FileSession Locking and Priority Error in CherryPy

Recently I ran into the following error when using file-based sessions in CherryPy v3.8:

AssertionError: The session load without being locked.
Check your tools' priority levels.

I was able to fix this with the following config setting:

'tools.sessions.locking': 'early'

Why is this? According to the CherryPy documentation on session locking, the default session locking setting is 'implicit'.

In _cptools.py, of the CherryPy session code, we see:

locking = conf.pop('locking', 'implicit')
if locking == 'implicit':
    hooks.attach('before_handler', self._lock_session)
elif locking == 'early':
    # Lock before the request body (but after _sessions.init runs!)
    hooks.attach('before_request_body', self._lock_session,
                 priority=60)

So, my problem was that I had another CherryPy tool that used session data, but the session had not yet been locked. Using early instead of implicit causes the session to be locked at the before_request_body hook point.