Fix data race against EditorFileSystem.scanning_changes_done

[Rubonnek]

Fixes the following report by ThreadSanitizer:

==================
WARNING: ThreadSanitizer: data race (pid=142244)
  Read of size 1 at 0x7b6c002a4b91 by main thread:
    #0 EditorFileSystem::_notification(int) /opt/godot/editor/editor_file_system.cpp:1272:10 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x588d21f) (BuildId: 3b2cca770c49afa3)
    #1 EditorFileSystem::_notificationv(int, bool) /opt/godot/editor/editor_file_system.h:146:2 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x58a530d) (BuildId: 3b2cca770c49afa3)
    #2 Object::notification(int, bool) /opt/godot/core/object/object.cpp:837:3 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0xa16bff3) (BuildId: 3b2cca770c49afa3)
    #3 SceneTree::_process_group(SceneTree::ProcessGroup*, bool) /opt/godot/scene/main/scene_tree.cpp (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x70e5d65) (BuildId: 3b2cca770c49afa3)
    #4 SceneTree::_process(bool) /opt/godot/scene/main/scene_tree.cpp:1028:8 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x70e27b1) (BuildId: 3b2cca770c49afa3)
    #5 SceneTree::process(double) /opt/godot/scene/main/scene_tree.cpp:508:2 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x70e3767) (BuildId: 3b2cca770c49afa3)
    #6 Main::iteration() /opt/godot/main/main.cpp:3939:44 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3ce42aa) (BuildId: 3b2cca770c49afa3)
    #7 OS_LinuxBSD::run() /opt/godot/platform/linuxbsd/os_linuxbsd.cpp:945:7 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3bb4604) (BuildId: 3b2cca770c49afa3)
    #8 main /opt/godot/platform/linuxbsd/godot_linuxbsd.cpp:86:6 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3ba46f4) (BuildId: 3b2cca770c49afa3)

  Previous write of size 1 at 0x7b6c002a4b91 by thread T58:
    #0 EditorFileSystem::_thread_func_sources(void*) /opt/godot/editor/editor_file_system.cpp:1186:29 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x588ca0b) (BuildId: 3b2cca770c49afa3)
    #1 Thread::callback(unsigned long, Thread::Settings const&, void (*)(void*), void*) /opt/godot/core/os/thread.cpp:64:3 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c47a8) (BuildId: 3b2cca770c49afa3)
    #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/13.2.1/../../../../include/c++/13.2.1/bits/invoke.h:61:14 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c4cb3) (BuildId: 3b2cca770c49afa3)
    #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/13.2.1/../../../../include/c++/13.2.1/bits/invoke.h:96:14 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c4cb3)
    #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/13.2.1/../../../../include/c++/13.2.1/bits/std_thread.h:292:13 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c4cb3)
    #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/13.2.1/../../../../include/c++/13.2.1/bits/std_thread.h:299:11 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c4cb3)
    #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/13.2.1/../../../../include/c++/13.2.1/bits/std_thread.h:244:13 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98c4cb3)
    #7 execute_native_thread_routine msdf-error-correction.cpp (godot.linuxbsd.editor.dev.x86_64.llvm.san+0xa692ee2) (BuildId: 3b2cca770c49afa3)

  Location is heap block of size 1680 at 0x7b6c002a4600 allocated by main thread:
    #0 malloc <null> (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3b35463) (BuildId: 3b2cca770c49afa3)
    #1 Memory::alloc_static(unsigned long, bool) /opt/godot/core/os/memory.cpp:75:14 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98bbcdd) (BuildId: 3b2cca770c49afa3)
    #2 operator new(unsigned long, char const*) /opt/godot/core/os/memory.cpp:40:9 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x98bbcdd)
    #3 EditorNode::EditorNode() /opt/godot/editor/editor_node.cpp:6456:26 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x5a501e0) (BuildId: 3b2cca770c49afa3)
    #4 Main::start() /opt/godot/main/main.cpp:3578:18 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3cdf856) (BuildId: 3b2cca770c49afa3)
    #5 main /opt/godot/platform/linuxbsd/godot_linuxbsd.cpp:84:6 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3ba46d9) (BuildId: 3b2cca770c49afa3)

  Thread T58 (tid=142443, finished) created by main thread at:
    #0 pthread_create <null> (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3b20336) (BuildId: 3b2cca770c49afa3)
    #1 std::thread::_M_start_thread(std::unique_ptr<std::thread::_State, std::default_delete<std::thread::_State>>, void (*)()) <null> (godot.linuxbsd.editor.dev.x86_64.llvm.san+0xa692fc9) (BuildId: 3b2cca770c49afa3)
    #2 EditorFileSystem::scan_changes() /opt/godot/editor/editor_file_system.cpp:1227:18 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x588cc60) (BuildId: 3b2cca770c49afa3)
    #3 EditorNode::_notification(int) /opt/godot/editor/editor_node.cpp:740:39 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x5a03f04) (BuildId: 3b2cca770c49afa3)
    #4 EditorNode::_notificationv(int, bool) /opt/godot/editor/editor_node.h:123:2 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x5a7b34d) (BuildId: 3b2cca770c49afa3)
    #5 Object::notification(int, bool) /opt/godot/core/object/object.cpp:837:3 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0xa16bff3) (BuildId: 3b2cca770c49afa3)
    #6 Node::propagate_notification(int) /opt/godot/scene/main/node.cpp:2288:2 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x7087066) (BuildId: 3b2cca770c49afa3)
    #7 Node::propagate_notification(int) /opt/godot/scene/main/node.cpp:2291:12 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x7087093) (BuildId: 3b2cca770c49afa3)
    #8 SceneTree::_notification(int) /opt/godot/scene/main/scene_tree.cpp (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x70fcf3a) (BuildId: 3b2cca770c49afa3)
    #9 SceneTree::_notificationv(int, bool) /opt/godot/scene/main/scene_tree.h:84:2 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x70fcf3a)
    #10 Object::notification(int, bool) /opt/godot/core/object/object.cpp:837:3 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0xa16bff3) (BuildId: 3b2cca770c49afa3)
    #11 DisplayServerX11::process_events() /opt/godot/platform/linuxbsd/x11/display_server_x11.cpp:4660:45 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3bf1c83) (BuildId: 3b2cca770c49afa3)
    #12 OS_LinuxBSD::run() /opt/godot/platform/linuxbsd/os_linuxbsd.cpp:941:35 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3bb45ef) (BuildId: 3b2cca770c49afa3)
    #13 main /opt/godot/platform/linuxbsd/godot_linuxbsd.cpp:86:6 (godot.linuxbsd.editor.dev.x86_64.llvm.san+0x3ba46f4) (BuildId: 3b2cca770c49afa3)

SUMMARY: ThreadSanitizer: data race /opt/godot/editor/editor_file_system.cpp:1272:10 in EditorFileSystem::_notification(int)
==================

[lyuma]

I think it makes sense. I'm not entirely sure why the original code didn't have the scanning code do a call_deferred to the main thread.... but it seems to me that as in this PR, polling an atomic bool in NOTIFICATION_PROCESS is roughly the same effect and should be correct.

[akien-mga]

Thanks!