Traceback (most recent call last):
  File "/usr/share/software-center/update-software-center-agent", line 70, in <module>
    db = xapian.WritableDatabase(pathname, xapian.DB_CREATE_OR_OVERWRITE)
  File "/usr/lib/python2.6/dist-packages/xapian.py", line 3195, in __init__
    _xapian.WritableDatabase_swiginit(self,_xapian.new_WritableDatabase(*args))
DatabaseLockError: Unable to acquire database write lock on /home/username/.cache/software-center/software-center-agent.db.tmp: already locked