Previously, we were returning redundant member count updates or encrypted
device updates from the /sync endpoint in some cases. The extra member
count updates are spec-compliant, but unnecessary, while the extra
encrypted device updates violate the spec.
The refactor necessary to fix this bug is also necessary to support
filtering on state events in sync.
Details:
Joined room incremental sync needs to examine state events for four
purposes:
1. determining whether we need to return an update to room member counts
2. determining the set of left/joined devices for encrypted rooms
(returned in `device_lists`)
3. returning state events to the client (in `rooms.joined.*.state`)
4. tracking which member events we have sent to the client, so they can
be omitted on future requests when lazy-loading is enabled.
The state events that we need to examine for the first two cases is member
events in the delta between `since` and the end of `timeline`. For the
second two cases, we need the delta between `since` and the start of
`timeline`, plus contextual member events for any senders that occur in
`timeline`. The second list is subject to filtering, while the first is
not.
Before this change, we were using the same set of state events that we are
returning to the client (cases 3/4) to do the analysis for cases 1/2.
In a compliant implementation, this would result in us missing some
relevant member events in 1/2 in addition to seeing redundant member
events. In current conduwuit this is not the case because the set of
events that we return to the client is always a superset of the set that
is needed for cases 1/2. This is because we don't support filtering, and
we have an existing bug[1] where we are returning the delta between
`since` and the end of `timeline` rather than the start.
[1]: https://github.com/girlbossceo/conduwuit/issues/361
Fixing this is necessary to implement filtering because otherwise
we would start missing some member events for member count or encrypted
device updates if the relevant member events are rejected by the filter.
This would be much worse than our current behavior.
This cache can serve invalid responses, and has an extremely low hit
rate.
It serves invalid responses because because it's only keyed off
the `since` parameter, but many of the other request parameters also
affect the response or it's side effects. This will become worse once we
implement filtering, because there will be a wider space of parameters
with different responses. This problem is fixable, but not worth it
because of the low hit rate.
The low hit rate is because normal clients will always issue the next
sync request with `since` set to the `prev_batch` value of the previous
response. The only time we expect to see multiple requests with the same
`since` is when the response is empty, but we don't cache empty
responses.
This was confirmed experimentally by logging cache hits and misses over
15 minutes with a wide variety of clients. This test was run on
matrix.computer.surgery, which has only a few active users, but a
large volume of sync traffic from many rooms. Over the test period, we
had 3 hits and 5309 misses. All hits occurred in the first minute, so I
suspect that they had something to do with client recovery from an
offline state. The clients that were connected during the test are:
- element web
- schildichat web
- iamb
- gomuks
- nheko
- fractal
- fluffychat web
- fluffychat android
- cinny web
- element android
- element X android
Fixes: #336
this project's codebase is so horrendous, im shocked that no one has ran
clippy at all. it had ~200 total lint warnings, some with performance
issues and unsoundness, and the rest just very ugly codebase. i have sat
down and fixed as many of these as possible and i am exhausted.
i haven't fixed some extremely complex ones, but i brought it down from
~200 to ~30.
i have also removed io_uring as a default feature due to it falling
under the same category as linux eBPF: major kernel attack surface for
minimal performance gains. this also makes it impossible to cross-compile
from macOS to Linux because io_uring does not exist in Darwin land.
there are far better ways to achieve better performance than io_uring on
the codebase level.
Signed-off-by: strawberry <june@girlboss.ceo>
