Megolm keys were not shared with all users in a room

[ara4n]

Steps to reproduce

1. Create a room and invite 9 people to it
2. One person joins, and sends a message
3. The others join, and discover that the user only sent [keys for] that message to the first 4 users that were invited - presumably we are using the LL heroes for targetting the encryption, rather than actually sending to all the users in the invite list.
4. Therefore, half the users in the room get UISIs for the messages sent before they joined.

Outcome

What did you expect?

Messages sent before users join a room should be reliable.

What happened instead?

UISIs.

Operating system

macOS

Application version

Nightly

How did you install the app?

Nightly

Homeserver

matrix.org

Will you send logs?

Yes

[richvdh]

presumably we are using the LL heroes for targetting the encryption, rather than actually sending to all the users in the invite list.

Possibly, though we certainly attempt to load the full user list before sending out any encrypted messages. It's rather odd.

It's worth noting that this code has changed a bit since September (matrix-org/matrix-js-sdk#2998, for example), and it will change a lot more with the switch to Element-R. I'm therefore not inclined to spend too much time trying to hunt it down right now.

What I have done is made some improvements to the logging in matrix-org/matrix-js-sdk#3043, which I hope will make this easier to debug if it recurs.

[richvdh]

Closing this for now as no further action is planned.