backend/drm: emit new_output after scanning connectors

This prevents receiving modesetting requests from the compositor
while we don't have the whole picture (ie. while we haven't yet
scanned all connectors).

This also makes connectors without CRTCs disabled (they can't be
enabled yet even if some CRTCs are free'd -- this is future work).

backend/drm/drm.c

@@ -564,7 +564,7 @@ static bool drm_connector_set_mode(struct wlr_output *output,
 	struct wlr_drm_crtc *crtc = conn->crtc;
 	if (!crtc) {
 		wlr_log(WLR_ERROR, "Unable to match %s with a CRTC", conn->output.name);
-		goto error_conn;
+		return false;
 	}
 	wlr_log(WLR_DEBUG, "%s: crtc=%td ovr=%td pri=%td cur=%td", conn->output.name,
 		crtc - drm->crtcs,
@@ -864,11 +864,13 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 		return;
 	}
 
-	size_t seen_len = wl_list_length(&drm->outputs);
+	size_t drm_outputs_len = wl_list_length(&drm->outputs);
 	// +1 so length can never be 0, which is undefined behaviour.
 	// Last element isn't used.
-	bool seen[seen_len + 1];
-	memset(seen, 0, sizeof(seen));
+	bool seen[drm_outputs_len + 1];
+	memset(seen, false, sizeof(seen));
+	size_t new_outputs_len = 0;
+	struct wlr_drm_connector *new_outputs[drm_outputs_len + 1];
 
 	for (int i = 0; i < res->count_connectors; ++i) {
 		drmModeConnector *drm_conn = drmModeGetConnector(drm->fd,
@@ -974,13 +976,10 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 				wl_list_insert(&wlr_conn->output.modes, &mode->wlr_mode.link);
 			}
 
-			wlr_output_update_enabled(&wlr_conn->output, true);
+			wlr_output_update_enabled(&wlr_conn->output, wlr_conn->crtc != NULL);
 
 			wlr_conn->state = WLR_DRM_CONN_NEEDS_MODESET;
-			wlr_log(WLR_INFO, "Sending modesetting signal for '%s'",
-				wlr_conn->output.name);
-			wlr_signal_emit_safe(&drm->backend.events.new_output,
-				&wlr_conn->output);
+			new_outputs[new_outputs_len++] = wlr_conn;
 		} else if (wlr_conn->state == WLR_DRM_CONN_CONNECTED &&
 				drm_conn->connection != DRM_MODE_CONNECTED) {
 			wlr_log(WLR_INFO, "'%s' disconnected", wlr_conn->output.name);
@@ -999,7 +998,7 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 	size_t index = wl_list_length(&drm->outputs);
 	wl_list_for_each_safe(conn, tmp_conn, &drm->outputs, link) {
 		index--;
-		if (index >= seen_len || seen[index]) {
+		if (index >= drm_outputs_len || seen[index]) {
 			continue;
 		}
 
@@ -1011,6 +1010,15 @@ void scan_drm_connectors(struct wlr_drm_backend *drm) {
 		wl_list_remove(&conn->link);
 		free(conn);
 	}
+
+	for (size_t i = 0; i < new_outputs_len; ++i) {
+		struct wlr_drm_connector *conn = new_outputs[i];
+
+		wlr_log(WLR_INFO, "Requesting modeset for '%s'",
+			conn->output.name);
+		wlr_signal_emit_safe(&drm->backend.events.new_output,
+			&conn->output);
+	}
 }
 
 static void page_flip_handler(int fd, unsigned seq,