[BUG] audio unit runtime exception on close

[TheTechnobear]

Short summary

audio unit is not shutting down successfully c++ runtime exception in log

Severity

major

Platform

macOS

Hardware

pico

Version

3.0.0-beta-1

Component

Agent

Agent

audio_unit

Steps to reproduce

load pico standard setup
load blank

in workbench, you'll see effects Audio Units still present.
these seem to use apple Audio Units, so are present

Logs / additional info

eigend: <audio_unit1>: Traceback (most recent call last):
eigend: <audio_unit1>:  File "/Applications/Eigenlabs/3.0.0-beta-1/pi/modules/pi/utils.py", line 85, in __do_nothrow
    return func(*args,**kwds)
eigend: <audio_unit1>:  File "/Applications/Eigenlabs/3.0.0-beta-1/pi/modules/pisession/workspace.py", line 201, in __unload
    self.module.unload(self.context.getenv(),self.agent,destroy)
    ~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
eigend: <audio_unit1>:  File "/Applications/Eigenlabs/3.0.0-beta-1/pi/modules/pi/agent.py", line 79, in __unload
    obj.unload(destroy)
    ~~~~~~~~~~^^^^^^^^^
eigend: <audio_unit1>:  File "/Applications/Eigenlabs/3.0.0-beta-1/pi/modules/pi/agent.py", line 506, in unload
    self.close_server()
    ~~~~~~~~~~~~~~~~~^^
eigend: <audio_unit1>:  File "/Applications/Eigenlabs/3.0.0-beta-1/pi/plugins/Eigenlabs/plg_host/host_plg.py", line 445, in close_server
    self.host.close();
    ~~~~~~~~~~~~~~~^^
eigend: <audio_unit1>:  RuntimeError: c++ exception in plugin_instance::close

[TheTechnobear]

I think underlying issues is agents are not closing down correctly - pre 3.0, these were just left hanging around loose, so didn't throw errors now we cleanup better, its showing underlying issues.
looking at logs, its already 'closed' this once - and native c++ code is tripping up, suspect timing issue across threads.
we can put some defence in, but doing deeper analysis to get root cause as well

[TheTechnobear]

analysis.md

[TheTechnobear]

testing shows, this appears to happen only IF a plugin is loaded, if no plugin is loaded then agent unloads.

which is why for standard setup rigs are ok, since the setup has invalid plugins (e.g. alchemy) for audio unit in rig, but effects in main rig are apple ones so are present.

Ive added some defensive code, but the above analysis will be useful for a deeper look when doing cleanup