wlr_scene: Fix texture reuse

Add private interface to ignore a buffer that's locking a client_buffer
for damage tracking. This should eventually be replaced by wlr_raster.

include/wlr/types/wlr_buffer.h

@@ -151,6 +151,8 @@ struct wlr_client_buffer {
 	// private state
 
 	struct wl_listener source_destroy;
+
+	size_t n_ignore_locks;
 };
 
 /**

types/buffer/client.c

@@ -84,7 +84,7 @@ struct wlr_client_buffer *wlr_client_buffer_create(struct wlr_buffer *buffer,
 
 bool wlr_client_buffer_apply_damage(struct wlr_client_buffer *client_buffer,
 		struct wlr_buffer *next, pixman_region32_t *damage) {
-	if (client_buffer->base.n_locks > 1) {
+	if (client_buffer->base.n_locks - client_buffer->n_ignore_locks > 1) {
 		// Someone else still has a reference to the buffer
 		return false;
 	}

types/scene/surface.c

@@ -1,3 +1,4 @@
+#include <assert.h>
 #include <stdlib.h>
 #include <wlr/types/wlr_scene.h>
 #include <wlr/types/wlr_presentation_time.h>
@@ -55,6 +56,28 @@ static void scene_surface_handle_surface_destroy(
 	wlr_scene_node_destroy(&surface->buffer->node);
 }
 
+// This is used for wlr_scene where it unconditionally locks buffers preventing
+// reuse of the existing texture for shm clients. With the usage pattern of
+// wlr_scene surface handling, we can mark its locked buffer as safe
+// for mutation.
+static void client_buffer_mark_next_can_damage(struct wlr_client_buffer *buffer) {
+	buffer->n_ignore_locks++;
+}
+
+static void scene_buffer_unmark_client_buffer(struct wlr_scene_buffer *scene_buffer) {
+	if (!scene_buffer->buffer) {
+		return;
+	}
+
+	struct wlr_client_buffer *buffer = wlr_client_buffer_get(scene_buffer->buffer);
+	if (!buffer) {
+		return;
+	}
+
+	assert(buffer->n_ignore_locks > 0);
+	buffer->n_ignore_locks--;
+}
+
 static void set_buffer_with_surface_state(struct wlr_scene_buffer *scene_buffer,
 		struct wlr_surface *surface) {
 	struct wlr_surface_state *state = &surface->current;
@@ -68,7 +91,11 @@ static void set_buffer_with_surface_state(struct wlr_scene_buffer *scene_buffer,
 	wlr_scene_buffer_set_dest_size(scene_buffer, state->width, state->height);
 	wlr_scene_buffer_set_transform(scene_buffer, state->transform);
 
+	scene_buffer_unmark_client_buffer(scene_buffer);
+
 	if (surface->buffer) {
+		client_buffer_mark_next_can_damage(surface->buffer);
+
 		wlr_scene_buffer_set_buffer_with_damage(scene_buffer,
 			&surface->buffer->base, &surface->buffer_damage);
 	} else {
@@ -109,6 +136,8 @@ static bool scene_buffer_point_accepts_input(struct wlr_scene_buffer *scene_buff
 static void surface_addon_destroy(struct wlr_addon *addon) {
 	struct wlr_scene_surface *surface = wl_container_of(addon, surface, addon);
 
+	scene_buffer_unmark_client_buffer(surface->buffer);
+
 	wlr_addon_finish(&surface->addon);
 
 	wl_list_remove(&surface->output_enter.link);