Make the commandline renderer work better

[PaulBatchelor]

Hello LMMS-ers,

It seems pretty clear to me based on issues like #3396 #3207 #2934 #2845 #3360 that there are fundamental issues with the LMMS audio engine, particularly when it comes to rendering audio to disk.

It is hope that one of my next contributions to LMMS will a step forward in solving the issues above by improving the command line renderer (aka "lmms -r").

To me, a successful CLI renderer is one that can bounce an audio file without any clicks or hiccups, regardless of what audio/MIDI drivers are being used. It would seem that this is not the case right now.

Based on my initial evaluations of the code, this will involve the following:

• Ensuring that no realtime audio driver starts (JACK/ALSA/PortAudio/CoreAudio etc..). I believe there are threading issues involving JACK and the Renderer class working at the same time
• Ensuring that no realtime MIDI starts up.
• If possible, remove the use of threads in the render loop (MixerWorkerThread, I'm looking at you)

As I write this, I am painfully aware of that mind-numbing tasks that await me, but this is the sort of thing that must be done. Since I'm still new to the LMMS codebase, I welcome any and all help/guidance on the matter.

All the best,

-P

[PaulBatchelor]

Getting some promising results. Posting my tweaks in the "cli_fixes" branch in my fork of LMMS:

https://github.com/paulbatchelor/lmms/tree/cli_fixes

It looks like I botched up some git rebasing. Don't worry, I plan on redoing my changes in a proper PR ;)

[PaulBatchelor]

Closing this to continue conversation in #3404