wayland: don't use wl_display_dispatch()

wl_display_dispatch() calls poll(), which is unnecessary since we
already know the FD is readable.

Use the more lower level wl_display_read_events() +
wl_display_dispatch_pending().

These require wl_display_prepare_read() to have been called.

The idea is to call wl_display_prepare_read() **before** calling
poll().

Thus, we do this more or less last in wayl_init(), and at the **end**
of the FDM handler.

However, having taking this lock also means we no longer can call
wl_display_roundtrip() directly (it will hang).

So, add a wrapper, wayl_roundtrip(), that cancels the read intent,
does the roundtrip, and then re-acquires the read intent.

terminal.c

@@ -659,9 +659,7 @@ term_init(const struct config *conf, struct fdm *fdm, struct wayland *wayl,
 
     /* Let the Wayland backend know we exist */
     tll_push_back(wayl->terms, term);
-
+    wayl_roundtrip(term->wl);
-    /* Roundtrip to ensure the wayland window have been configured */
-    wl_display_roundtrip(term->wl->display);
 
     term_set_window_title(term, "foot");
 

wayland.c

@@ -559,10 +559,17 @@ static bool
 fdm_wayl(struct fdm *fdm, int fd, int events, void *data)
 {
     struct wayland *wayl = data;
-    int event_count = wl_display_dispatch(wayl->display);
+    int event_count = 0;
+
+    if (events & EPOLLIN)
+        wl_display_read_events(wayl->display);
+
+    while (wl_display_prepare_read(wayl->display) != 0)
+        wl_display_dispatch_pending(wayl->display);
 
     if (events & EPOLLHUP) {
         LOG_WARN("disconnected from Wayland");
+        wl_display_cancel_read(wayl->display);
         return false;
     }
 
@@ -711,6 +718,11 @@ wayl_init(const struct config *conf, struct fdm *fdm)
         goto out;
     }
 
+    if (wl_display_prepare_read(wayl->display) != 0) {
+        LOG_ERRNO("failed to prepare for reading wayland events");
+        goto out;
+    }
+
     if (!fdm_add(fdm, wl_display_get_fd(wayl->display), EPOLLIN, &fdm_wayl, wayl) ||
         !fdm_add(fdm, wayl->kbd.repeat.fd, EPOLLIN, &fdm_repeat, wayl))
     {
@@ -889,12 +901,12 @@ wayl_win_destroy(struct wl_window *win)
     /* Scrollback search */
     wl_surface_attach(win->search_surface, NULL, 0, 0);
     wl_surface_commit(win->search_surface);
-    wl_display_roundtrip(win->term->wl->display);
+    wayl_roundtrip(win->term->wl);
 
     /* Main window */
     wl_surface_attach(win->surface, NULL, 0, 0);
     wl_surface_commit(win->surface);
-    wl_display_roundtrip(win->term->wl->display);
+    wayl_roundtrip(win->term->wl);
 
     tll_free(win->on_outputs);
     if (win->search_sub_surface != NULL)
@@ -912,8 +924,7 @@ wayl_win_destroy(struct wl_window *win)
     if (win->surface != NULL)
         wl_surface_destroy(win->surface);
 
-    wl_display_roundtrip(win->term->wl->display);
+    wayl_roundtrip(win->term->wl);
-
     free(win);
 }
 
@@ -1006,3 +1017,14 @@ wayl_terminal_from_surface(struct wayland *wayl, struct wl_surface *surface)
     LOG_WARN("surface %p doesn't map to a terminal", surface);
     return NULL;
 }
+
+void
+wayl_roundtrip(struct wayland *wayl)
+{
+    wl_display_cancel_read(wayl->display);
+    wl_display_roundtrip(wayl->display);
+
+    while (wl_display_prepare_read(wayl->display) != 0)
+        wl_display_dispatch_pending(wayl->display);
+    wl_display_flush(wayl->display);
+}

wayland.h

@@ -178,6 +178,8 @@ struct wayland {
 struct wayland *wayl_init(const struct config *conf, struct fdm *fdm);
 void wayl_destroy(struct wayland *wayl);
 
+void wayl_roundtrip(struct wayland *wayl);
+
 struct terminal *wayl_terminal_from_surface(
     struct wayland *wayl, struct wl_surface *surface);