I run a homeserver whose logs show a high number of incoming empty
presence EDUs originating from the user agent "Conduwuit/0.4.4". They
arrive at a rate of about 2 queries per second per Conduwuit server.
The empty EDUs all look the same, only with `origin_server_ts`
increasing:
```
{"origin":"example.com","origin_server_ts":1720266475601,"edus":[{"edu_type":"m.presence","content":{"push":[]}}]}
```
These updates are unnecessary because they don't do anything. They
only increase network traffic and CPU usage on both sides.
After this commit, the empty presence updates are no longer inserted
into the outgoing event queue.
this `real_users_cache` cache seems weird, and i have no idea what
prompted its creation upstream. perhaps they did this because
sqlite was very slow and their rocksdb setup is very poor, so
a "solution" was to stick member counts in memory.
slow iterators, scanning, etc do not apply to conduwuit where
our rocksdb is extremely tuned, and i seriously doubt something
like this would have any real world net-positive performance impact.
also for some reason, there is suspicious logic where we
overwrite the entire push target collection.
both of these things could be a potential cause for receiving
notifications in rooms we've left.
Signed-off-by: strawberry <strawberry@puppygock.gay>
