User Interactive auth sessions are only stored in memory, so if you restart server mid-auth, auth will fail

[ara4n]

This is particularly unpleasant given restarting the server whilst someone is trying to validate 3PIDs during registration will cause their registration to fail.

This is based on looking at

synapse/synapse/handlers/auth.py

Line 832 in da1010c

def _save_session(self, session):

[clokep]

This was fixed by #7302.