output: introduce wlr_output_lock_attach_render

This allows screen shooters and screen grabbers to ensure rendering will be
used instead of direct scan-out.

include/wlr/types/wlr_output.h

@@ -137,6 +137,8 @@ struct wlr_output {
 	struct wl_event_source *idle_frame;
 	struct wl_event_source *idle_done;
 
+	int attach_render_locks; // number of locks forcing rendering
+
 	struct wl_list cursors; // wlr_output_cursor::link
 	struct wlr_output_cursor *hardware_cursor;
 	int software_cursor_locks; // number of locks forcing software cursors
@@ -287,6 +289,13 @@ bool wlr_output_set_gamma(struct wlr_output *output, size_t size,
 bool wlr_output_export_dmabuf(struct wlr_output *output,
 	struct wlr_dmabuf_attributes *attribs);
 struct wlr_output *wlr_output_from_resource(struct wl_resource *resource);
+/**
+ * Locks the output to only use rendering instead of direct scan-out. This is
+ * useful if direct scan-out needs to be temporarily disabled (e.g. during
+ * screen capture). There must be as many unlocks as there have been locks to
+ * restore the original state. There should never be an unlock before a lock.
+ */
+void wlr_output_lock_attach_render(struct wlr_output *output, bool lock);
 /**
  * Locks the output to only use software cursors instead of hardware cursors.
  * This is useful if hardware cursors need to be temporarily disabled (e.g.

types/wlr_export_dmabuf_v1.c

@@ -34,6 +34,7 @@ static void frame_destroy(struct wlr_export_dmabuf_frame_v1 *frame) {
 	if (frame == NULL) {
 		return;
 	}
+	wlr_output_lock_attach_render(frame->output, false);
 	if (frame->cursor_locked) {
 		wlr_output_lock_software_cursors(frame->output, false);
 	}
@@ -126,6 +127,7 @@ static void manager_handle_capture_output(struct wl_client *client,
 		return;
 	}
 
+	wlr_output_lock_attach_render(frame->output, true);
 	if (overlay_cursor) {
 		wlr_output_lock_software_cursors(frame->output, true);
 		frame->cursor_locked = true;

types/wlr_output.c

@@ -497,6 +497,9 @@ bool wlr_output_attach_buffer(struct wlr_output *output,
 	if (!output->impl->attach_buffer) {
 		return false;
 	}
+	if (output->attach_render_locks > 0) {
+		return false;
+	}
 
 	// If the output has at least one software cursor, refuse to attach the
 	// buffer
@@ -614,6 +617,18 @@ struct wlr_output *wlr_output_from_resource(struct wl_resource *resource) {
 	return wl_resource_get_user_data(resource);
 }
 
+void wlr_output_lock_attach_render(struct wlr_output *output, bool lock) {
+	if (lock) {
+		++output->attach_render_locks;
+	} else {
+		assert(output->attach_render_locks > 0);
+		--output->attach_render_locks;
+	}
+	wlr_log(WLR_DEBUG, "%s direct scan-out on output '%s' (locks: %d)",
+		lock ? "Disabling" : "Enabling", output->name,
+		output->attach_render_locks);
+}
+
 static void output_cursor_damage_whole(struct wlr_output_cursor *cursor);
 
 void wlr_output_lock_software_cursors(struct wlr_output *output, bool lock) {

types/wlr_screencopy_v1.c

@@ -23,6 +23,7 @@ static void frame_destroy(struct wlr_screencopy_frame_v1 *frame) {
 	if (frame == NULL) {
 		return;
 	}
+	wlr_output_lock_attach_render(frame->output, false);
 	if (frame->cursor_locked) {
 		wlr_output_lock_software_cursors(frame->output, false);
 	}
@@ -143,6 +144,7 @@ static void frame_handle_copy(struct wl_client *client,
 	output->needs_frame = true;
 	wlr_output_schedule_frame(output);
 
+	wlr_output_lock_attach_render(output, true);
 	if (frame->overlay_cursor) {
 		wlr_output_lock_software_cursors(output, true);
 		frame->cursor_locked = true;