Require user to have left room to forget room

This dramatically simplifies the forget API code - in particular it no
longer generates a leave event.

synapse/handlers/room.py

@@ -416,8 +416,6 @@ class RoomMemberHandler(BaseHandler):
         effective_membership_state = action
         if action in ["kick", "unban"]:
             effective_membership_state = "leave"
-        elif action == "forget":
-            effective_membership_state = "leave"
 
         if third_party_signed is not None:
             replication = self.hs.get_replication_layer()
@@ -473,9 +471,6 @@ class RoomMemberHandler(BaseHandler):
             remote_room_hosts=remote_room_hosts,
         )
 
-        if action == "forget":
-            yield self.forget(requester.user, room_id)
-
     @defer.inlineCallbacks
     def send_membership_event(
             self,
@@ -935,8 +930,23 @@ class RoomMemberHandler(BaseHandler):
         display_name = data["display_name"]
         defer.returnValue((token, public_keys, fallback_public_key, display_name))
 
+    @defer.inlineCallbacks
     def forget(self, user, room_id):
-        return self.store.forget(user.to_string(), room_id)
+        user_id = user.to_string()
+
+        member = yield self.state_handler.get_current_state(
+            room_id=room_id,
+            event_type=EventTypes.Member,
+            state_key=user_id
+        )
+        membership = member.membership if member else None
+
+        if membership is not None and membership != Membership.LEAVE:
+            raise SynapseError(400, "User %s in room %s" % (
+                user_id, room_id
+            ))
+
+        yield self.store.forget(user_id, room_id)
 
 
 class RoomListHandler(BaseHandler):

synapse/rest/client/v1/room.py

@@ -405,6 +405,43 @@ class RoomEventContext(ClientV1RestServlet):
         defer.returnValue((200, results))
 
 
+class RoomForgetRestServlet(ClientV1RestServlet):
+    def register(self, http_server):
+        # /rooms/$roomid/[invite|join|leave]
+        PATTERNS = ("/rooms/(?P<room_id>[^/]*)/forget")
+        register_txn_path(self, PATTERNS, http_server)
+
+    @defer.inlineCallbacks
+    def on_POST(self, request, room_id, txn_id=None):
+        requester = yield self.auth.get_user_by_req(
+            request,
+            allow_guest=False,
+        )
+
+        yield self.handlers.room_member_handler.forget(
+            user=requester.user,
+            room_id=room_id,
+        )
+
+        defer.returnValue((200, {}))
+
+    @defer.inlineCallbacks
+    def on_PUT(self, request, room_id, txn_id):
+        try:
+            defer.returnValue(
+                self.txns.get_client_transaction(request, txn_id)
+            )
+        except KeyError:
+            pass
+
+        response = yield self.on_POST(
+            request, room_id, txn_id
+        )
+
+        self.txns.store_client_transaction(request, txn_id, response)
+        defer.returnValue(response)
+
+
 # TODO: Needs unit testing
 class RoomMembershipRestServlet(ClientV1RestServlet):
 
@@ -624,6 +661,7 @@ def register_servlets(hs, http_server):
     RoomMemberListRestServlet(hs).register(http_server)
     RoomMessageListRestServlet(hs).register(http_server)
     JoinRoomAliasServlet(hs).register(http_server)
+    RoomForgetRestServlet(hs).register(http_server)
     RoomMembershipRestServlet(hs).register(http_server)
     RoomSendEventRestServlet(hs).register(http_server)
     PublicRoomListRestServlet(hs).register(http_server)