For an installation using PHP 7 and memcached (3.x) for sessions, MDL-53947 mis-configures the the memcached.sess_lock_wait_max value (default 2 sec.) to be acquiretimeout * 1000 (effectively 120 sec.).

Unfortunately, the docs are not .... what they could be, and were misinterpreted.

The names for the memcached configs were not simply altered; the entire scheme of polling for a session lock was changed, and the two _max and _min configs have entirely different uses. There is no longer a config that correlates directly to how long you wait for a session lock before giving up.

Reviewing the code change, the _min and _max values determine the initial (_min) and subsequent (_max) values passed to usleep()--with a twist. Between each attempt, the argument is adjusted to be the minimum of the _max value or 2 X the last used value.

Using the values from config-dist.php in conjunction with the MDL-53947 code modification results in a max lock timeout of 31 seconds (rather than the 120 you expect); this reduced timeout value (not matching the acquiretimeout value) breaks MDL-53713--for PHP7 + memcached sessions.