Fix announcing session establishment feature

An XMPP client won't try the old session establishment step if it doesn't see it
announced in the server features, even when the server is strictly waiting for
it, so this would deadlock all connections.

src/c2s/mongoose_c2s.erl

@@ -451,7 +451,7 @@ stream_start_features_before_auth(#c2s_data{host_type = HostType, lserver = LSer
 stream_start_features_after_auth(#c2s_data{host_type = HostType, lserver = LServer,
                                            lang = Lang, listener_opts = LOpts} = S) ->
     send_header(S, LServer, <<"1.0">>, Lang),
-    StreamFeatures = mongoose_c2s_stanzas:stream_features_after_auth(HostType, LServer),
+    StreamFeatures = mongoose_c2s_stanzas:stream_features_after_auth(HostType, LServer, LOpts),
     send_element_from_server_jid(S, StreamFeatures),
     {next_state, {wait_for_feature_after_auth, ?BIND_RETRIES}, S, state_timeout(LOpts)}.
 

src/c2s/mongoose_c2s_stanzas.erl

@@ -7,7 +7,7 @@
          stream_header/4,
          stream_features_before_auth/4,
          tls_proceed/0,
-         stream_features_after_auth/2,
+         stream_features_after_auth/3,
          sasl_success_stanza/1,
          sasl_failure_stanza/1,
          sasl_challenge_stanza/1,
@@ -87,11 +87,19 @@ tls_proceed() ->
     #xmlel{name = <<"proceed">>,
            attrs = [{<<"xmlns">>, ?NS_TLS}]}.
 
--spec stream_features_after_auth(mongooseim:host_type(), jid:lserver()) -> exml:element().
-stream_features_after_auth(HostType, LServer) ->
+-spec stream_features_after_auth(mongooseim:host_type(), jid:lserver(), mongoose_listener:options()) ->
+    exml:element().
+stream_features_after_auth(HostType, LServer, #{backwards_compatible_session := false}) ->
     Features = [#xmlel{name = <<"bind">>,
                        attrs = [{<<"xmlns">>, ?NS_BIND}]}
                 | hook_enabled_features(HostType, LServer)],
+    stream_features(Features);
+stream_features_after_auth(HostType, LServer, #{backwards_compatible_session := true}) ->
+    Features = [#xmlel{name = <<"session">>,
+                       attrs = [{<<"xmlns">>, ?NS_SESSION}]},
+                #xmlel{name = <<"bind">>,
+                       attrs = [{<<"xmlns">>, ?NS_BIND}]}
+                | hook_enabled_features(HostType, LServer)],
     stream_features(Features).
 
 hook_enabled_features(HostType, LServer) ->