Postpone log callback to Unity to the main thread

Also added AddCallbackWithThreadCheck() to execute the callback immediately if it is called from the callback thread.

Mono would attempt to kill the thread which called any C# function before.  As a result, if any C++ thread call any log function, Mono would attempt to send SIGINT signal to that thread.  Once the signal is sent, any call from C++->C# would cause hang.
This postpone the C++->C# call to the main thread to avoid this issue.

PiperOrigin-RevId: 267512154

Author: chkuang-g

app/src/callback.cc

@@ -242,6 +242,20 @@ void* AddCallback(Callback* callback) {
   return g_callback_dispatcher->AddCallback(callback);
 }
 
+// TODO(chkuang): remove this once we properly implement C++->C# log callback.
+void* AddCallbackWithThreadCheck(Callback* callback) {
+  if (g_callback_thread_id_initialized &&
+      Thread::IsCurrentThread(g_callback_thread_id)) {
+    // If we are on the callback thread, we can safely execute the callback
+    // right away and blocking would be a deadlock, so we run it.
+    callback->Run();
+    delete callback;
+    return nullptr;
+  } else {
+    return AddCallback(callback);
+  }
+}
+
 // A blocking callback posts a semaphore after being done.
 // This allows the caller to wait for its completion.
 class BlockingCallback : public Callback {

app/src/callback.h

@@ -368,6 +368,16 @@ Callback* NewCallback(CallbackAndArgs... callback_and_args) {
 /// callback is executed.
 void* AddCallback(Callback* callback);
 
+/// Adds a Callback to be called on the next PollCallbacks call.
+/// This function returns a reference in the queue that can be used to remove
+/// the callback from the queue.  The reference is only valid until the
+/// callback is executed.
+/// If AddCallbackWithThreadCheck is called from the callback thread,
+/// the new callback is executed immediately to avoid deadlocks and return a
+/// nullptr.
+/// This is a temporary solution for C++->C# log callback.
+void* AddCallbackWithThreadCheck(Callback* callback);
+
 /// Adds a Callback to be called on the next PollCallbacks call.
 /// This function blocks until the callback has been executed or removed from
 /// the queue.  If AddBlockingCallback is called from the callback thread,