Make it impossible to have more than one main thread.

(cherry picked from commit 2ff661cd8d9b229b6a8d0e64ccd9cb52e92c1315)

core/os/thread.cpp

@@ -88,6 +88,20 @@ void Thread::wait_to_finish() {
 	id = UNASSIGNED_ID;
 }
 
+void Thread::make_main_thread() {
+	if (caller_id == MAIN_ID) {
+		return; // We're already the main thread
+	}
+	CRASH_COND_MSG(!is_main_thread_assigned.set_if_clear(), "A second thread attempted to become the main thread.");
+	caller_id = MAIN_ID;
+}
+
+void Thread::release_main_thread() {
+	CRASH_COND_MSG(caller_id != MAIN_ID, "Trying to release main thread from a thread that isn't main.");
+	CRASH_COND(!is_main_thread_assigned.clear_if_set());
+	caller_id = id_counter.increment();
+}
+
 Error Thread::set_name(const String &p_name) {
 	if (platform_functions.set_name) {
 		return platform_functions.set_name(p_name);

core/os/thread.h

@@ -107,15 +107,13 @@ private:
 
 	ID id = UNASSIGNED_ID;
 
+	static inline SafeFlag is_main_thread_assigned{ false };
 	static SafeNumeric<uint64_t> id_counter;
 	static thread_local ID caller_id;
 	THREADING_NAMESPACE::thread thread;
 
 	static void callback(ID p_caller_id, const Settings &p_settings, Thread::Callback p_callback, void *p_userdata);
 
-	static void make_main_thread() { caller_id = MAIN_ID; }
-	static void release_main_thread() { caller_id = id_counter.increment(); }
-
 public:
 	static void _set_platform_functions(const PlatformFunctions &p_functions);
 
@@ -130,6 +128,8 @@ public:
 	_FORCE_INLINE_ static ID get_main_id() { return MAIN_ID; }
 
 	_FORCE_INLINE_ static bool is_main_thread() { return caller_id == MAIN_ID; } // Gain a tiny bit of perf here because there is no need to validate caller_id here, because only main thread will be set as 1.
+	static void make_main_thread();
+	static void release_main_thread();
 
 	static Error set_name(const String &p_name);
 
@@ -182,9 +182,6 @@ private:
 
 	static PlatformFunctions platform_functions;
 
-	static void make_main_thread() {}
-	static void release_main_thread() {}
-
 public:
 	static void _set_platform_functions(const PlatformFunctions &p_functions);
 
@@ -193,6 +190,8 @@ public:
 	_FORCE_INLINE_ static ID get_main_id() { return MAIN_ID; }
 
 	_FORCE_INLINE_ static bool is_main_thread() { return true; }
+	static void make_main_thread() {}
+	static void release_main_thread() {}
 
 	static Error set_name(const String &p_name) { return ERR_UNAVAILABLE; }
 

drivers/apple/thread_apple.cpp

@@ -62,6 +62,20 @@ void *Thread::thread_callback(void *p_data) {
 	return nullptr;
 }
 
+void Thread::make_main_thread() {
+	if (caller_id == MAIN_ID) {
+		return; // We're already the main thread
+	}
+	CRASH_COND_MSG(!is_main_thread_assigned.set_if_clear(), "A second thread attempted to become the main thread.");
+	caller_id = MAIN_ID;
+}
+
+void Thread::release_main_thread() {
+	CRASH_COND_MSG(caller_id != MAIN_ID, "Trying to release main thread from a thread that isn't main.");
+	CRASH_COND(!is_main_thread_assigned.clear_if_set());
+	caller_id = id_counter.increment();
+}
+
 Error Thread::set_name(const String &p_name) {
 	int err = pthread_setname_np(p_name.utf8().get_data());
 	return err == 0 ? OK : ERR_INVALID_PARAMETER;

drivers/apple/thread_apple.h

@@ -76,14 +76,12 @@ private:
 	ID id = UNASSIGNED_ID;
 	pthread_t pthread;
 
+	static inline SafeFlag is_main_thread_assigned{ false };
 	static SafeNumeric<uint64_t> id_counter;
 	static thread_local ID caller_id;
 
 	static void *thread_callback(void *p_data);
 
-	static void make_main_thread() { caller_id = MAIN_ID; }
-	static void release_main_thread() { caller_id = id_counter.increment(); }
-
 public:
 	_FORCE_INLINE_ static void yield() { pthread_yield_np(); }
 
@@ -96,6 +94,8 @@ public:
 	_FORCE_INLINE_ static ID get_main_id() { return MAIN_ID; }
 
 	_FORCE_INLINE_ static bool is_main_thread() { return caller_id == MAIN_ID; }
+	static void make_main_thread();
+	static void release_main_thread();
 
 	static Error set_name(const String &p_name);
 

main/main.cpp

@@ -745,6 +745,7 @@ void Main::test_cleanup() {}
 // are initialized here. This also combines `Main::setup2()` initialization.
 Error Main::test_setup() {
 	Thread::make_main_thread();
+
 	set_current_thread_safe_for_nodes(true);
 
 	OS::get_singleton()->initialize();
@@ -884,6 +885,8 @@ Error Main::test_setup() {
 void Main::test_cleanup() {
 	ERR_FAIL_COND(!_start_success);
 
+	Thread::make_main_thread();
+
 	// Printing in the usual way can become problematic during/after cleanup.
 	CoreGlobals::print_ready = false;
 
@@ -971,6 +974,8 @@ void Main::test_cleanup() {
 	unregister_core_types();
 
 	OS::get_singleton()->finalize_core();
+
+	Thread::release_main_thread();
 }
 #endif // TESTS_ENABLED
 
@@ -3004,6 +3009,8 @@ error:
 	OS::get_singleton()->finalize_core();
 	locale = String();
 
+	Thread::release_main_thread();
+
 	return exit_err;
 }
 
@@ -5210,6 +5217,8 @@ void Main::force_redraw() {
  * The order matters as some of those steps are linked with each other.
  */
 void Main::cleanup(bool p_force) {
+	Thread::make_main_thread();
+
 	GodotProfileZone("cleanup");
 	OS::get_singleton()->benchmark_begin_measure("Shutdown", "Main::Cleanup");
 	if (!p_force) {
@@ -5395,4 +5404,6 @@ void Main::cleanup(bool p_force) {
 	OS::get_singleton()->benchmark_dump();
 
 	OS::get_singleton()->finalize_core();
+
+	Thread::release_main_thread();
 }