Source

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

public function set_changeset_lock( $changeset_post_id, $take_over = false ) {
		if ( $changeset_post_id ) {
			$can_override = ! (bool) get_post_meta( $changeset_post_id, '_edit_lock', true );

			if ( $take_over ) {
				$can_override = true;
			}

			if ( $can_override ) {
				$lock = sprintf( '%s:%s', time(), get_current_user_id() );
				update_post_meta( $changeset_post_id, '_edit_lock', $lock );
			} else {
				$this->refresh_changeset_lock( $changeset_post_id );
			}
		}
	}