Rotated megolm session not shared to all members

[BillCarsonFr]

See this Rageshake:
https://github.com/matrix-org/element-web-rageshakes/issues/6602

Previous session shared to members in 9 slices
Rotated session only shared in 3 slices

=> Lost a lot of users

We don't have enough information to find exactly where is the problem.
Potential sources:

matrix-js-sdk/src/crypto/algorithms/megolm.ts

Line 988 in b288c97

public prepareToEncrypt(room: Room): void {

• await this.getDevicesInRoom(room); Returns the wrong number of members?
• this.removeUnknownDevices(devicesInRoom) did drop wrong devices?

Also from the log the reason for rotation was not printed out.

As we don't know exactly what can be the reason, maybe we can improve the logs to:

• Logs informations on devices in the rooms #getDevicesInRoom
• Ensure to print reason for session rotation (here there was no reason printed out)
• More logs on loadMembersIfNeeded in room (and put roomId in the trace)

[uhoreg]

It looks like the cause of this is that the js-sdk's state storage had stale information. So when Element was restarted, it only had a partial membership list. I haven't found a cause for this stale data, but I've created #1998 so that it will reload the membership list for encrypted rooms when it gets restarted, to work around it. Debugging the stale data is hard due to element-hq/element-web#6291.

I'm going to stop working on this for now, but leave the issue open until we figure out why the data can be stale.