Federation: history is broken when an invited member joins a room that he left previously

[giomfo]

TL;DR: history may be missing between a federated user being re-invited to a room and re-joining it.

Description

A part of the room history is missing when an invited member joins a room that he left previously,
if the room has been created on another homeserver than the member's one.
Note: The e2e encryption is disabled in this room.

Steps to reproduce

• Consider 2 users userA and userB, created on 2 different Homeservers.
• userA creates a room, history_visibility is shared (default value)
• userA invites userB
• userA sends some messages
• userB joins, he can read the messages OK
• userA and userB send some messages OK
• userB leaves the room
• userA sends a message M1
• userA invites again userB
• userA sends a message M2
• userB joins, the messages M1 and M2 are missing in the room history (I checked we did not get them in the server sync response after userB joined the room) NOK

Note1: The room history is restored correctly if we trigger an initial sync. Indeed the messages M1 and M2 are then provided in the server sync response.

Note2: You can send more than 1 message with the message M2. The problem is the same, userB don't receive them when he joins. He gets them only by triggering an initial sync.

Note3: This issue is not reproduced when userA and userB belong to the same homeserver.

[ara4n]

@giomfo does M2 ever arrive? I'm wondering if userB has to speak in the room to 'pull in' the missing message.

[giomfo]

@ara4n yes M2 arrived
no, the missing message is not pulled in If userB speaks in the room

[ara4n]

ok, what happened to cause M2 to arrive?

[giomfo]

what happened to cause M2 to arrive?

I triggered a clear cache on the userB client, which involves an initial sync.

To sum up:

• The room history (10 last events of the timeline) is wrong in the server sync response after userB joined again the room.
• This history (10 last events of the timeline) is correct when I force an initial sync for this room