Memcached sessions lose lock early

Memcached sessions will loose their lock after max_execution_time seconds, regardless of if Moodle later extends the time using set_time_limit(). This is because the memcached session handler uses max_execution_time to set the max lock duration at session_start().

This means that long running scripts (like backups and restores) won't hold their session lock for long (only 30 seconds in a default php install).

Now, because of this, there is a potential solution, you could do something like:

public static function start() {

    $default = ini_get('max_execution_time');

    set_time_limit($CFG->session_memcached_lock_expire);

    parent::start();

    set_time_limit($default);

}

I fully acknowledge that this is probably horrible in many ways.

You can see the memcached session locking logic at:
https://github.com/php-memcached-dev/php-memcached/blob/master/php_memcached_session.c