fix(unix): Add foreign Qt threds in legacy POSIX thread impl

src/lib/arch/unix/ArchMultithreadPosix.cpp

@@ -517,13 +517,27 @@ void ArchMultithreadPosix::startSignalHandler()
 
 ArchThreadImpl *ArchMultithreadPosix::find(pthread_t thread)
 {
-  ArchThreadImpl *impl = findNoRef(thread);
+  ArchThreadImpl *impl = findNoRefOrInsert(thread);
   if (impl != nullptr) {
     refThread(impl);
   }
   return impl;
 }
 
+ArchThreadImpl *ArchMultithreadPosix::findNoRefOrInsert(pthread_t thread)
+{
+  ArchThreadImpl *impl = findNoRef(thread);
+  if (impl == nullptr) {
+    // create thread for calling thread which isn't in our list and
+    // add it to the list. this can happen when a foreign thread
+    // (e.g. a Qt thread) calls into the arch layer.
+    impl = new ArchThreadImpl;
+    impl->m_thread = thread;
+    insert(impl);
+  }
+  return impl;
+}
+
 ArchThreadImpl *ArchMultithreadPosix::findNoRef(pthread_t thread)
 {
   // linear search

src/lib/arch/unix/ArchMultithreadPosix.h

@@ -84,6 +84,7 @@ private:
 
   ArchThreadImpl *find(pthread_t thread);
   ArchThreadImpl *findNoRef(pthread_t thread);
+  ArchThreadImpl *findNoRefOrInsert(pthread_t thread);
   void insert(ArchThreadImpl *thread);
   void erase(const ArchThreadImpl *thread);