Synapse will hide expired events from clients even if message retention policy support isn't enabled

[babolivier]

If retention.enabled is set to False in the configuration file, and a m.room.retention event is inserted into the state of a room, instead of only storing it (and ignoring it otherwise) Synapse will hide "expired" events from clients once their age gets higher than the retention policy's max_lifetime.

Practically, this means this happens with any Synapse, rather than only a Synapse with the feature enabled:

(clear cache + reload was needed because Element doesn't implement this feature yet)

The impact of this bug isn't too high since we don't actually delete any event, however from a user perspective it strongly implies their messages get deleted when they don't which can be pretty confusing.

The issue lies within filter_events_for_client, which isn't aware of whether the feature is enabled whereas it should be. We would also need to change the 30 or so calls to this function throughout the Synapse codebase to give it that info (given we can't blindly assume it with a default value since it depends on the config, not on the context of the call).

[babolivier]

Note that synapse-dinsic doesn't have this issue, because if the feature isn't enabled it returns a retention policy with None for both min_lifetime and max_lifetime: https://github.com/matrix-org/synapse-dinsic/blob/4737a7fd11060935279c0ed12c2eb27ab830d03d/synapse/storage/databases/main/room.py#L675-L679

This works but is a bit hacky, I'd be more in favour of giving filter_events_for_client the homeserver's config so it can check for itself (and would prevent similar issues from happening with other features).