We still have a bunch of "global" C variables (static globals, static locals, maybe thread-local storage) in our code-base that break the isolation between interpreters. I added Tools/c-globals/check-c-globals.py a while back to help identify such variables, however more have crept in. I also did not take static locals or thread-locals into account.
To address the above, we should do the following:
1. update check-c-globals.py to identify static locals (and thread-locals)
2. deal with any identified globals
* move them to _PyRuntimeState (or thread-locals to PyThreadState, etc.)
* ignore them by adding them to Tools/c-globals/ignored-globals.txt
3. add check-c-globals.py to "make check"
4. (if "make check" isn't already there), ensure check-c-globals.py is run at some point in CI
Separately, we should move fields out of _PyRuntimeState into PyInterpreterState wherever possible. That can also be done at step 2 if it's not too much work.