Fix crash due to concurrent access in MidiController (Alternative)

[daschuer]

This is an alternative to fix #13940

I have not yet fully understand the issue though. But this should help to uncover it.

[daschuer]

@mxmilkiib Can you check out his?

[daschuer]

@mxmilkiib I have added mutex with debug Information.
This will hopefully finally fix the crash. However It needs some polishing.
Please Test and provide the mixxx.log. (or the backtrace if it is still crashing)

[daschuer]

@mxmilkiib Does this still crash?

[mxmilkiib]

yes

maaaybe this is config file related, removing files manually but Mixxx not forgetting

can't remember the state from before though

[daschuer]

Can you please share the mixxx.log of the crash?
A backtrace is also helpful.

[mxmilkiib]

ah this is the older issue

I'll see if I can replicate now, not had any recent ones for this issue though, building from this branch that is

[daschuer]

Cool, please also share the mixxx.log from a short test run without crash. This will help me polishing.

[mxmilkiib]

https://gist.github.com/mxmilkiib/b223874ec20f85b07840b7e0d2a5ba55

I managed to make it crash, on clicking play though.

DEBUG ASSERT: "it == cbegin() || it == cend() || *it >= position" in function mixxx::Beats::ConstIterator mixxx::Beats::iteratorFrom(mixxx::audio::FramePos) const at ./src/track/beats.cpp:470

Otherwise, no problem experienced (except the waveforms were lo res, idk).