Support for roster versioning (EJAB-964)

Introduces two options for mod_roster and mod_roster_odbc:
- {versioning, true | false}   Enable or disable roster versioning on ejabberd.
- {store_current_id, true | false}   If true, the current roster version is stored on DB (internal or odbc). Otherwise it is calculated on the fly each time.

Performance:
Setting store_current_id to true should help in reducing the load for both ejabberd and the DB.

Details: 
If store_current_id is false,  the roster version is a hash of the entire roster. If store_current_id is true, the roster version is a hash, but of the current time
(this has to do with transactional semantics; we need to perform both the roster update and the version update on the same transaction, but we don't   
have the entire roster when we are changing a single item on DB. Loading it there requires significant changes to be introduced, so I opted for this simpler approach).

In either case, there is no difference for the clients, the roster version ID is opaque.

IMPORTANT:
mod_shared_roster is not compatible with the option 'store_current_id'.  Shared roster and roster versioning can be both enabled, but store_current_id MUST be set to false.

SVN Revision: 2428

1 files changed, 6 insertions, 1 deletions

diff --git a/src/ejabberd_c2s.erl b/src/ejabberd_c2s.erl
index f77c35a9d..e61b9e987 100644
--- a/src/ejabberd_c2s.erl
+++ b/src/ejabberd_c2s.erl
@@ -310,12 +310,17 @@ wait_for_stream({xmlstreamstart, #xmlel{ns = NS} = Opening}, StateData) ->
 				_ ->
 				    case StateData#state.resource of
 					undefined ->
+					    RosterVersioningFeature =
+					    	case roster_versioning:is_enabled(ServerB) of
+							true -> [roster_versioning:stream_feature()];
+							false -> []
+						end,
 					    send_element(
 					      StateData,
 					      exmpp_stream:features([
 						  exmpp_server_binding:feature(),
 						  exmpp_server_session:feature()
-						])),
+						| RosterVersioningFeature])),
 					    fsm_next_state(wait_for_bind,
 						       StateData#state{
 							 server = ServerB,