Ensure mono_gc_thread_detach is always called

[joncham]

The previous logic would not call mono_gc_thread_detach for a number of scenarios. One example:

start_wrapper
- mono_thread_info_attach - thread is now live in mono threads layer
- start_wrapper_internal
-- mono_thread_attach_internal - mono attached to vm thread layer. GC handle is set via mono_thread_info_set_internal_thread_gchandle
-- mono_thread_detach_internal - detached from vm thread layer. GC handle cleared via call to mono_thread_info_unset_internal_thread_gchandle
- mono_thread_info_exit
-- mono_thread_info_detach
--- unregister_thread
---- thread_detach callback - checks if gc handle is valid and returns if not via mono_thread_info_try_get_internal_thread_gchandle. We've already cleared above so we never call mono_gc_thread_detach.

This change ensures mono_gc_thread_detach is always called even the GC handle for the thread has already been cleared.

[lateralusX]

I assume this is needed on other GC's than sgen that implements logic in mono_gc_thread_detach, sgen's implementation of that function is empty. I can see that Boehm GC implementation does, GC_unregister_my_thread as part of its implementation of mono_gc_thread_detach, so there it will have effects. Might be worth keeping in mind that all CI runs using sgen will then probably not detect any side effects due to this change, but I assume you run plenty on your end using Boehm GC.

[lambdageek]

/azp run sync-runtime-to-mono

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[joncham]

I assume this is needed on other GC's than sgen that implements logic in mono_gc_thread_detach, sgen's implementation of that function is empty. I can see that Boehm GC implementation does, GC_unregister_my_thread as part of its implementation of mono_gc_thread_detach, so there it will have effects. Might be worth keeping in mind that all CI runs using sgen will then probably not detect any side effects due to this change, but I assume you run plenty on your end using Boehm GC.

Yes, this is an issue I encountered in Boehm as part of our upgrading Mono to more recent version. We handle all testing for Boehm usage on our side.

[akoeplinger]

Android issue is #47850.