Recently, we discussed this merge request, and you (Tim) said you do not want
to merge code that locks a mutex during audio processing due to **priority
inversion**.
So, what does the lock protect here? There is one global fluid_synth instance
that is used by both, the soundfont repo and the soundfont osc code. There is
also a bunch of data shared between the soundfont osc modules that is protected
by the lock. Locks are acquired
* by the soundfont repo when loading/unloading soundfonts
* by the soundfont osc modules when rendering audio
Like the wave repo code, we never load/unload soundfonts if the repo is
prepared, so effectively the soundfont repo will not lock the global mutex
while the project is playing.
The soundfont osc modules are
* one common module that does the actual rendering (1)
* per osc modules that just copy the data out, and maybe change the preset of
the current voice (2)
But these modules are all executed within the engine process() callback so they
effectively run with the highest priority we have. So **there is no priority
inversion possible here**. Also note that while (1) takes a while because it
renders the data, (2) is really cheap, because the audio data is already
available.
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