Thread issues to fix Fall quarter 2025

[zelenski]

Reminder of issues for students are running into:

1. scary messages "QThreadStorage destroyed before end of thread"
2. seg fault on thread exit Windows mingw
3. single-step in lldb not sticking to the active thread

[zelenski]

1&2 are due to misordering of destructor calls, basically need to ensure thread-local storage is only deleted after thread exits. The dtors at end of student thread are ones cause problems. Situation is made more complex because QApplication intentionally takes on deletion for all QThreads. My original plan was to use _Exit as panic button (i.e. stop here, no destructors) but sadly _Exit implementation mingw is not to spec (does call destructors) Starting student thread as std not QThread almost works, but if thread converts to QAdoptedThread it goes into the mix for QApplication management (and I found it near impossible to avoid adoption in all cases). Instead I changed student thread to be a true QThread and made it play nice with QApplication.

_Exit mingw https://github.com/qt/qtbase/blob/fd1e3fe0d77e1faf6572790d4c8cef4fabc6e54c/src/corelib/global/qassert.cpp#L29C58-L34C34

[zelenski]

3 is due to lldbridge.py using bogus strategy to decide which thread to activate when stopped. It finds first thread stopped for any reason and activates it, but instead should first look for a thread stopped due to stepping and prefer that, and only if none found, fall back to any stoppped thread. The threads are searched in order they were created and since our student thread is created later that rest of thread pool, the original approach will erroneously activate a thread from pool when one happens to be in idle/stop state.

Note that when running in lldb itself, it never get this wrong, only goes astray when using python bridge. A good fix for Qt Creator would be to ask lldb which thread is active, rather than try to suss it out itself.

I added a monkey patch in stanfordtypes.py to replace findFirstStoppedThread with version that acts as described above.