Faster room joins: Outgoing device list updates not sent if we leave the room before join completes

[squahtx]

When a local user updates their device list while in a partial state room, their device list updates are buffered up, since it's not known for sure which servers are in the room and expecting device list updates. #13891 ensures that buffered device list updates are sent once the room transitions to full state.

However, if the room is left before then, there is no transition to full state and some remote homeservers will miss updates.

eg.

1. @alice:local partial state joins a room
2. The local homeserver thinks @elsie:remote is not in the room and does not send them device list updates, either because the /send_join servers_in_room response was incorrect or it has calculated incorrect state.
3. @elsie:remote queries @alice:local's device list and caches it.
4. @alice:local updates their device list.
5. @elsie:remote joins another room shared with @alice:local.
6. @alice:local leaves the partial state room.
7. (bad) @elsie:remote thinks their cached device list for @alice:local is up to date when it's actually stale.

We're missing complement tests for this too.

related:

• Faster joins: fixes to device list tracking #12993
• Faster joins: Handle leaving a room while re-syncing state #12802
• Make outgoing device list tracking work after we partial-join #13891

[richvdh]

Is this tested at all by matrix-org/complement#476?

[squahtx]

Those tests are for incoming device list updates rather than outgoing ones. We're missing tests for this issue.

[richvdh]

We think this is an edge-case, and no worse than existing bugs in device-list tracking (such as #13651), so not making it a priority for a fix.