[shib_auth] class registry in drupal 7 conflicting with shib sessions

[Mike Cammilleri]

We are using the shibboleth module with D7 which works, most of the 
time. Users can click on the shibboleth login link, be directed to the 
Idp, authenticated successfully and returned to D7, at which point 
.htaccess allows them in with

AuthType Shibboleth
ShibRequireSession Off
require shibboleth
ShibUseHeaders On

About an hour or so goes by and all of a sudden D7 stops allowing the 
shib-authenticated user in. The user can still authenticate to the shib 
Idp, but when returned to the D7 site, drupal just behaves as if they 
never logged in at all. No error page, no 404 not found, no white page 
of death, just back to the D7 site and the login button for shib is 
still sitting there as if they never clicked on it.

In order to fix this, all I need to do is flush the Class Registry from 
the Flush All Caches menu. Simply flushing that cache corrects the 
behavior and once again D7 will allow users to log in via shib. Until 
another hour goes by and D7 stops allowing shib users, and I need to 
flush the Class Registry again.

Has anyone experienced this? What is in the Class Registry that could be 
causing D7 to think users aren't logged in? (Local drupal accounts work 
fine by the way). I turned off all caching in the Performance section, 
except for block caching which is greyed out because I'm using the 
Access Control module. This still has not fixed the problem.

Any hints would be greatly appreciated.