backend/wayland: re-use wl_buffers

Instead of re-importing a buffer each time we submit a new frame, re-use
the wl_buffer objects if possible.

backend/wayland/backend.c

@@ -210,6 +210,11 @@ static void backend_destroy(struct wlr_backend *backend) {
 
 	wlr_drm_format_set_finish(&wl->linux_dmabuf_v1_formats);
 
+	struct wlr_wl_buffer *buffer, *tmp_buffer;
+	wl_list_for_each_safe(buffer, tmp_buffer, &wl->buffers, link) {
+		destroy_wl_buffer(buffer);
+	}
+
 	destroy_wl_seats(wl);
 	if (wl->zxdg_decoration_manager_v1) {
 		zxdg_decoration_manager_v1_destroy(wl->zxdg_decoration_manager_v1);
@@ -270,6 +275,7 @@ struct wlr_backend *wlr_wl_backend_create(struct wl_display *display,
 	wl_list_init(&wl->devices);
 	wl_list_init(&wl->outputs);
 	wl_list_init(&wl->seats);
+	wl_list_init(&wl->buffers);
 
 	wl->remote_display = wl_display_connect(remote);
 	if (!wl->remote_display) {

backend/wayland/output.c

@@ -133,24 +133,33 @@ static bool output_attach_render(struct wlr_output *wlr_output,
 	return true;
 }
 
-static void destroy_wl_buffer(struct wlr_wl_buffer *buffer) {
+void destroy_wl_buffer(struct wlr_wl_buffer *buffer) {
 	if (buffer == NULL) {
 		return;
 	}
+	wl_list_remove(&buffer->buffer_destroy.link);
+	wl_list_remove(&buffer->link);
 	wl_buffer_destroy(buffer->wl_buffer);
-	wlr_buffer_unlock(buffer->buffer);
 	free(buffer);
 }
 
 static void buffer_handle_release(void *data, struct wl_buffer *wl_buffer) {
 	struct wlr_wl_buffer *buffer = data;
-	destroy_wl_buffer(buffer);
+	buffer->released = true;
+	wlr_buffer_unlock(buffer->buffer); // might free buffer
 }
 
 static const struct wl_buffer_listener buffer_listener = {
 	.release = buffer_handle_release,
 };
 
+static void buffer_handle_buffer_destroy(struct wl_listener *listener,
+		void *data) {
+	struct wlr_wl_buffer *buffer =
+		wl_container_of(listener, buffer, buffer_destroy);
+	destroy_wl_buffer(buffer);
+}
+
 static bool test_buffer(struct wlr_wl_backend *wl,
 		struct wlr_buffer *wlr_buffer) {
 	struct wlr_dmabuf_attributes attribs;
@@ -207,12 +216,33 @@ static struct wlr_wl_buffer *create_wl_buffer(struct wlr_wl_backend *wl,
 	}
 	buffer->wl_buffer = wl_buffer;
 	buffer->buffer = wlr_buffer_lock(wlr_buffer);
+	wl_list_insert(&wl->buffers, &buffer->link);
 
 	wl_buffer_add_listener(wl_buffer, &buffer_listener, buffer);
 
+	buffer->buffer_destroy.notify = buffer_handle_buffer_destroy;
+	wl_signal_add(&wlr_buffer->events.destroy, &buffer->buffer_destroy);
+
 	return buffer;
 }
 
+static struct wlr_wl_buffer *get_or_create_wl_buffer(struct wlr_wl_backend *wl,
+		struct wlr_buffer *wlr_buffer) {
+	struct wlr_wl_buffer *buffer;
+	wl_list_for_each(buffer, &wl->buffers, link) {
+		// We can only re-use a wlr_wl_buffer if the parent compositor has
+		// released it, because wl_buffer.release is per-wl_buffer, not per
+		// wl_surface.commit.
+		if (buffer->buffer == wlr_buffer && buffer->released) {
+			buffer->released = false;
+			wlr_buffer_lock(buffer->buffer);
+			return buffer;
+		}
+	}
+
+	return create_wl_buffer(wl, wlr_buffer);
+}
+
 static bool output_test(struct wlr_output *wlr_output) {
 	struct wlr_wl_output *output =
 		get_wl_output_from_output(wlr_output);
@@ -287,7 +317,7 @@ static bool output_commit(struct wlr_output *wlr_output) {
 		}
 
 		struct wlr_wl_buffer *buffer =
-			create_wl_buffer(output->backend, wlr_buffer);
+			get_or_create_wl_buffer(output->backend, wlr_buffer);
 		if (buffer == NULL) {
 			return false;
 		}

include/backend/wayland.h

@@ -25,9 +25,11 @@ struct wlr_wl_backend {
 	struct wlr_renderer *renderer;
 	struct wlr_drm_format *format;
 	struct wlr_allocator *allocator;
+	struct wl_list buffers; // wlr_wl_buffer.link
 	size_t requested_outputs;
 	size_t last_output_num;
 	struct wl_listener local_display_destroy;
+
 	/* remote state */
 	struct wl_display *remote_display;
 	struct wl_event_source *remote_display_src;
@@ -47,6 +49,9 @@ struct wlr_wl_backend {
 struct wlr_wl_buffer {
 	struct wlr_buffer *buffer;
 	struct wl_buffer *wl_buffer;
+	bool released;
+	struct wl_list link; // wlr_wl_backend.buffers
+	struct wl_listener buffer_destroy;
 };
 
 struct wlr_wl_presentation_feedback {
@@ -130,6 +135,7 @@ struct wlr_wl_input_device *create_wl_input_device(
 	struct wlr_wl_seat *seat, enum wlr_input_device_type type);
 bool create_wl_seat(struct wl_seat *wl_seat, struct wlr_wl_backend *wl);
 void destroy_wl_seats(struct wlr_wl_backend *wl);
+void destroy_wl_buffer(struct wlr_wl_buffer *buffer);
 
 extern const struct wl_seat_listener seat_listener;