egl/wayland: Use roundtrips when awaiting buffer release

In get_back_bo, we use wl_display_dispatch_queue() to block and wait for
a buffer release event. However, not all Wayland compositors flush the
client socket on posting a buffer-release event, so by only blocking
client-side, we may block indefinitely, or at least need to wait for an
input event / frame completion to arrive for the compositor to flush.

We now use dispatch_queue as a first pass, but if our entire buffer pool
is exhausted, use a roundtrip (an immediately-triggered wl_callback) to
ensure that the compositor flushes out our release event immediately.

[daniels: Modified comment and commit message.]

Signed-off-by: Kai Chen <kai.chen@intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
CC: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 151188d1e3)

src/egl/drivers/dri2/platform_wayland.c

@@ -403,8 +403,13 @@ get_back_bo(struct dri2_egl_surface *dri2_surf)
          break;
 
       /* If we don't have a buffer, then block on the server to release one for
-       * us, and try again. */
-      if (wl_display_dispatch_queue(dri2_dpy->wl_dpy, dri2_surf->wl_queue) < 0)
+       * us, and try again. wl_display_dispatch_queue will process any pending
+       * events, however not all servers flush on issuing a buffer release
+       * event. So, we spam the server with roundtrips as they always cause a
+       * client flush.
+       */
+      if (wl_display_roundtrip_queue(dri2_dpy->wl_dpy,
+                                     dri2_surf->wl_queue) < 0)
           return -1;
    }