Source

File: wp-includes/class-wp-customize-manager.php

public function refresh_changeset_lock( $changeset_post_id ) {
		if ( ! $changeset_post_id ) {
			return;
		}
		$lock = get_post_meta( $changeset_post_id, '_edit_lock', true );
		$lock = explode( ':', $lock );

		if ( $lock && ! empty( $lock[1] ) ) {
			$user_id         = intval( $lock[1] );
			$current_user_id = get_current_user_id();
			if ( $user_id === $current_user_id ) {
				$lock = sprintf( '%s:%s', time(), $user_id );
				update_post_meta( $changeset_post_id, '_edit_lock', $lock );
			}
		}
	}