core: don't create direct connections in group (#2996)

docs/protocol/diagrams/group.mmd

@@ -3,24 +3,31 @@ sequenceDiagram
   participant A as Alice
   participant B as Bob
   participant C as Existing<br>contact
-  
+
   note over A, B: 1. send and accept group invitation
   A ->> B: x.grp.inv<br>invite Bob to group<br>(via contact connection)
-  B ->> A: x.grp.acpt<br>accept invitation<br>(via member connection)
-  B ->> A: establish group member connection
+  B ->> A: x.grp.acpt<br>accept invitation<br>(via member connection)<br>establish group member connection
 
   note over M, B: 2. introduce new member Bob to all existing members
   A ->> M: x.grp.mem.new<br>"announce" Bob<br>to existing members<br>(via member connections)
-  A ->> B: x.grp.mem.intro * N<br>"introduce" members<br>(via member connection)
-  B ->> A: x.grp.mem.inv * N<br>"invitations" to connect<br>for all members<br>(via member connection)
-  A ->> M: x.grp.mem.fwd<br>forward "invitations"<br>to all members<br>(via member connections)
+  loop batched
+    A ->> B: x.grp.mem.intro * N<br>"introduce" members and<br>their chat protocol versions<br>(via member connection)
+    note over B: prepare group member connections
+    opt chat protocol compatible version < 2
+      note over B: prepare direct connections
+    end
+    B ->> A: x.grp.mem.inv * N<br>"invitations" to connect<br>for all members<br>(via member connection)
+  end
+  A ->> M: x.grp.mem.fwd<br>forward "invitations" and<br>Bob's chat protocol version<br>to all members<br>(via member connections)
 
   note over M, B: 3. establish direct and group member connections
   M ->> B: establish group member connection
-  M ->> B: establish direct connection
 
-  note over M, C: 4. deduplicate new contact
-  B ->> M: x.info.probe<br>"probe" is sent to all new members
-  B ->> C: x.info.probe.check<br>"probe" hash,<br>in case contact and<br>member profiles match
-  C ->> B: x.info.probe.ok<br> original "probe",<br> in case contact and member<br>are the same user
-  note over B: merge existing and new contacts if received and sent probe hashes match
+  opt chat protocol compatible version < 2
+    M ->> B: establish direct connection
+    note over M, C: 4. deduplicate new contact
+    B ->> M: x.info.probe<br>"probe" is sent to all new members
+    B ->> C: x.info.probe.check<br>"probe" hash,<br>in case contact and<br>member profiles match
+    C ->> B: x.info.probe.ok<br> original "probe",<br> in case contact and member<br>are the same user
+    note over B: merge existing and new contacts if received and sent probe hashes match
+  end