Fix race in ScriptServer.

Fixes godotengine#76581.

TSAN flagged this issue on starting the editor:

1. main calls register_core_types, which calls IP::create(), which calls Thread::start on the resolver thread
2. Thread::callback calls ScriptServer::thread_enter(), as "Scripts may need to attach a stack."
3. ScriptServer::thread_enter() accesses ScriptServer::_languages, which is still being initialized on the main thread by initialize_gdscript_module

This fixes the issue by skipping thread enter/exit notifications if languages have not finished initializing yet.
I'm assuming that notifying un-initialized languages of thread starts/stops would have been pointless anyways. If we need to somehow notify languages of threads before the languages initialize, we'll need a different solution.

```
Godot Engine v4.0.2.stable.custom_build.7a0977ce2 - https://godotengine.org
==================
WARNING: ThreadSanitizer: data race (pid=9426)
  Write of size 4 at 0x55615b187cd0 by main thread:
    #0 ScriptServer::register_language(ScriptLanguage*) /home/rcorre/src/godot/godot/core/object/script_language.cpp:177:28 (godot.linuxbsd.editor.x86_64.llvm.san+0x9e52ab9) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    #1 initialize_gdscript_module(ModuleInitializationLevel) /home/rcorre/src/godot/godot/modules/gdscript/register_types.cpp:118:3 (godot.linuxbsd.editor.x86_64.llvm.san+0x36f9c6f) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#2 initialize_modules(ModuleInitializationLevel) /home/rcorre/src/godot/godot/modules/register_module_types.gen.cpp:93:2 (godot.linuxbsd.editor.x86_64.llvm.san+0x2f50499) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#3 Main::setup2(unsigned long) /home/rcorre/src/godot/godot/main/main.cpp:1961:2 (godot.linuxbsd.editor.x86_64.llvm.san+0x2f1d40d) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#4 Main::setup(char const*, int, char**, bool) /home/rcorre/src/godot/godot/main/main.cpp:1879:10 (godot.linuxbsd.editor.x86_64.llvm.san+0x2f16370) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#5 main /home/rcorre/src/godot/godot/platform/linuxbsd/godot_linuxbsd.cpp:61:14 (godot.linuxbsd.editor.x86_64.llvm.san+0x2e67e1f) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)

  Previous read of size 4 at 0x55615b187cd0 by thread T1:
    #0 ScriptServer::thread_enter() /home/rcorre/src/godot/godot/core/object/script_language.cpp:244:22 (godot.linuxbsd.editor.x86_64.llvm.san+0x9e54aed) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    #1 Thread::callback(unsigned long, Thread::Settings const&, void (*)(void*), void*) /home/rcorre/src/godot/godot/core/os/thread.cpp:61:2 (godot.linuxbsd.editor.x86_64.llvm.san+0x9464ab0) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#2 void std::__invoke_impl<void, void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>(std::__invoke_other, void (*&&)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long&&, Thread::Settings&&, void (*&&)(void*), void*&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:61:14 (godot.linuxbsd.editor.x86_64.llvm.san+0x9465283) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#3 std::__invoke_result<void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>::type std::__invoke<void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>(void (*&&)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long&&, Thread::Settings&&, void (*&&)(void*), void*&&) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/invoke.h:96:14 (godot.linuxbsd.editor.x86_64.llvm.san+0x9465283)
    godotengine#4 void std::thread::_Invoker<std::tuple<void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>>::_M_invoke<0ul, 1ul, 2ul, 3ul, 4ul>(std::_Index_tuple<0ul, 1ul, 2ul, 3ul, 4ul>) /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:258:13 (godot.linuxbsd.editor.x86_64.llvm.san+0x9465283)
    godotengine#5 std::thread::_Invoker<std::tuple<void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>>::operator()() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:265:11 (godot.linuxbsd.editor.x86_64.llvm.san+0x9465283)
    godotengine#6 std::thread::_State_impl<std::thread::_Invoker<std::tuple<void (*)(unsigned long, Thread::Settings const&, void (*)(void*), void*), unsigned long, Thread::Settings, void (*)(void*), void*>>>::_M_run() /usr/bin/../lib64/gcc/x86_64-pc-linux-gnu/12.2.1/../../../../include/c++/12.2.1/bits/std_thread.h:210:13 (godot.linuxbsd.editor.x86_64.llvm.san+0x9465283)
    godotengine#7 execute_native_thread_routine /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:82:18 (libstdc++.so.6+0xd72c2) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)

  Location is global 'ScriptServer::_language_count' of size 4 at 0x55615b187cd0 (godot.linuxbsd.editor.x86_64.llvm.san+0xbf46cd0)

  Thread T1 (tid=9431, running) created by main thread at:
    #0 pthread_create <null> (godot.linuxbsd.editor.x86_64.llvm.san+0x2de5776) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    #1 __gthread_create /usr/src/debug/gcc/gcc-build/x86_64-pc-linux-gnu/libstdc++-v3/include/x86_64-pc-linux-gnu/bits/gthr-default.h:663:35 (libstdc++.so.6+0xd73a9) (BuildId: 6fe66a2d539a78c993bd2d377e00fad389220963)
    godotengine#2 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State>>, void (*)()) /usr/src/debug/gcc/gcc/libstdc++-v3/src/c++11/thread.cc:147:37 (libstdc++.so.6+0xd73a9)
    godotengine#3 IP::IP() /home/rcorre/src/godot/godot/core/io/ip.cpp:347:19 (godot.linuxbsd.editor.x86_64.llvm.san+0x962cbcd) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#4 IPUnix::IPUnix() /home/rcorre/src/godot/godot/drivers/unix/ip_unix.cpp:261:9 (godot.linuxbsd.editor.x86_64.llvm.san+0x4aee599) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#5 IPUnix::_create_unix() /home/rcorre/src/godot/godot/drivers/unix/ip_unix.cpp:258:9 (godot.linuxbsd.editor.x86_64.llvm.san+0x4aee599)
    godotengine#6 IP::create() /home/rcorre/src/godot/godot/core/io/ip.cpp:339:9 (godot.linuxbsd.editor.x86_64.llvm.san+0x962ca5e) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#7 register_core_types() /home/rcorre/src/godot/godot/core/register_core_types.cpp:279:7 (godot.linuxbsd.editor.x86_64.llvm.san+0x93e2333) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#8 Main::setup(char const*, int, char**, bool) /home/rcorre/src/godot/godot/main/main.cpp:690:2 (godot.linuxbsd.editor.x86_64.llvm.san+0x2f08a49) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)
    godotengine#9 main /home/rcorre/src/godot/godot/platform/linuxbsd/godot_linuxbsd.cpp:61:14 (godot.linuxbsd.editor.x86_64.llvm.san+0x2e67e1f) (BuildId: 780a9db7c37d88e78d5ee659c4fa1cd378abd048)

SUMMARY: ThreadSanitizer: data race /home/rcorre/src/godot/godot/core/object/script_language.cpp:177:28 in ScriptServer::register_language(ScriptLanguage*)
```

Co-authored-by: Pedro J. Estébanez <[email protected]>

core/object/script_language.cpp

@@ -42,7 +42,7 @@ int ScriptServer::_language_count = 0;
 
 bool ScriptServer::scripting_enabled = true;
 bool ScriptServer::reload_scripts_on_save = false;
-bool ScriptServer::languages_finished = false;
+SafeFlag ScriptServer::languages_finished; // Used until GH-76581 is fixed properly.
 ScriptEditRequestFunction ScriptServer::edit_request_func = nullptr;
 
 void Script::_notification(int p_what) {
@@ -228,7 +228,7 @@ void ScriptServer::finish_languages() {
 		_languages[i]->finish();
 	}
 	global_classes_clear();
-	languages_finished = true;
+	languages_finished.set();
 }
 
 void ScriptServer::set_reload_scripts_on_save(bool p_enable) {
@@ -240,12 +240,18 @@ bool ScriptServer::is_reload_scripts_on_save_enabled() {
 }
 
 void ScriptServer::thread_enter() {
+	if (!languages_finished.is_set()) {
+		return;
+	}
 	for (int i = 0; i < _language_count; i++) {
 		_languages[i]->thread_enter();
 	}
 }
 
 void ScriptServer::thread_exit() {
+	if (!languages_finished.is_set()) {
+		return;
+	}
 	for (int i = 0; i < _language_count; i++) {
 		_languages[i]->thread_exit();
 	}

core/object/script_language.h

@@ -35,6 +35,7 @@
 #include "core/io/resource.h"
 #include "core/templates/pair.h"
 #include "core/templates/rb_map.h"
+#include "core/templates/safe_refcount.h"
 #include "core/variant/typed_array.h"
 
 class ScriptLanguage;
@@ -52,7 +53,7 @@ class ScriptServer {
 	static int _language_count;
 	static bool scripting_enabled;
 	static bool reload_scripts_on_save;
-	static bool languages_finished;
+	static SafeFlag languages_finished; // Used until GH-76581 is fixed properly.
 
 	struct GlobalScriptClass {
 		StringName language;
@@ -97,7 +98,7 @@ class ScriptServer {
 	static void init_languages();
 	static void finish_languages();
 
-	static bool are_languages_finished() { return languages_finished; }
+	static bool are_languages_finished() { return languages_finished.is_set(); }
 };
 
 class ScriptInstance;