session: Don’t bind if it is optional.

See https://tools.ietf.org/html/draft-cridland-xmpp-session-01

slixmpp/features/feature_session/session.py

@@ -43,10 +43,11 @@ class FeatureSession(BasePlugin):
         Arguments:
             feature -- The stream features element.
         """
-        iq = self.xmpp.Iq()
-        iq['type'] = 'set'
-        iq.enable('session')
-        yield from iq.send(callback=self._on_start_session_response)
+        if not features['session']['optional']:
+            iq = self.xmpp.Iq()
+            iq['type'] = 'set'
+            iq.enable('session')
+            yield from iq.send(callback=self._on_start_session_response)
 
     def _on_start_session_response(self, response):
         self.xmpp.features.add('session')

slixmpp/features/feature_session/stanza.py

@@ -6,7 +6,7 @@
     See the file LICENSE for copying permission.
 """
 
-from slixmpp.xmlstream import ElementBase
+from slixmpp.xmlstream import ElementBase, ET
 
 
 class Session(ElementBase):
@@ -16,5 +16,19 @@ class Session(ElementBase):
 
     name = 'session'
     namespace = 'urn:ietf:params:xml:ns:xmpp-session'
-    interfaces = set()
+    interfaces = {'optional'}
     plugin_attrib = 'session'
+
+    def get_optional(self):
+        return self.xml.find('{%s}optional' % self.namespace) is not None
+
+    def set_optional(self, value):
+        if value:
+            optional = ET.Element('{%s}optional' % self.namespace)
+            self.xml.append(optional)
+        else:
+            self.del_optional()
+
+    def del_optional(self):
+        optional = self.xml.find('{%s}optional' % self.namespace)
+        self.xml.remove(optional)