In Python 3.11a3 on Linux/x86_64 (failed to replicate on macOS, not attempted on Windows) the interpreter non-deterministically segfaults when running some code under coverage. This did not occur under 3.11a2. Looking at the backtrace from a core dump I see:

#0  _PyFrame_FastToLocalsWithError (frame=0x7fedf9e1f608) at Objects/frameobject.c:903
#1  0x00007fedfa15f593 in call_trampoline (tstate=0x55b767a44080, callback=0x7fedf8bbd9c0, 

This is the trace received if I use pure Python coverage (sys.settrace) while I get one inside coverage's ctracer if I use the native library. However, at the moment I don't believe the bug resides within coverage.

Since stack frame optimization has been a focus in 3.11 could something have changed that is causing issues with sys.settrace/PyEval_SetTrace?

I haven't managed to reduce this test case much but here's a somewhat messy dockerfile that can demonstrate it:

FROM ubuntu:focal
RUN apt-get update && apt-get install -y build-essential git cargo libffi-dev libssl-dev libsqlite3-dev zlib1g-dev curl
RUN curl -OL https://www.python.org/ftp/python/3.11.0/Python-3.11.0a3.tgz && \
    tar zxf Python-3.11* && \
    cd Python-3.11* && \
    ./configure --prefix=/opt && \
    make -j4 && make install
RUN /opt/bin/pip3 install tox && git clone https://github.com/pyca/cryptography
RUN cd cryptography && /opt/bin/tox -e py311