output-damage: stop relying on output pending state on commit

References: https://github.com/swaywm/wlroots/issues/2098

include/wlr/types/wlr_output_damage.h

@@ -42,6 +42,8 @@ struct wlr_output_damage {
 	pixman_region32_t previous[WLR_OUTPUT_DAMAGE_PREVIOUS_LEN];
 	size_t previous_idx;
 
+	enum wlr_output_state_buffer_type pending_buffer_type;
+
 	struct {
 		struct wl_signal frame;
 		struct wl_signal destroy;
@@ -52,6 +54,7 @@ struct wlr_output_damage {
 	struct wl_listener output_needs_frame;
 	struct wl_listener output_damage;
 	struct wl_listener output_frame;
+	struct wl_listener output_precommit;
 	struct wl_listener output_commit;
 };
 

types/wlr_output_damage.c

@@ -44,6 +44,18 @@ static void output_handle_frame(struct wl_listener *listener, void *data) {
 	wlr_signal_emit_safe(&output_damage->events.frame, output_damage);
 }
 
+static void output_handle_precommit(struct wl_listener *listener, void *data) {
+	struct wlr_output_damage *output_damage =
+		wl_container_of(listener, output_damage, output_precommit);
+	struct wlr_output *output = output_damage->output;
+
+	if (output->pending.committed & WLR_OUTPUT_STATE_BUFFER) {
+		// TODO: find a better way to access this info without a precommit
+		// handler
+		output_damage->pending_buffer_type = output->pending.buffer_type;
+	}
+}
+
 static void output_handle_commit(struct wl_listener *listener, void *data) {
 	struct wlr_output_damage *output_damage =
 		wl_container_of(listener, output_damage, output_commit);
@@ -58,7 +70,7 @@ static void output_handle_commit(struct wl_listener *listener, void *data) {
 	}
 
 	pixman_region32_t *prev;
-	switch (output_damage->output->pending.buffer_type) {
+	switch (output_damage->pending_buffer_type) {
 	case WLR_OUTPUT_STATE_BUFFER_RENDER:
 		// render-buffers have been swapped, rotate the damage
 
@@ -106,6 +118,8 @@ struct wlr_output_damage *wlr_output_damage_create(struct wlr_output *output) {
 	output_damage->output_damage.notify = output_handle_damage;
 	wl_signal_add(&output->events.frame, &output_damage->output_frame);
 	output_damage->output_frame.notify = output_handle_frame;
+	wl_signal_add(&output->events.precommit, &output_damage->output_precommit);
+	output_damage->output_precommit.notify = output_handle_precommit;
 	wl_signal_add(&output->events.commit, &output_damage->output_commit);
 	output_damage->output_commit.notify = output_handle_commit;
 
@@ -122,6 +136,7 @@ void wlr_output_damage_destroy(struct wlr_output_damage *output_damage) {
 	wl_list_remove(&output_damage->output_needs_frame.link);
 	wl_list_remove(&output_damage->output_damage.link);
 	wl_list_remove(&output_damage->output_frame.link);
+	wl_list_remove(&output_damage->output_precommit.link);
 	wl_list_remove(&output_damage->output_commit.link);
 	pixman_region32_fini(&output_damage->current);
 	for (size_t i = 0; i < WLR_OUTPUT_DAMAGE_PREVIOUS_LEN; ++i) {