is a PEP 523 issue specifically.

Is there anything left to do on this issue? It seems like a question more than a bug report.