XEP-0313: Fix off-by-one-page RSM fetching

Add a "results" interface to mam_fin, and fix some things in RSM

Items just received were not taken into account because:
- RSM code is checking iq['mam_fin']['results'], results were at
  iq['mam']['results']
- RSM handler was run after checking the number

slixmpp/plugins/xep_0059/rsm.py

@@ -135,6 +135,9 @@ class ResultIterator(AsyncIterator):
                not r[self.recv_interface]['rsm']['last']:
                 raise StopAsyncIteration
 
+            if self.post_cb:
+                self.post_cb(r)
+
             if r[self.recv_interface]['rsm']['count'] and \
                r[self.recv_interface]['rsm']['first_index']:
                 count = int(r[self.recv_interface]['rsm']['count'])
@@ -147,9 +150,6 @@ class ResultIterator(AsyncIterator):
                 self.start = r[self.recv_interface]['rsm']['first']
             else:
                 self.start = r[self.recv_interface]['rsm']['last']
-
-            if self.post_cb:
-                self.post_cb(r)
             return r
         except XMPPError:
             raise StopAsyncIteration

slixmpp/plugins/xep_0313/mam.py

@@ -112,6 +112,7 @@ class XEP_0313(BasePlugin):
             results = cb_data['collector'].stop()
             if result['type'] == 'result':
                 result['mam']['results'] = results
+                result['mam_fin']['results'] = results
 
         if iterator:
             return self.xmpp['xep_0059'].iterate(
@@ -185,6 +186,7 @@ class XEP_0313(BasePlugin):
             results = cb_data['collector'].stop()
             if result['type'] == 'result':
                 result['mam']['results'] = results
+                result['mam_fin']['results'] = results
 
         iterator = self.xmpp['xep_0059'].iterate(
             iq, 'mam', 'results', amount=amount,

slixmpp/plugins/xep_0313/stanza.py

@@ -187,6 +187,24 @@ class Fin(ElementBase):
     name = 'fin'
     namespace = 'urn:xmpp:mam:2'
     plugin_attrib = 'mam_fin'
+    interfaces = {'results'}
+
+    def setup(self, xml=None):
+        ElementBase.setup(self, xml)
+        self._results: List[Message] = []
+
+    # The results interface is meant only as an easy
+    # way to access the set of collected message responses
+    # from the query.
+
+    def get_results(self) -> List[Message]:
+        return self._results
+
+    def set_results(self, values: List[Message]):
+        self._results = values
+
+    def del_results(self):
+        self._results = []
 
 
 class Result(ElementBase):