Complement Existing members see new members' presence is flaky #13199
Description
It took me 363 runs (~3 hours of CI) to reproduce this, so maybe it's not the highest priority flake around.
The issue seems to be that Bob's presence information is not coming down sync.
- Complement test link: https://github.com/matrix-org/complement/blob/d784f7d96677e8b4267da9729d5c601c1e99e741/tests/csapi/rooms_members_local_test.go#L38-L61
- Equivalent SyTest link: https://github.com/matrix-org/sytest/blob/a94cd1dc2d6102e5a9e94659e2dca243e9b72208/tests/30rooms/02members-local.pl#L104-L117
- Logs for failing test: TestMembersLocal.log
I believe the reason we expect Bob's presence to come down sync is because /join involves sending an event and that should update his presence.
I can't see the code that should trigger this in Synapse, but since this succeeds most of the time it must be there.
In the test logs for this test, I didn't spot any requests to /_synapse/replication/presence_set_state/ for Bob, which I would have expected to see. (I do see some for Alice.)
For this reason, I hesitate to brush it off as 'slow replication'. I also note that the requests in the log don't seem to be that slow around the time of the failure, so I don't suspect CPU contention.