List of changes:
* New OOP API using PHP namespace \core\session\.
* All handlers now update the sessions table consistently.
* Experimental DB session support in Oracle.
* Full support for session file handler (filesystem locking required).
* New option for alternative session directory.
* Official memcached session handler support.
* Workaround for memcached version with non-functional gc.
* Improved security - forced session id regeneration.
* Improved compatibility with recent PHP releases.
* Fixed borked CSS during install in debug mode.
* Switched to file based sessions in new installs.
* DB session setting disappears if DB does not support sessions.
* DB session setting disappears if session handler specified in config.php.
* Fast purging of sessions used in request only.
* No legacy distinction - file, database and memcached support the same functionality.
* Session handler name included in performance info.
* Fixed user_loggedin and user_loggedout event triggering.
* Other minor bugfixing and improvements.
* Fixed database session segfault if MUC disposed before $DB.
Limitations:
* Session access time is now updated right after session start.
* Support for $CFG->sessionlockloggedinonly was removed.
* First request does not update userid in sessions table.
* The timeouts may break badly if server hosting forces PHP.ini session settings.
* The session GC is a lot slower, we do not rely on external session timeouts.
* There cannot be any hooks triggered at the session write time.
* File and memcached handlers do not support session lock acquire timeouts.
* Some low level PHP session functions can not be used directly in Moodle code.
Refactoring and improvements of the accesslib.php library including prevention of access for not-logged-in users when forcelogin enabled, improved context caching, OOP refactoring of contexts, fixed context loading, deduplication of role definitions in user sessions, installation improvements, decoupling of enrolment checking from capability loading, added detection of deleted and non-existent users in has_capability(), new function accesslib test, auth and enrol upgrade notes.
More details are available in tracker subtasks.
Juan Leyva