backend/drm: don't destroy previous DRM master blobs

On startup, we fetch the previous MODE_ID blob ID so that
compositors can keep using the previous mode if they want to.
However, that blob doesn't belong to us, it belongs to the
previous DRM master. As a result, we get an error when trying to
destroy it.

Fix this by tracking whether the blob belongs to us or not.

Closes: https://gitlab.freedesktop.org/wlroots/wlroots/-/issues/3811

backend/drm/atomic.c

@@ -379,6 +379,10 @@ static bool atomic_crtc_commit(struct wlr_drm_connector *conn,
 	atomic_finish(&atom);
 
 	if (ok && !test_only) {
+		if (!crtc->own_mode_id) {
+			crtc->mode_id = 0; // don't try to delete previous master's blobs
+		}
+		crtc->own_mode_id = true;
 		commit_blob(drm, &crtc->mode_id, mode_id);
 		commit_blob(drm, &crtc->gamma_lut, gamma_lut);
 

backend/drm/drm.c

@@ -329,7 +329,7 @@ void finish_drm_resources(struct wlr_drm_backend *drm) {
 	for (size_t i = 0; i < drm->num_crtcs; ++i) {
 		struct wlr_drm_crtc *crtc = &drm->crtcs[i];
 
-		if (crtc->mode_id) {
+		if (crtc->mode_id && crtc->own_mode_id) {
 			drmModeDestroyPropertyBlob(drm->fd, crtc->mode_id);
 		}
 		if (crtc->gamma_lut) {
@@ -1483,6 +1483,7 @@ static bool connect_drm_connector(struct wlr_drm_connector *wlr_conn,
 			get_drm_prop(drm->fd, wlr_conn->crtc->id,
 				wlr_conn->crtc->props.mode_id, &mode_id);
 
+			wlr_conn->crtc->own_mode_id = false;
 			wlr_conn->crtc->mode_id = mode_id;
 			wlr_conn->refresh = calculate_refresh_rate(current_modeinfo);
 		}

backend/drm/libliftoff.c

@@ -475,6 +475,10 @@ out:
 	drmModeAtomicFree(req);
 
 	if (ok && !test_only) {
+		if (!crtc->own_mode_id) {
+			crtc->mode_id = 0; // don't try to delete previous master's blobs
+		}
+		crtc->own_mode_id = true;
 		commit_blob(drm, &crtc->mode_id, mode_id);
 		commit_blob(drm, &crtc->gamma_lut, gamma_lut);
 

include/backend/drm/drm.h

@@ -61,6 +61,7 @@ struct wlr_drm_crtc {
 	struct wl_list layers; // wlr_drm_layer.link
 
 	// Atomic modesetting only
+	bool own_mode_id;
 	uint32_t mode_id;
 	uint32_t gamma_lut;