[Standards] XEP-0045 join when already in MUC

Georg Lukas georg at op-co.de
Thu Jun 23 22:35:11 UTC 2016


Hi,

TL;DR: whenever a client (re)sends a join, the MUC service should return
everything a newly joining client needs to know.

According to XEP-0045, an initial join presence needs to contain an 'x'
element with 'http://jabber.org/protocol/muc' namespace (§7.2.2),
whereas a regular presence update to the MUC does not (§7.7).

In §7.2.3 it follows up then with "If the service is able to add the
user to the room, it MUST send presence from all the existing
participants' occupant JIDs to the new occupant's full JID, ..."

I would like to propose the following add-on clarification:

"The service MUST send to the client everything that would be sent on a
join even if the user already is joined to the MUC with the same full
JID. This is required because a client (or the user's server) might have
been restarted without informing the MUC by means of an 'unavailable'
presence."

I was told that this behavior is already implemented in prosody-trunk
(though not in older versions) and that the sky hasn't fallen down yet.

When on mobile data, I often see a join only answered by a reflection of
my own presence, without a list of other participants and without a room
subject (I'm running prosody stable). This happens because the MUC
assumes that I'm still inside and only want to update my status.

Without the proposed change, there is no way for a client to reliably
sync a MUC - the only indication for a failed join is that no room
subject is received, and sending the subject is a SHALL, not a MUST
(§7.2.16). And even if the client could rely on a subject being sent on
a proper join, it would have to setup a timeout (the subject comes after
the reflected presence), and then to leave and re-join the MUC if no
subject is received.

The only semi-acceptable workaround that comes to my mind is to prepend
each join with a directed presence-unavailable to the MUC to enforce a
clean state.



Georg
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