render: throttle xcursor updates

With a bad behaving client (e.g. 'less' with mouse support enabled),
we can end up with a *lot* of xcursor updates (so much, that we
flooded the wayland socket before we implemented a blocking
wayl_flush()).

Since there's little point in updating the cursor more than once per
frame interval, use frame callbacks to throttle the updates.

This works more or lesslike normal terminal refreshes:

render_xcursor_set() stores the last terminal (window) that had (and
updated) the cursor.

The renderer's FDM hook checks if we have such a pending terminal set,
and if so, tries to refresh the cursor.

This is done by first checking if we're already waiting for a callback
from a previous cursor update, and if so we do nothing; the callback
will update the cursor for the next frame. If we're *not* already
waiting for a callback, we update the cursor immediately.

render.c

@@ -34,25 +34,7 @@ static struct {
     size_t two;   /* commits presented in two or more frame intervals */
 } presentation_statistics = {0};
 
-static void
-fdm_hook_refresh_pending_terminals(struct fdm *fdm, void *data)
-{
-    struct renderer *renderer = data;
-    tll_foreach(renderer->wayl->terms, it) {
-        struct terminal *term = it->item;
-
-        if (!term->render.refresh_needed)
-            continue;
-
-        assert(term->window->is_configured);
-        term->render.refresh_needed = false;
-
-        if (term->window->frame_callback == NULL)
-            grid_render(term);
-        else
-            term->render.pending = true;
-    }
-}
+static void fdm_hook_refresh_pending_terminals(struct fdm *fdm, void *data);
 
 struct renderer *
 render_init(struct fdm *fdm, struct wayland *wayl)
@@ -601,7 +583,7 @@ static const struct wl_callback_listener frame_listener = {
     .done = &frame_callback,
 };
 
-void
+static void
 grid_render(struct terminal *term)
 {
     if (term->is_shutting_down)
@@ -1110,6 +1092,102 @@ render_resize(struct terminal *term, int width, int height)
     render_refresh(term);
 }
 
+static void xcursor_callback(
+    void *data, struct wl_callback *wl_callback, uint32_t callback_data);
+
+static const struct wl_callback_listener xcursor_listener = {
+    .done = &xcursor_callback,
+};
+
+static void
+render_xcursor_update(struct wayland *wayl, const struct terminal *term)
+{
+    wayl->pointer.pending_terminal = NULL;
+
+    /* If called from a frame callback, we may no longer have mouse focus */
+    if (wayl->mouse_focus != term)
+        return;
+
+    wayl->pointer.cursor = wl_cursor_theme_get_cursor(wayl->pointer.theme, term->xcursor);
+    if (wayl->pointer.cursor == NULL) {
+        LOG_ERR("%s: failed to load xcursor pointer '%s'",
+                wayl->pointer.theme_name, term->xcursor);
+        return;
+    }
+
+    wayl->pointer.xcursor = term->xcursor;
+
+    const int scale = term->scale;
+    struct wl_cursor_image *image = wayl->pointer.cursor->images[0];
+
+    wl_surface_attach(
+        wayl->pointer.surface, wl_cursor_image_get_buffer(image), 0, 0);
+
+    wl_pointer_set_cursor(
+        wayl->pointer.pointer, wayl->pointer.serial,
+        wayl->pointer.surface,
+        image->hotspot_x / scale, image->hotspot_y / scale);
+
+    wl_surface_damage_buffer(
+        wayl->pointer.surface, 0, 0, INT32_MAX, INT32_MAX);
+
+    wl_surface_set_buffer_scale(wayl->pointer.surface, scale);
+
+    assert(wayl->pointer.xcursor_callback == NULL);
+    wayl->pointer.xcursor_callback = wl_surface_frame(wayl->pointer.surface);
+    wl_callback_add_listener(wayl->pointer.xcursor_callback, &xcursor_listener, wayl);
+
+    wl_surface_commit(wayl->pointer.surface);
+    wayl_flush(wayl);
+}
+
+static void
+render_xcursor_refresh(struct wayland *wayl)
+{
+    if (wayl->pointer.pending_terminal == NULL)
+        return;
+
+    if (wayl->pointer.xcursor_callback == NULL)
+        render_xcursor_update(wayl, wayl->pointer.pending_terminal);
+    else {
+        /* Frame callback will call render_xcursor_update() */
+    }
+}
+
+static void
+xcursor_callback(void *data, struct wl_callback *wl_callback, uint32_t callback_data)
+{
+    struct wayland *wayl = data;
+
+    assert(wayl->pointer.xcursor_callback == wl_callback);
+    wl_callback_destroy(wl_callback);
+    wayl->pointer.xcursor_callback = NULL;
+
+    render_xcursor_refresh(wayl);
+}
+
+static void
+fdm_hook_refresh_pending_terminals(struct fdm *fdm, void *data)
+{
+    struct renderer *renderer = data;
+    tll_foreach(renderer->wayl->terms, it) {
+        struct terminal *term = it->item;
+
+        if (!term->render.refresh_needed)
+            continue;
+
+        assert(term->window->is_configured);
+        term->render.refresh_needed = false;
+
+        if (term->window->frame_callback == NULL)
+            grid_render(term);
+        else
+            term->render.pending = true;
+    }
+
+    render_xcursor_refresh(renderer->wayl);
+}
+
 void
 render_set_title(struct terminal *term, const char *_title)
 {
@@ -1133,3 +1211,30 @@ render_refresh(struct terminal *term)
 {
     term->render.refresh_needed = true;
 }
+
+bool
+render_xcursor_set(struct terminal *term)
+{
+    struct wayland *wayl = term->wl;
+
+    if (wayl->pointer.theme == NULL)
+        return false;
+
+    if (wayl->mouse_focus == NULL) {
+        wayl->pointer.xcursor = NULL;
+        wayl->pointer.pending_terminal = NULL;
+        return true;
+    }
+
+    if (wayl->mouse_focus != term) {
+        /* This terminal doesn't have mouse focus */
+        return true;
+    }
+
+    if (wayl->pointer.xcursor == term->xcursor)
+        return true;
+
+    /* FDM hook takes care of actual rendering */
+    wayl->pointer.pending_terminal = term;
+    return true;
+}

render.h

@@ -8,10 +8,10 @@ struct renderer;
 struct renderer *render_init(struct fdm *fdm, struct wayland *wayl);
 void render_destroy(struct renderer *renderer);
 
-void grid_render(struct terminal *term);
 void render_resize(struct terminal *term, int width, int height);
 void render_set_title(struct terminal *term, const char *title);
 void render_refresh(struct terminal *term);
+bool render_xcursor_set(struct terminal *term);
 
 void render_search_box(struct terminal *term);
 

terminal.c

@@ -1674,7 +1674,7 @@ term_xcursor_update(struct terminal *term)
         selection_enabled(term) ? XCURSOR_TEXT :
         XCURSOR_HAND2;
 
-    wayl_cursor_set(term->wl, term);
+    render_xcursor_set(term);
 }
 
 void

wayland.c

@@ -32,7 +32,7 @@
 #define max(x, y) ((x) > (y) ? (x) : (y))
 
 static bool wayl_reload_cursor_theme(
-    struct wayland *wayl, const struct terminal *term);
+    struct wayland *wayl, struct terminal *term);
 
 static void
 shm_format(void *data, struct wl_shm *wl_shm, uint32_t format)
@@ -769,6 +769,9 @@ wayl_destroy(struct wayland *wayl)
         tll_remove(wayl->monitors, it);
     }
 
+    if (wayl->pointer.xcursor_callback != NULL)
+        wl_callback_destroy(wayl->pointer.xcursor_callback);
+
     if (wayl->xdg_output_manager != NULL)
         zxdg_output_manager_v1_destroy(wayl->xdg_output_manager);
     if (wayl->shell != NULL)
@@ -940,56 +943,8 @@ wayl_win_destroy(struct wl_window *win)
     free(win);
 }
 
-bool
-wayl_cursor_set(struct wayland *wayl, const struct terminal *term)
-{
-    if (wayl->pointer.theme == NULL)
-        return false;
-
-    if (wayl->mouse_focus == NULL) {
-        wayl->pointer.xcursor = NULL;
-        return true;
-    }
-
-    if (wayl->mouse_focus != term) {
-        /* This terminal doesn't have mouse focus */
-        return true;
-    }
-
-    if (wayl->pointer.xcursor == term->xcursor)
-        return true;
-
-    wayl->pointer.cursor = wl_cursor_theme_get_cursor(wayl->pointer.theme, term->xcursor);
-    if (wayl->pointer.cursor == NULL) {
-        LOG_ERR("%s: failed to load xcursor pointer '%s'",
-                wayl->pointer.theme_name, term->xcursor);
-        return false;
-    }
-
-    wayl->pointer.xcursor = term->xcursor;
-
-    const int scale = term->scale;
-    struct wl_cursor_image *image = wayl->pointer.cursor->images[0];
-
-    wl_surface_attach(
-        wayl->pointer.surface, wl_cursor_image_get_buffer(image), 0, 0);
-
-    wl_pointer_set_cursor(
-        wayl->pointer.pointer, wayl->pointer.serial,
-        wayl->pointer.surface,
-        image->hotspot_x / scale, image->hotspot_y / scale);
-
-    wl_surface_damage_buffer(
-        wayl->pointer.surface, 0, 0, INT32_MAX, INT32_MAX);
-
-    wl_surface_set_buffer_scale(wayl->pointer.surface, scale);
-    wl_surface_commit(wayl->pointer.surface);
-    wayl_flush(wayl);
-    return true;
-}
-
 static bool
-wayl_reload_cursor_theme(struct wayland *wayl, const struct terminal *term)
+wayl_reload_cursor_theme(struct wayland *wayl, struct terminal *term)
 {
     if (wayl->pointer.size == 0)
         return true;
@@ -1011,7 +966,7 @@ wayl_reload_cursor_theme(struct wayland *wayl, const struct terminal *term)
         return false;
     }
 
-    return wayl_cursor_set(wayl, term);
+    return render_xcursor_set(term);
 }
 
 struct terminal *

wayland.h

@@ -152,6 +152,9 @@ struct wayland {
         int size;
         char *theme_name;
         const char *xcursor;
+
+        const struct terminal *pending_terminal;
+        struct wl_callback *xcursor_callback;
     } pointer;
 
     struct {
@@ -184,8 +187,5 @@ void wayl_roundtrip(struct wayland *wayl);
 struct terminal *wayl_terminal_from_surface(
     struct wayland *wayl, struct wl_surface *surface);
 
-/* TODO: pass something other than 'term'? Need scale... */
-bool wayl_cursor_set(struct wayland *wayl, const struct terminal *term);
-
 struct wl_window *wayl_win_init(struct terminal *term);
 void wayl_win_destroy(struct wl_window *win);