Annotations for state_deltas.py

[DMRobertson]

I was sad that I couldn't do better for
_curr_state_delta_stream_cache. At least it's explicitly called out in
a comment with #TODO.

[clokep]

See #11322 instead. 😄

[reivilibre]

Hacky abstract thing is up to you, everything else LGTM (though the mypy.ini change is soon going to be obsolete one way or the other ...)

[reivilibre]

Can we make this an abstract class perhaps, with this being an abstract.. field... if that exists ...

It doesn't seem like it does, but I would be tempted to try an abstract property:

    @property
    @abstractmethod
    def _curr_state_delta_stream_cache(self) -> StreamChangeCache:
        ...

Not super clean, but curious if that'd do the trick (and if it's allowed to just overwrite it with a field later)

[DMRobertson]

I got

synapse/app/generic_worker.py:222: error: Definition of "_curr_state_delta_stream_cache" in base class "StateDeltasStore" is incompatible with definition in base class "SlavedEventStore"  [misc]

after

class StateDeltasStore(SQLBaseStore, abc.ABC):
    @property
    @abc.abstractmethod
    def _curr_state_delta_stream_cache(self) -> StreamChangeCache:
        """This class must be mixed in with a child class which provides the
        following attribute."""
        ...

[DMRobertson]

You might be interested in the discussion here: python/mypy#4426