[shib_auth] Authentication errors under load

[Nate Klingenstein]

A site that uses Shibboleth, Drupal, and shib_auth under heavy load is experiencing occasional single authentication failures.  The error message is generated by shib_auth, which makes me think there may be a concurrency issue in shib_auth.

The exact error condition is one of these two(same error message, can't modify code in place to disambiguate):

shib_auth.module:

 424     else shib_auth_error('Couldn\'t login user: ' . $authmap_username['name     ']);
 425   }
 426   else shib_auth_error('Couldn\'t login user: ' . $authmap_username['name']     ); 

Can you think of anything that would cause this?