Fix general keyboard input lag on X11 display server

This change makes keyboard inputs more responsive on Linux, especially
when the FPS is lower on slower configurations.

Polling events from the x server is done on a separate thread to avoid a
frame delay with inputs, due to first sending the event to the input
manager with XFilterEvent then processing the new event only on the next
frame.

Calls to Input Manager functions like XSetICFocus, XUnsetICFocus and
XSetICValues use a mutex, because they are polling events internally and
would otherwise interfere with our own thread process for polling events
which can cause a deadlock in some cases.

XUnsetICFocus is called instead of XSetICFocus on FocusOut events,
so the input manager can be properly notified of focus changes.

clipboard_get now uses a blocking call to poll for a specific event type
when waiting for a SelectionNotify event, instead of polling all events
and filtering them afterwards.

2 files changed, 165 insertions, 61 deletions

diff --git a/platform/linuxbsd/display_server_x11.cpp b/platform/linuxbsd/display_server_x11.cpp
index e3cf992302..9fa9d57b68 100644
--- a/platform/linuxbsd/display_server_x11.cpp
+++ b/platform/linuxbsd/display_server_x11.cpp
@@ -322,23 +322,19 @@ bool DisplayServerX11::_refresh_device_info() {
 }
 
 void DisplayServerX11::_flush_mouse_motion() {
-	while (true) {
-		if (XPending(x11_display) > 0) {
-			XEvent event;
-			XPeekEvent(x11_display, &event);
-
-			if (XGetEventData(x11_display, &event.xcookie) && event.xcookie.type == GenericEvent && event.xcookie.extension == xi.opcode) {
-				XIDeviceEvent *event_data = (XIDeviceEvent *)event.xcookie.data;
-
-				if (event_data->evtype == XI_RawMotion) {
-					XNextEvent(x11_display, &event);
-				} else {
-					break;
-				}
-			} else {
-				break;
+	// Block events polling while flushing motion events.
+	MutexLock mutex_lock(events_mutex);
+
+	for (uint32_t event_index = 0; event_index < polled_events.size(); ++event_index) {
+		XEvent &event = polled_events[event_index];
+		if (XGetEventData(x11_display, &event.xcookie) && event.xcookie.type == GenericEvent && event.xcookie.extension == xi.opcode) {
+			XIDeviceEvent *event_data = (XIDeviceEvent *)event.xcookie.data;
+			if (event_data->evtype == XI_RawMotion) {
+				XFreeEventData(x11_display, &event.xcookie);
+				polled_events.remove(event_index--);
+				continue;
 			}
-		} else {
+			XFreeEventData(x11_display, &event.xcookie);
 			break;
 		}
 	}
@@ -456,7 +452,15 @@ void DisplayServerX11::clipboard_set(const String &p_text) {
 	XSetSelectionOwner(x11_display, XInternAtom(x11_display, "CLIPBOARD", 0), windows[MAIN_WINDOW_ID].x11_window, CurrentTime);
 }
 
-static String _clipboard_get_impl(Atom p_source, Window x11_window, ::Display *x11_display, String p_internal_clipboard, Atom target) {
+Bool DisplayServerX11::_predicate_clipboard_selection(Display *display, XEvent *event, XPointer arg) {
+	if (event->type == SelectionNotify && event->xselection.requestor == *(Window *)arg) {
+		return True;
+	} else {
+		return False;
+	}
+}
+
+String DisplayServerX11::_clipboard_get_impl(Atom p_source, Window x11_window, Atom target) const {
 	String ret;
 
 	Atom type;
@@ -467,20 +471,24 @@ static String _clipboard_get_impl(Atom p_source, Window x11_window, ::Display *x
 	Window Sown = XGetSelectionOwner(x11_display, p_source);
 
 	if (Sown == x11_window) {
-		return p_internal_clipboard;
+		return internal_clipboard;
 	};
 
 	if (Sown != None) {
-		XConvertSelection(x11_display, p_source, target, selection,
-				x11_window, CurrentTime);
-		XFlush(x11_display);
-		while (true) {
+		{
+			// Block events polling while processing selection events.
+			MutexLock mutex_lock(events_mutex);
+
+			XConvertSelection(x11_display, p_source, target, selection,
+					x11_window, CurrentTime);
+
+			XFlush(x11_display);
+
+			// Blocking wait for predicate to be True
+			// and remove the event from the queue.
 			XEvent event;
-			XNextEvent(x11_display, &event);
-			if (event.type == SelectionNotify && event.xselection.requestor == x11_window) {
-				break;
-			};
-		};
+			XIfEvent(x11_display, &event, _predicate_clipboard_selection, (XPointer)&x11_window);
+		}
 
 		//
 		// Do not get any data, see how much data is there
@@ -514,14 +522,14 @@ static String _clipboard_get_impl(Atom p_source, Window x11_window, ::Display *x
 	return ret;
 }
 
-static String _clipboard_get(Atom p_source, Window x11_window, ::Display *x11_display, String p_internal_clipboard) {
+String DisplayServerX11::_clipboard_get(Atom p_source, Window x11_window) const {
 	String ret;
 	Atom utf8_atom = XInternAtom(x11_display, "UTF8_STRING", True);
 	if (utf8_atom != None) {
-		ret = _clipboard_get_impl(p_source, x11_window, x11_display, p_internal_clipboard, utf8_atom);
+		ret = _clipboard_get_impl(p_source, x11_window, utf8_atom);
 	}
-	if (ret == "") {
-		ret = _clipboard_get_impl(p_source, x11_window, x11_display, p_internal_clipboard, XA_STRING);
+	if (ret.empty()) {
+		ret = _clipboard_get_impl(p_source, x11_window, XA_STRING);
 	}
 	return ret;
 }
@@ -530,11 +538,11 @@ String DisplayServerX11::clipboard_get() const {
 	_THREAD_SAFE_METHOD_
 
 	String ret;
-	ret = _clipboard_get(XInternAtom(x11_display, "CLIPBOARD", 0), windows[MAIN_WINDOW_ID].x11_window, x11_display, internal_clipboard);
+	ret = _clipboard_get(XInternAtom(x11_display, "CLIPBOARD", 0), windows[MAIN_WINDOW_ID].x11_window);
 
-	if (ret == "") {
-		ret = _clipboard_get(XA_PRIMARY, windows[MAIN_WINDOW_ID].x11_window, x11_display, internal_clipboard);
-	};
+	if (ret.empty()) {
+		ret = _clipboard_get(XA_PRIMARY, windows[MAIN_WINDOW_ID].x11_window);
+	}
 
 	return ret;
 }
@@ -1649,10 +1657,16 @@ void DisplayServerX11::window_set_ime_active(const bool p_active, WindowID p_win
 		return;
 	}
 
+	// Block events polling while changing input focus
+	// because it triggers some event polling internally.
 	if (p_active) {
-		XSetICFocus(wd.xic);
+		{
+			MutexLock mutex_lock(events_mutex);
+			XSetICFocus(wd.xic);
+		}
 		window_set_ime_position(wd.im_position, p_window);
 	} else {
+		MutexLock mutex_lock(events_mutex);
 		XUnsetICFocus(wd.xic);
 	}
 }
@@ -1673,7 +1687,14 @@ void DisplayServerX11::window_set_ime_position(const Point2i &p_pos, WindowID p_
 	spot.x = short(p_pos.x);
 	spot.y = short(p_pos.y);
 	XVaNestedList preedit_attr = XVaCreateNestedList(0, XNSpotLocation, &spot, nullptr);
-	XSetICValues(wd.xic, XNPreeditAttributes, preedit_attr, nullptr);
+
+	{
+		// Block events polling during this call
+		// because it triggers some event polling internally.
+		MutexLock mutex_lock(events_mutex);
+		XSetICValues(wd.xic, XNPreeditAttributes, preedit_attr, nullptr);
+	}
+
 	XFree(preedit_attr);
 }
 
@@ -1993,7 +2014,7 @@ unsigned int DisplayServerX11::_get_mouse_button_state(unsigned int p_x11_button
 	return last_button_state;
 }
 
-void DisplayServerX11::_handle_key_event(WindowID p_window, XKeyEvent *p_event, bool p_echo) {
+void DisplayServerX11::_handle_key_event(WindowID p_window, XKeyEvent *p_event, LocalVector<XEvent> &p_events, uint32_t &p_event_index, bool p_echo) {
 	WindowData wd = windows[p_window];
 	// X11 functions don't know what const is
 	XKeyEvent *xkeyevent = p_event;
@@ -2130,7 +2151,7 @@ void DisplayServerX11::_handle_key_event(WindowID p_window, XKeyEvent *p_event,
 	/* Phase 4, determine if event must be filtered */
 
 	// This seems to be a side-effect of using XIM.
-	// XEventFilter looks like a core X11 function,
+	// XFilterEvent looks like a core X11 function,
 	// but it's actually just used to see if we must
 	// ignore a deadkey, or events XIM determines
 	// must not reach the actual gui.
@@ -2164,17 +2185,16 @@ void DisplayServerX11::_handle_key_event(WindowID p_window, XKeyEvent *p_event,
 
 	// Echo characters in X11 are a keyrelease and a keypress
 	// one after the other with the (almot) same timestamp.
-	// To detect them, i use XPeekEvent and check that their
-	// difference in time is below a threshold.
+	// To detect them, i compare to the next event in list and
+	// check that their difference in time is below a threshold.
 
 	if (xkeyevent->type != KeyPress) {
 		p_echo = false;
 
 		// make sure there are events pending,
 		// so this call won't block.
-		if (XPending(x11_display) > 0) {
-			XEvent peek_event;
-			XPeekEvent(x11_display, &peek_event);
+		if (p_event_index + 1 < p_events.size()) {
+			XEvent &peek_event = p_events[p_event_index + 1];
 
 			// I'm using a threshold of 5 msecs,
 			// since sometimes there seems to be a little
@@ -2189,9 +2209,9 @@ void DisplayServerX11::_handle_key_event(WindowID p_window, XKeyEvent *p_event,
 				KeySym rk;
 				XLookupString((XKeyEvent *)&peek_event, str, 256, &rk, nullptr);
 				if (rk == keysym_keycode) {
-					XEvent event;
-					XNextEvent(x11_display, &event); //erase next event
-					_handle_key_event(p_window, (XKeyEvent *)&event, true);
+					// Consume to next event.
+					++p_event_index;
+					_handle_key_event(p_window, (XKeyEvent *)&peek_event, p_events, p_event_index, true);
 					return; //ignore current, echo next
 				}
 			}
@@ -2358,6 +2378,56 @@ void DisplayServerX11::_send_window_event(const WindowData &wd, WindowEvent p_ev
 	}
 }
 
+void DisplayServerX11::_poll_events_thread(void *ud) {
+	DisplayServerX11 *display_server = (DisplayServerX11 *)ud;
+	display_server->_poll_events();
+}
+
+Bool DisplayServerX11::_predicate_all_events(Display *display, XEvent *event, XPointer arg) {
+	// Just accept all events.
+	return True;
+}
+
+void DisplayServerX11::_poll_events() {
+	int x11_fd = ConnectionNumber(x11_display);
+	fd_set in_fds;
+
+	while (!events_thread_done) {
+		XFlush(x11_display);
+
+		FD_ZERO(&in_fds);
+		FD_SET(x11_fd, &in_fds);
+
+		struct timeval tv;
+		tv.tv_usec = 0;
+		tv.tv_sec = 1;
+
+		// Wait for next event or timeout.
+		int num_ready_fds = select(x11_fd + 1, &in_fds, NULL, NULL, &tv);
+		if (num_ready_fds < 0) {
+			ERR_PRINT("_poll_events: select error: " + itos(errno));
+		}
+
+		// Process events from the queue.
+		{
+			MutexLock mutex_lock(events_mutex);
+
+			// Non-blocking wait for next event
+			// and remove it from the queue.
+			XEvent ev;
+			while (XCheckIfEvent(x11_display, &ev, _predicate_all_events, nullptr)) {
+				// Check if the input manager wants to process the event.
+				if (XFilterEvent(&ev, None)) {
+					// Event has been filtered by the Input Manager,
+					// it has to be ignored and a new one will be received.
+					continue;
+				}
+				polled_events.push_back(ev);
+			}
+		}
+	}
+}
+
 void DisplayServerX11::process_events() {
 	_THREAD_SAFE_METHOD_
 
@@ -2401,13 +2471,16 @@ void DisplayServerX11::process_events() {
 	xi.tilt = Vector2();
 	xi.pressure_supported = false;
 
-	while (XPending(x11_display) > 0) {
-		XEvent event;
-		XNextEvent(x11_display, &event);
+	LocalVector<XEvent> events;
+	{
+		// Block events polling while flushing events.
+		MutexLock mutex_lock(events_mutex);
+		events = polled_events;
+		polled_events.clear();
+	}
 
-		if (XFilterEvent(&event, None)) {
-			continue;
-		}
+	for (uint32_t event_index = 0; event_index < events.size(); ++event_index) {
+		XEvent &event = events[event_index];
 
 		WindowID window_id = MAIN_WINDOW_ID;
 
@@ -2666,6 +2739,9 @@ void DisplayServerX11::process_events() {
 				}*/
 #endif
 				if (wd.xic) {
+					// Block events polling while changing input focus
+					// because it triggers some event polling internally.
+					MutexLock mutex_lock(events_mutex);
 					XSetICFocus(wd.xic);
 				}
 
@@ -2715,7 +2791,10 @@ void DisplayServerX11::process_events() {
 				xi.state.clear();
 #endif
 				if (wd.xic) {
-					XSetICFocus(wd.xic);
+					// Block events polling while changing input focus
+					// because it triggers some event polling internally.
+					MutexLock mutex_lock(events_mutex);
+					XUnsetICFocus(wd.xic);
 				}
 			} break;
 
@@ -2834,11 +2913,11 @@ void DisplayServerX11::process_events() {
 						break;
 					}
 
-					if (XPending(x11_display) > 0) {
-						XEvent tevent;
-						XPeekEvent(x11_display, &tevent);
-						if (tevent.type == MotionNotify) {
-							XNextEvent(x11_display, &event);
+					if (event_index + 1 < events.size()) {
+						const XEvent &next_event = events[event_index + 1];
+						if (next_event.type == MotionNotify) {
+							++event_index;
+							event = next_event;
 						} else {
 							break;
 						}
@@ -2969,7 +3048,7 @@ void DisplayServerX11::process_events() {
 
 				// key event is a little complex, so
 				// it will be handled in its own function.
-				_handle_key_event(window_id, (XKeyEvent *)&event);
+				_handle_key_event(window_id, (XKeyEvent *)&event, events, event_index);
 			} break;
 			case SelectionRequest: {
 				XSelectionRequestEvent *req;
@@ -3424,6 +3503,10 @@ DisplayServerX11::WindowID DisplayServerX11::_create_window(WindowMode p_mode, u
 		XChangeProperty(x11_display, wd.x11_window, xdnd_aware, XA_ATOM, 32, PropModeReplace, (unsigned char *)&xdnd_version, 1);
 
 		if (xim && xim_style) {
+			// Block events polling while changing input focus
+			// because it triggers some event polling internally.
+			MutexLock mutex_lock(events_mutex);
+
 			wd.xic = XCreateIC(xim, XNInputStyle, xim_style, XNClientWindow, wd.x11_window, XNFocusWindow, wd.x11_window, (char *)nullptr);
 			if (XGetICValues(wd.xic, XNFilterEvents, &im_event_mask, nullptr) != nullptr) {
 				WARN_PRINT("XGetICValues couldn't obtain XNFilterEvents value");
@@ -3926,12 +4009,19 @@ DisplayServerX11::DisplayServerX11(const String &p_rendering_driver, WindowMode
 		}
 	}
 
+	events_thread = Thread::create(_poll_events_thread, this);
+
 	_update_real_mouse_position(windows[MAIN_WINDOW_ID]);
 
 	r_error = OK;
 }
 
 DisplayServerX11::~DisplayServerX11() {
+	events_thread_done = true;
+	Thread::wait_to_finish(events_thread);
+	memdelete(events_thread);
+	events_thread = nullptr;
+
 	//destroy all windows
 	for (Map<WindowID, WindowData>::Element *E = windows.front(); E; E = E->next()) {
 #ifdef VULKAN_ENABLED
diff --git a/platform/linuxbsd/display_server_x11.h b/platform/linuxbsd/display_server_x11.h
index e894cf515c..51af02f3cc 100644
--- a/platform/linuxbsd/display_server_x11.h
+++ b/platform/linuxbsd/display_server_x11.h
@@ -36,6 +36,7 @@
 #include "servers/display_server.h"
 
 #include "core/input/input.h"
+#include "core/local_vector.h"
 #include "drivers/alsa/audio_driver_alsa.h"
 #include "drivers/alsamidi/midi_driver_alsamidi.h"
 #include "drivers/pulseaudio/audio_driver_pulseaudio.h"
@@ -202,7 +203,10 @@ class DisplayServerX11 : public DisplayServer {
 	MouseMode mouse_mode;
 	Point2i center;
 
-	void _handle_key_event(WindowID p_window, XKeyEvent *p_event, bool p_echo = false);
+	void _handle_key_event(WindowID p_window, XKeyEvent *p_event, LocalVector<XEvent> &p_events, uint32_t &p_event_index, bool p_echo = false);
+
+	String _clipboard_get_impl(Atom p_source, Window x11_window, Atom target) const;
+	String _clipboard_get(Atom p_source, Window x11_window) const;
 
 	//bool minimized;
 	//bool window_has_focus;
@@ -252,6 +256,16 @@ class DisplayServerX11 : public DisplayServer {
 	static void _dispatch_input_events(const Ref<InputEvent> &p_event);
 	void _dispatch_input_event(const Ref<InputEvent> &p_event);
 
+	mutable Mutex events_mutex;
+	Thread *events_thread = nullptr;
+	bool events_thread_done = false;
+	LocalVector<XEvent> polled_events;
+	static void _poll_events_thread(void *ud);
+	void _poll_events();
+
+	static Bool _predicate_all_events(Display *display, XEvent *event, XPointer arg);
+	static Bool _predicate_clipboard_selection(Display *display, XEvent *event, XPointer arg);
+
 protected:
 	void _window_changed(XEvent *event);