When kernels are culled, the kernel is terminated in the background,
unbeknownst to the session management. As a result, invalid sessions
can be produced that appear to exist, yet cannot produce a model from
the persisted row due to the associated kernel no longer being active.
Prior to this change, these sessions, when encountered via a subsequent
call to `get_session()`, would be deleted and a KeyError would be raised.
This change updates the existence check to tolerate those kinds of sessions.
It removes such sessions (as would happen previously), but rather than
raise a KeyError when attempting to convert the row to a dictionary,
it logs a warning and returns None, which then allows `session_exists()`
to return False since the session was removed (as was ultimately the
case previously).
Calls to `get_session()` remain just as before and have the potential
to raise `KeyError` in such cases. The difference now being that the
`KeyError` is accompanied by a message indicating the cause.
Fixes#4209
Kevin Bates
0e63ebb1db