Something new has come up. If I put two instances of my MTPlugin test AU plug-in on separate tracks in Logic Pro X, then click back and forth between the tracks to enable one and then the other, AUHostingServiceXPC_arrow will eventually crash with a different error:
Thread 6 Crashed:: AUOOPRenderingServer-11887798
0 libobjc.A.dylib 0x186a77f00 objc_retain + 16
1 caulk 0x190461c3c caulk::mach::details::retain_os_object(void*) + 20
2 AudioToolboxCore 0x188c15130 void* caulk::thread_proxy<std::__1::tuple<caulk::attributes, AUOOPRenderingServer::AUOOPRenderingServer(int, int, int, std::__1::vector<AudioStreamBasicDescription, std::__1::allocator> const&, unsigned int, unsigned int, applesauce::xpc::dict const&, std::__1::shared_ptrauoop::WorkgroupMirror)::$_0, std::__1::tuple<>>>(void*) + 1152
3 libsystem_pthread.dylib 0x186e13fa8 _pthread_start + 148
4 libsystem_pthread.dylib 0x186e0eda0 thread_start + 8
The crashing seems to be triggered only when some MIDI has been sent to one of the tracks. I’m scratching my head about this. The plug-in code does virtually nothing, so I’m thinking it might be something in the JUCE AU client code??