x11: Use XCheckIfEvent() instead of XNextEvent() for thread-safety

A racing reader could read from our fd between SDL_IOReady()/X11_Pending()
and our call to XNextEvent() which will cause XNextEvent() to block for
more data. Avoid this by using XCheckIfEvent() which will never block.

This also fixes a bug where we could poll() for data, even when events were
already read and pending in the queue. Unlike the Wayland implementation,
this isn't totally thread-safe because nothing prevents a racing reader
from reading events into the queue between our XCheckIfEvent() and
SDL_IOReady() calls, but I think this is the best we can do with Xlib.

src/video/x11/SDL_x11events.c

@@ -1557,23 +1557,21 @@ X11_HandleFocusChanges(_THIS)
         }
     }
 }
-/* Ack!  X11_XPending() actually performs a blocking read if no events available */
-static int
-X11_Pending(Display * display)
+
+static Bool
+isAnyEvent(Display *display, XEvent *ev, XPointer arg)
 {
-    /* Flush the display connection and look to see if events are queued */
-    X11_XFlush(display);
-    if (X11_XEventsQueued(display, QueuedAlready)) {
-        return (1);
-    }
+    return True;
+}
 
-    /* More drastic measures are required -- see if X is ready to talk */
-    if (SDL_IOReady(ConnectionNumber(display), SDL_IOR_READ, 0)) {
-        return (X11_XPending(display));
+static SDL_bool
+X11_PollEvent(Display *display, XEvent *event)
+{
+    if (!X11_XCheckIfEvent(display, event, isAnyEvent, NULL)) {
+        return SDL_FALSE;
     }
 
-    /* Oh well, nothing is ready .. */
-    return (0);
+    return SDL_TRUE;
 }
 
 void
@@ -1611,17 +1609,21 @@ X11_WaitEventTimeout(_THIS, int timeout)
 
     SDL_zero(xevent);
 
-    if (timeout == 0) {
-        if (X11_Pending(display)) {
-            X11_XNextEvent(display, &xevent);
-        } else {
-            return 0;
-        }
+    /* Flush and poll to grab any events already read and queued */
+    X11_XFlush(display);
+    if (X11_PollEvent(display, &xevent)) {
+        /* Fall through */
+    } else if (timeout == 0) {
+        return 0;
     } else {
         /* Use SDL_IOR_NO_RETRY to ensure SIGINT will break us out of our wait */
         int err = SDL_IOReady(ConnectionNumber(display), SDL_IOR_READ | SDL_IOR_NO_RETRY, timeout);
         if (err > 0) {
-            X11_XNextEvent(display, &xevent);
+            if (!X11_PollEvent(display, &xevent)) {
+                /* Someone may have beat us to reading the fd. Return 1 here to
+                 * trigger the normal spurious wakeup logic in the event core. */
+                return 1;
+            }
         } else if (err == 0) {
             /* Timeout */
             return 0;
@@ -1679,8 +1681,7 @@ X11_PumpEvents(_THIS)
     SDL_zero(xevent);
 
     /* Keep processing pending events */
-    while (X11_Pending(data->display)) {
-        X11_XNextEvent(data->display, &xevent);
+    while (X11_PollEvent(data->display, &xevent)) {
         X11_DispatchEvent(_this, &xevent);
     }
 

src/video/x11/SDL_x11sym.h

@@ -97,7 +97,6 @@ SDL_X11_SYM(int,XMapRaised,(Display* a,Window b),(a,b),return)
 SDL_X11_SYM(Status,XMatchVisualInfo,(Display* a,int b,int c,int d,XVisualInfo* e),(a,b,c,d,e),return)
 SDL_X11_SYM(int,XMissingExtension,(Display* a,_Xconst char* b),(a,b),return)
 SDL_X11_SYM(int,XMoveWindow,(Display* a,Window b,int c,int d),(a,b,c,d),return)
-SDL_X11_SYM(int,XNextEvent,(Display* a,XEvent* b),(a,b),return)
 SDL_X11_SYM(Display*,XOpenDisplay,(_Xconst char* a),(a),return)
 SDL_X11_SYM(Status,XInitThreads,(void),(),return)
 SDL_X11_SYM(int,XPeekEvent,(Display* a,XEvent* b),(a,b),return)