How to debug C++ code that is called from Python that is called from MATLAB, using the MSVC debugger?

Question

My setup:

• Given some Legacy™ C++ code
• Using wrappers that use the Python ↔ C/C++ API, this is compiled into a .pyd file
• There is a bunch of Legacy™ Python code that uses this .pyd file
• This Python code is then called from MATLAB
• MATLAB R2018a / Python 3.6 / C++11 / Windows 10 / MSVC 2017 Community
• This setup is rigid, i.e., all this code is used by 100s of people in all sorts of different contexts; this non-ideal setup is already the best possible trade-off

The problem:

• MATLAB crashes due to an Access Violation® somewhere in the C++ code.

Obviously, MATLAB can't "look through" the .pyd binary to determine the root cause, so this is all I have to go on.

What I've tried:

1. • Using MSVC2017, build the .pyd in Debug mode (setup.py build --debug).
   • In MATLAB: pyversion 'c:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\python.exe'
   • Be really annoyed by having to restart MATLAB before you can actually do that.
   • After restarting MATLAB and running said command, in MSVC: Debug → Attach to Process → select MATLAB.exe
   • Run the MATLAB code that causes the crash.
   • MATLAB/Python complains: Python Error: ImportError: DLL load failed: The specified module could not be found.
2. • Try the same as in 1., this time renaming the my_pylib.pyd file to my_pylib_d.pyd (as found here of all places...)
   • MATLAB/Python complains: Python Error: ImportError: cannot import name 'my_pylib'
3. • Try the same as 2., this time stating pyversion 'c:\Program Files (x86)\Microsoft Visual Studio\Shared\Python36_64\python_d.exe' (first making sure that a Python3.6 debugging environment has been installed in the MSVC context).
   • Be REALLY annoyed by having to restart MATLAB Every. Fucken. Time. you run that command!
   • Go to The MathWorks Support site and put in a Feature Request to try and get that fixed in R2019a
   • Restart MATLAB, re-attach MSVC to MATLAB.exe and repeat from the top
   • MATLAB/Python still complains: Python Error: ImportError: cannot import name 'my_pylib'
4. • Repeat 3., this time forcing MATLAB to also use python36_d.dll because somehow that mechanism seems broken in MATLAB.
   • SCREAM IN RAGE because you have to restart MATLAB AGAIN and not forget to re-attach to the MATLAB.exe process in MSVC AGAIN.
   • This time it at least continues with Python code execution, but it trips on every imported module (numpy, scipy, etc.) with the same ImportError as before ...
5. • Give up on Python3.6.
   • Try again, from the top, using a fresh, non-MSVC installation of Python 3.7, including debugging symbols etc.
   • in MATLAB: pyversion 'c:\wherever\Python37_64\python.exe'
   • <speak_angrily_through_teeth> Oh. Yeah. I forgot. Restart MATLAB AGAIN, run pyversion command above AGAIN, run offending MATLAB code, SCREAM WHILE PULLING OUT HAIR because you forgot to re-attach MSVC, restart MATLAB AGAIN because it obviously crashed, re-attach MSVC, run offending MATLAB code </speak_angrily_through_teeth>

   • Success! MSVC goes to my API code after MATLAB triggers a breakpoint. The breakpoint is triggered here:

     PyObject *module = PyModule_Create(&moduledef);
     

     where

     static struct PyModuleDef moduledef = 
     {
         PyModuleDef_HEAD_INIT,
         "my_pylib",
         NULL,
         sizeof(struct module_state),
         my_pylib_methods,
         NULL,
         my_pylib_traverse,
         my_pylib_clear,
         NULL
     };
     

     After downloading the Python 3.7 source code, I can dig a bit deeper. The PyModule_Create call is a wrapper that calls the following function in Objects/moduleobject.c:

     PyObject *
     PyModule_Create2(struct PyModuleDef* module, int module_api_version)
     {
         if (!_PyImport_IsInitialized(PyThreadState_GET()->interp))
             Py_FatalError("Python import machinery not initialized");
         return _PyModule_CreateInitialized(module, module_api_version);
     }
     

     where the breakpoint is inside the if() clause. This means that the moduledef doesn't even matter. _PyImport_IsInitialized() is a function in Python/import.c:

     int
     _PyImport_IsInitialized(PyInterpreterState *interp)
     {
         if (interp->modules == NULL)
             return 0;
         return 1;
     }
     

     which doesn't seem like a very likely candidate to be causing my Access Violation®. Going into the PyThreadState_GET() finally made me realize: I'm actually debugging the Python/C++ API instead of my code...

Questions:

• What the heck!! Why is debugging in MATLAB using Python3.6 so damn difficult?! What is the "proper" way to do it? I can't find much in the (online) documentation for it...
• Is there a known issue with the Python 3.7 C/C++ API that could be causing this?
• Am I doing something wrong / stupidly? Any tips/pointers on how I can find the problem in my C++ code more effectively?