Receiving two Olm messages in the same sync can cause the second to be undecryptable

[turt2live]

In my test case, sending a dummy event followed by a room key causes the room key to be undecryptable, thus UISI.

[germain-gg]

Could you provide some reproducible steps so that someone look into this in the future?
Maybe sharing your test case could help

[turt2live]

A test case is a bit difficult to provide as it means setting up 3/4s of encryption, but the summary was:

1. Create Olm session
2. Notify Element Web of that session with an m.dummy
3. Prepare an outbound group session and notify Element Web of that with an m.room_key
4. Watch when Element Web fails to decrypt due to bad MAC on the room_key (it seems to get as far as knowing it's a room_key, but not as far as actually using it).

There might be some timing issues at play here, where in my case the Element Web happened to receive both in the same /sync request but it's entirely possible that the server wakes the sync loop between the messages. However, considering I managed to get it to reliably be an issue on a localhost server using a regular Element Web, I'm not convinced it'll fall to the "safe" side of 2 syncs very often.