Checks for errors when using cookie-based authentication.

WordPress’ built-in cookie authentication is always active
for logged in users. However, the API has to check nonces
for each request to ensure users are not vulnerable to CSRF.

function rest_cookie_check_errors( $result ) {
	if ( ! empty( $result ) ) {
		return $result;
	}

	global $wp_rest_auth_cookie;

	/*
	 * Is cookie authentication being used? (If we get an auth
	 * error, but we're still logged in, another authentication
	 * must have been used).
	 */
	if ( true !== $wp_rest_auth_cookie && is_user_logged_in() ) {
		return $result;
	}

	// Determine if there is a nonce.
	$nonce = null;

	if ( isset( $_REQUEST['_wpnonce'] ) ) {
		$nonce = $_REQUEST['_wpnonce'];
	} elseif ( isset( $_SERVER['HTTP_X_WP_NONCE'] ) ) {
		$nonce = $_SERVER['HTTP_X_WP_NONCE'];
	}

	if ( null === $nonce ) {
		// No nonce at all, so act as if it's an unauthenticated request.
		wp_set_current_user( 0 );
		return true;
	}

	// Check the nonce.
	$result = wp_verify_nonce( $nonce, 'wp_rest' );

	if ( ! $result ) {
		return new WP_Error( 'rest_cookie_invalid_nonce', __( 'Cookie check failed' ), array( 'status' => 403 ) );
	}

	// Send a refreshed nonce in header.
	rest_get_server()->send_header( 'X-WP-Nonce', wp_create_nonce( 'wp_rest' ) );

	return true;
}