MUC: Add support for presence probes (fixes #1535)

The following patch allows Prosody to respond to `probe` presences and send out the probed occupant's current presence.

This is based on line 17.3 in XEP-0045:

    A MUC service MAY handle presence probes sent to the room JID <room@service> or an occupant JID <room@service/nick>
    (e.g, these might be sent by an occupant's home server to determine if the room is still online or to synchronize
    presence information if the user or the user's server has gone offline temporarily or has started sharing presence again,
    as for instance when Stanza Interception and Filtering Technology (XEP-0273) is used).

1 files changed, 115 insertions, 0 deletions

diff --git a/spec/scansion/muc_presence_probe.scs b/spec/scansion/muc_presence_probe.scs
new file mode 100644
index 00000000..ffd61dba
--- /dev/null
+++ b/spec/scansion/muc_presence_probe.scs
@@ -0,0 +1,115 @@
+# #1535 Let MUCs respond to presence probes
+
+[Client] Romeo
+	jid: user@localhost
+	password: password
+
+[Client] Juliet
+	jid: user2@localhost
+	password: password
+
+[Client] Mercutio
+	jid: user3@localhost
+	password: password
+
+-----
+
+Romeo connects
+
+Romeo sends:
+	<presence to="room@conference.localhost/Romeo">
+		<x xmlns="http://jabber.org/protocol/muc"/>
+	</presence>
+
+Romeo receives:
+	<presence from='room@conference.localhost/Romeo'>
+		<x xmlns='http://jabber.org/protocol/muc#user'>
+			<status code='201'/>
+			<item jid="${Romeo's full JID}" affiliation='owner' role='moderator'/>
+			<status code='110'/>
+		</x>
+	</presence>
+
+Romeo receives:
+	<message type='groupchat' from='room@conference.localhost'><subject/></message>
+
+# Disable presences for non-mods
+Romeo sends:
+	<iq id='config1' to='room@conference.localhost' type='set'>
+		<query xmlns='http://jabber.org/protocol/muc#owner'>
+			<x xmlns='jabber:x:data' type='submit'>
+				<field var='FORM_TYPE'>
+					<value>http://jabber.org/protocol/muc#roomconfig</value>
+				</field>
+				<field var='muc#roomconfig_presencebroadcast'>
+					<value>moderator</value>
+				</field>
+			</x>
+		</query>
+	</iq>
+
+Romeo receives:
+	<iq id="config1" from="room@conference.localhost" type="result">
+	</iq>
+
+# Juliet connects, and joins the room
+Juliet connects
+
+Juliet sends:
+	<presence to="room@conference.localhost/Juliet">
+		<x xmlns="http://jabber.org/protocol/muc"/>
+	</presence>
+
+Juliet receives:
+	<presence from="room@conference.localhost/Romeo" />
+
+Juliet receives:
+	<presence from="room@conference.localhost/Juliet" />
+
+# Romeo probes Juliet
+
+Romeo sends:
+	<presence to="room@conference.localhost/Juliet" type="probe">
+		<x xmlns="http://jabber.org/protocol/muc"/>
+	</presence>
+
+Romeo receives:
+	<presence from='room@conference.localhost/Juliet'>
+		<x xmlns='http://jabber.org/protocol/muc#user'>
+			<item jid="${Juliet's full JID}" affiliation='none' role='participant'/>
+		</x>
+	</presence>
+
+# Romeo makes Mercutio a member and registers his nickname
+
+Romeo sends:
+	<iq id='member1' to='room@conference.localhost' type='set'>
+		<query xmlns='http://jabber.org/protocol/muc#admin'>
+			<item affiliation='member' jid="${Mercutio's JID}" nick="Mercutio"/>
+		</query>
+	</iq>
+
+Romeo receives:
+	<message from='room@conference.localhost'>
+		<x xmlns='http://jabber.org/protocol/muc#user'>
+			<item jid="${Mercutio's JID}" affiliation='member' />
+		</x>
+	</message>
+
+Romeo receives:
+	<iq from='room@conference.localhost' id='member1' type='result'/>
+
+
+# Romeo probes Mercutio, even though he's unavailable
+
+Romeo sends:
+	<presence to="room@conference.localhost/Mercutio" type="probe">
+		<x xmlns="http://jabber.org/protocol/muc"/>
+	</presence>
+
+Romeo receives:
+	<presence from='room@conference.localhost/Mercutio' type="unavailable">
+		<x xmlns='http://jabber.org/protocol/muc#user'>
+			<item nick="Mercutio" affiliation='member' role='none' jid="${Mercutio's JID}" />
+		</x>
+	</presence>