Linux: X-Plane frozen during FMOD shutdown / after FMOD error "not unit length, or perpendicular"

[TwinFan]

Describe the bug User report: X-Plane froze, last line in log is

LT/XPMP2 ERROR Sound.cpp:835/SoundUpdatesDone: FMOD Error 36 - 'The vectors passed in are not unit length, or perpendicular.' in FMOD_System_Set3DListenerAttributes

To Reproduce / Source Code Example Unclear. But can probably be forced in debugger by sending wrong coordinates into FMOD_System_Set3DListenerAttributes.

Expected behavior

1. Error shall not freeze X-Plane, check error handling.
2. Error shall not happen.

Version

• OS: Reported on Linux
• X-Plane: 12.00rc1
• using LiveTraffic 3.2.0

Log.txt Log-Snd_Freeze.txt

Additional context Original user report is a comment to LiveTraffic. More discussion then tracked in a support thread.

[TwinFan]

I could not recreate any problem. The error does not appear to me "just so".

I can certainly enforce it via debugger or by adding code that consciously passes non-unit length vectors to FMOD_System_Set3DListenerAttributes, but all that happens then is as per design:

• the above error message is logged
• with logging settings set to DEBUG, another message is logged stating that the sound system has now been disabled and otherwise everything just goes on as normal, no interruption to LiveTraffic's planes or X-Plane. Tested on Mac and Linux.

Can only ask affected user to try a debug version that provokes the error to see if once it happens his system is freezing again.

[TwinFan]

Started a support thread for discussion with affected users. A xPilot user joined in with similar issue with xPilot, which also uses XPMP2, also tracked as xpilot-project/xpilot#137.

Turns out: It is not actually the error 36 itself that causes issues, but calling FMOD_System_Release while handling the original error: Currently XPMP2 treats any error returned by the call to FMOD_System_Set3DListenerAttributes as fatal and shuts down the sound system to avoid further errors. Only the shutdown actually causes the freeze.

The very same freeze happens during a normal exit of the plugin at the same place, ie. during the call to FMOD_System_Release.

Users see a connection to the use of the PipeWire module (in absence of PulseAudio) in certain Linux distros. I still can't reproduce in a standard Ubuntu 22.04 installation and hence not confirm the suspicion.

Fix/Workaround: It's easy not to treat error 36 as a fatal error, so that no sound shutdown happens thereafter. We've tested that in an analysis version of LiveTraffic.

But I'm not convinced to remove the final call to FMOD_System_Release during plugin shutdown, as that would mean not to properly clean up FMOD resources. So for the moment the few affected users might need to live with a freeze during shutdown if using XPMP2 sound.