When I upgrade 1.8 dev (around 2007/01/22) to 1.9 dev, upgrading fails.
The cause seems to be in group/db/upgrade.php.

The problems are:

• When I need to delete group table for upgrading, I cannot continue to upgrade after the message shows up like bellow:
  'Upgrading process is already running in this session.' (I forget correct sentence though).
• The record of group module in 'mdl_log_display' table is not correctly installed.
  (Error message indicates that the record already exists).
• When group table is empty, it is treated as an errors even though just the table is empty.

I attached my patch fixing this problem even though I am not sure if my fix is correct or not.