wayland: Use multi-thread event reading APIs

Wayland provides the prepare_read()/read_events() family of APIs for
reading from the display fd in a deadlock-free manner across multiple
threads in a multi-threaded application. Let's use those instead of
trying to roll our own solution using a mutex.

This fixes an issue where a call to SDL_GL_SwapWindow() doesn't swap
buffers if it happens to collide with SDL_PumpEvents() in the main
thread. It also allows coexistence with other code or toolkits in
our process that may want read and dispatch events themselves.

src/video/wayland/SDL_waylandevents.c

@@ -231,20 +231,18 @@ Wayland_PumpEvents(_THIS)
         keyboard_repeat_handle(&input->keyboard_repeat, now);
     }
 
-    /* If we're trying to dispatch the display in another thread,
+    /* wl_display_prepare_read() will return -1 if the default queue is not empty.
-     * we could trigger a race condition and end up blocking
+     * If the default queue is empty, it will prepare us for our SDL_IOReady() call. */
-     * in wl_display_dispatch() */
+    if (WAYLAND_wl_display_prepare_read(d->display) == 0) {
-    if (SDL_TryLockMutex(d->display_dispatch_lock) != 0) {
+        if (SDL_IOReady(WAYLAND_wl_display_get_fd(d->display), SDL_FALSE, 0) > 0) {
-        return;
+            WAYLAND_wl_display_read_events(d->display);
-    }
-
-    if (SDL_IOReady(WAYLAND_wl_display_get_fd(d->display), SDL_FALSE, 0)) {
-        err = WAYLAND_wl_display_dispatch(d->display);
         } else {
-        err = WAYLAND_wl_display_dispatch_pending(d->display);
+            WAYLAND_wl_display_cancel_read(d->display);
+        }
     }
 
-    SDL_UnlockMutex(d->display_dispatch_lock);
+    /* Dispatch any pre-existing pending events or new events we may have read */
+    err = WAYLAND_wl_display_dispatch_pending(d->display);
 
     if (err == -1 && !d->display_disconnected) {
         /* Something has failed with the Wayland connection -- for example,

src/video/wayland/SDL_waylandopengles.c

@@ -136,33 +136,34 @@ Wayland_GLES_SwapWindow(_THIS, SDL_Window *window)
         while (SDL_AtomicGet(&data->swap_interval_ready) == 0) {
             Uint32 now;
 
-            /* !!! FIXME: this is just the crucial piece of Wayland_PumpEvents */
             WAYLAND_wl_display_flush(display);
-            if (WAYLAND_wl_display_dispatch_queue_pending(display, data->frame_event_queue) > 0) {
+
-                /* We dispatched some pending events. Check if the frame callback happened. */
+            /* wl_display_prepare_read_queue() will return -1 if the event queue is not empty.
+             * If the event queue is empty, it will prepare us for our SDL_IOReady() call. */
+            if (WAYLAND_wl_display_prepare_read_queue(display, data->frame_event_queue) != 0) {
+                /* We have some pending events. Check if the frame callback happened. */
+                WAYLAND_wl_display_dispatch_queue_pending(display, data->frame_event_queue);
                 continue;
             }
 
+            /* Beyond this point, we must either call wl_display_cancel_read() or wl_display_read_events() */
+
             now = SDL_GetTicks();
             if (SDL_TICKS_PASSED(now, max_wait)) {
-                /* Timeout expired */
+                /* Timeout expired. Cancel the read. */
+                WAYLAND_wl_display_cancel_read(display);
                 break;
             }
 
-            /* Make sure we're not competing with SDL_PumpEvents() for any new
-             * events, or one of us may end up blocking in wl_display_dispatch */
-            if (SDL_TryLockMutex(videodata->display_dispatch_lock) != 0) {
-                continue;
-            }
-
             if (SDL_IOReady(WAYLAND_wl_display_get_fd(display), SDL_FALSE, max_wait - now) <= 0) {
-                /* Error or timeout expired without any events for us */
+                /* Error or timeout expired without any events for us. Cancel the read. */
-                SDL_UnlockMutex(videodata->display_dispatch_lock);
+                WAYLAND_wl_display_cancel_read(display);
                 break;
             }
 
-            WAYLAND_wl_display_dispatch_queue(display, data->frame_event_queue);
+            /* We have events. Read and dispatch them. */
-            SDL_UnlockMutex(videodata->display_dispatch_lock);
+            WAYLAND_wl_display_read_events(display);
+            WAYLAND_wl_display_dispatch_queue_pending(display, data->frame_event_queue);
         }
         SDL_AtomicSet(&data->swap_interval_ready, 0);
     }

src/video/wayland/SDL_waylandsym.h

@@ -56,6 +56,10 @@ SDL_WAYLAND_SYM(int, wl_display_dispatch, (struct wl_display *))
 SDL_WAYLAND_SYM(int, wl_display_dispatch_queue, (struct wl_display *, struct wl_event_queue *))
 SDL_WAYLAND_SYM(int, wl_display_dispatch_queue_pending, (struct wl_display *, struct wl_event_queue *))
 SDL_WAYLAND_SYM(int, wl_display_dispatch_pending, (struct wl_display *))
+SDL_WAYLAND_SYM(int, wl_display_prepare_read, (struct wl_display *))
+SDL_WAYLAND_SYM(int, wl_display_prepare_read_queue, (struct wl_display *, struct wl_event_queue *))
+SDL_WAYLAND_SYM(int, wl_display_read_events, (struct wl_display *))
+SDL_WAYLAND_SYM(void, wl_display_cancel_read, (struct wl_display *))
 SDL_WAYLAND_SYM(int, wl_display_get_error, (struct wl_display *))
 SDL_WAYLAND_SYM(int, wl_display_flush, (struct wl_display *))
 SDL_WAYLAND_SYM(int, wl_display_roundtrip, (struct wl_display *))

src/video/wayland/SDL_waylandvideo.c

@@ -169,7 +169,6 @@ Wayland_DeleteDevice(SDL_VideoDevice *device)
         WAYLAND_wl_display_flush(data->display);
         WAYLAND_wl_display_disconnect(data->display);
     }
-    SDL_DestroyMutex(data->display_dispatch_lock);
     SDL_free(data);
     SDL_free(device);
     SDL_WAYLAND_UnloadSymbols();
@@ -202,8 +201,6 @@ Wayland_CreateDevice(int devindex)
 
     data->display = display;
 
-    data->display_dispatch_lock = SDL_CreateMutex();
-
     /* Initialize all variables that we clean on shutdown */
     device = SDL_calloc(1, sizeof(SDL_VideoDevice));
     if (!device) {

src/video/wayland/SDL_waylandvideo.h

@@ -86,7 +86,6 @@ typedef struct {
     char *classname;
 
     int relative_mouse_mode;
-    SDL_mutex *display_dispatch_lock;
 } SDL_VideoData;
 
 typedef struct {