calling Debug.Log from unmanaged thread crashes on experimental dotnet 4.6 runtime

EDIT: I should make it clear. platform is OSX UnityEditor.

hi, I have a plugin which uses unmanaged thread internally (using c++11 std::thread)

these threads call UnityEngine.Debug.Log via [UnmanagedFunctionPointer(CallingConvention.Cdecl)] delegate to show some information on Editor console.

when it runs under stable scripting runtime (dotnet 3.5), no problem occurs. but when I use experimental runtime (dotnet 4.6), it crashes inside Domain Unloader thread on entering play mode twice.

I could narrow down the problem to below little example.

when it runs on dotnet 3.5, it always shows "called back" 5 times then shows "done" .

but for dotnet 4.6, first time it runs ok, but crash on 2nd time (means stop 1st time run and restart).

native plugin (built as die.bundle)

 #include <pthread.h>
 #include <stdlib.h>
 #include <unistd.h>
 
 static pthread_t *threads;
 static int n_threads = 0;
 void *proc(void *a) {
     while (1) {
         ((void (*)())a)();
         sleep(1);
     }
     return NULL;
 }
 extern void start_thread(int n, void (*func)()) {
     threads = malloc(n * sizeof(pthread_t));
     for (int i = 0; i < n; i++) {
         pthread_create(threads + i, NULL, proc, func);
     }
     n_threads = n;
 }
 
 extern void stop_thread() {
     for (int i = 0; i < n_threads; i++) {
         pthread_cancel(threads[i]);
         pthread_join(threads[i], NULL);
     }
     free(threads);
 }
 

MonoBehaviour uses this plugin

 using System;
 using System.Runtime.InteropServices;
 using System.Threading;
 using System.Collections;
 using UnityEngine;
 
 class Crash : MonoBehaviour
 {
     [UnmanagedFunctionPointer(CallingConvention.Cdecl)]
     public delegate void ThreadCB();
 
     [DllImport ("die")]
     extern static void start_thread(int n, ThreadCB cb);
 
     [DllImport ("die")]
     extern static void stop_thread();
 
     bool stopped;
 
 
 
     void Start()
     {
         start_thread(1, () => {
             Debug.Log("called back");
         });
         stopped = false;
     }
 
     void Update() {
         if (Time.time > 5.0f && !stopped) {
             stop_thread();
             Debug.Log("done");
             stopped = true;
         }
     }
 }

I suspect that it relate with this bug https://bugzilla.xamarin.com/show_bug.cgi?id=50537 and need cleanup mono thread state somehow (I estimate mono runtime auto attached thread state when unmanaged thread entering it, as MS dotnet runtime does). but check with mono_thread_internal_current_is_attached shows these threads does not seems to be attached MonoInternalThread. so now I completely stuck.

does anyone got same issue? if does, can solve this problem? any suggestions are welcome.

regards,