wayland: prevent hang when terminating due to broken Wayland connection

If the call to fdm_wayl() with EPOLLHUP was followed by calls to
wayl_roundtrip(), foot would hang.

The reason for this is that the EPOLLHUP handler in fdm_wayl() would
call wl_display_cancel_read().

wayl_roundtrip() also calls wl_display_cancel_read() (since we
normally have called wl_display_prepare_read()), before calling
wl_display_roundtrip().

When calling wl_display_cancel_read() two times in a row, without a
wl_display_prepare_read() in between, wl_display_roundtrip() hangs.

Fix by not calling wl_display_cancel_read() in fdm_wayl(). This
ensures our invariant holds: wl_display_prepare_read() is *always* in
effect outside of fdm_wayl().

Closes #651

CHANGELOG.md

@@ -48,6 +48,8 @@
 * Crash in scrollback search mode when selection has been canceled due
   to terminal content updates
   (https://codeberg.org/dnkl/foot/issues/644).
+* Foot process not terminating when the Wayland connection is broken
+  (https://codeberg.org/dnkl/foot/issues/651).
 
 
 ### Security

wayland.c

@@ -1163,7 +1163,15 @@ fdm_wayl(struct fdm *fdm, int fd, int events, void *data)
 
     if (events & EPOLLHUP) {
         LOG_WARN("disconnected from Wayland");
-        wl_display_cancel_read(wayl->display);
+        /*
+         * Do *not* call wl_display_cancel_read() here.
+         *
+         * Doing so causes later calls to wayl_roundtrip() (called
+         * from term_destroy() -> wayl_win_destroy()) to hang
+         * indefinitely.
+         *
+         * https://codeberg.org/dnkl/foot/issues/651
+         */
         return false;
     }