Server notices: Dissociate room creation/lookup from invite (#7199)

Fixes #6815

Before figuring out whether we should alert a user on MAU, we call get_notice_room_for_user to get some info on the existing server notices room for this user. This function, if the room doesn't exist, creates it and invites the user in it. This means that, if we decide later that no server notice is needed, the user gets invited in a room with no message in it. This happens at every restart of the server, since the room ID returned by get_notice_room_for_user is cached.

This PR fixes that by moving the inviting bit to a dedicated function, that's only called when the server actually needs to send a notice to the user. A potential issue with this approach is that the room that's created by get_notice_room_for_user doesn't match how that same function looks for an existing room (i.e. it creates a room that doesn't have an invite or a join for the current user in it, so it could lead to a new room being created each time a user syncs), but I'm not sure this is a problem given it's cached until the server restarts, so that function won't run very often.

It also renames get_notice_room_for_user into get_or_create_notice_room_for_user to make what it does clearer.

1 files changed, 108 insertions, 12 deletions

diff --git a/tests/server_notices/test_resource_limits_server_notices.py b/tests/server_notices/test_resource_limits_server_notices.py
index 0d27b92a86..93eb053b8c 100644
--- a/tests/server_notices/test_resource_limits_server_notices.py
+++ b/tests/server_notices/test_resource_limits_server_notices.py
@@ -19,6 +19,9 @@ from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, LimitBlockingTypes, ServerNoticeMsgType
 from synapse.api.errors import ResourceLimitError
+from synapse.rest import admin
+from synapse.rest.client.v1 import login, room
+from synapse.rest.client.v2_alpha import sync
 from synapse.server_notices.resource_limits_server_notices import (
     ResourceLimitsServerNotices,
 )
@@ -67,7 +70,7 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
         # self.server_notices_mxid_avatar_url = None
         # self.server_notices_room_name = "Server Notices"
 
-        self._rlsn._server_notices_manager.get_notice_room_for_user = Mock(
+        self._rlsn._server_notices_manager.get_or_create_notice_room_for_user = Mock(
             returnValue=""
         )
         self._rlsn._store.add_tag_to_room = Mock()
@@ -215,6 +218,26 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
 
 
 class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
+    servlets = [
+        admin.register_servlets,
+        login.register_servlets,
+        room.register_servlets,
+        sync.register_servlets,
+    ]
+
+    def default_config(self):
+        c = super().default_config()
+        c["server_notices"] = {
+            "system_mxid_localpart": "server",
+            "system_mxid_display_name": None,
+            "system_mxid_avatar_url": None,
+            "room_name": "Test Server Notice Room",
+        }
+        c["limit_usage_by_mau"] = True
+        c["max_mau_value"] = 5
+        c["admin_contact"] = "mailto:user@test.com"
+        return c
+
     def prepare(self, reactor, clock, hs):
         self.store = self.hs.get_datastore()
         self.server_notices_sender = self.hs.get_server_notices_sender()
@@ -228,18 +251,8 @@ class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
         if not isinstance(self._rlsn, ResourceLimitsServerNotices):
             raise Exception("Failed to find reference to ResourceLimitsServerNotices")
 
-        self.hs.config.limit_usage_by_mau = True
-        self.hs.config.hs_disabled = False
-        self.hs.config.max_mau_value = 5
-        self.hs.config.server_notices_mxid = "@server:test"
-        self.hs.config.server_notices_mxid_display_name = None
-        self.hs.config.server_notices_mxid_avatar_url = None
-        self.hs.config.server_notices_room_name = "Test Server Notice Room"
-
         self.user_id = "@user_id:test"
 
-        self.hs.config.admin_contact = "mailto:user@test.com"
-
     def test_server_notice_only_sent_once(self):
         self.store.get_monthly_active_count = Mock(return_value=1000)
 
@@ -253,7 +266,7 @@ class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
         # Now lets get the last load of messages in the service notice room and
         # check that there is only one server notice
         room_id = self.get_success(
-            self.server_notices_manager.get_notice_room_for_user(self.user_id)
+            self.server_notices_manager.get_or_create_notice_room_for_user(self.user_id)
         )
 
         token = self.get_success(self.event_source.get_current_token())
@@ -273,3 +286,86 @@ class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
             count += 1
 
         self.assertEqual(count, 1)
+
+    def test_no_invite_without_notice(self):
+        """Tests that a user doesn't get invited to a server notices room without a
+        server notice being sent.
+
+        The scenario for this test is a single user on a server where the MAU limit
+        hasn't been reached (since it's the only user and the limit is 5), so users
+        shouldn't receive a server notice.
+        """
+        self.register_user("user", "password")
+        tok = self.login("user", "password")
+
+        request, channel = self.make_request("GET", "/sync?timeout=0", access_token=tok)
+        self.render(request)
+
+        invites = channel.json_body["rooms"]["invite"]
+        self.assertEqual(len(invites), 0, invites)
+
+    def test_invite_with_notice(self):
+        """Tests that, if the MAU limit is hit, the server notices user invites each user
+        to a room in which it has sent a notice.
+        """
+        user_id, tok, room_id = self._trigger_notice_and_join()
+
+        # Sync again to retrieve the events in the room, so we can check whether this
+        # room has a notice in it.
+        request, channel = self.make_request("GET", "/sync?timeout=0", access_token=tok)
+        self.render(request)
+
+        # Scan the events in the room to search for a message from the server notices
+        # user.
+        events = channel.json_body["rooms"]["join"][room_id]["timeline"]["events"]
+        notice_in_room = False
+        for event in events:
+            if (
+                event["type"] == EventTypes.Message
+                and event["sender"] == self.hs.config.server_notices_mxid
+            ):
+                notice_in_room = True
+
+        self.assertTrue(notice_in_room, "No server notice in room")
+
+    def _trigger_notice_and_join(self):
+        """Creates enough active users to hit the MAU limit and trigger a system notice
+        about it, then joins the system notices room with one of the users created.
+
+        Returns:
+            user_id (str): The ID of the user that joined the room.
+            tok (str): The access token of the user that joined the room.
+            room_id (str): The ID of the room that's been joined.
+        """
+        user_id = None
+        tok = None
+        invites = []
+
+        # Register as many users as the MAU limit allows.
+        for i in range(self.hs.config.max_mau_value):
+            localpart = "user%d" % i
+            user_id = self.register_user(localpart, "password")
+            tok = self.login(localpart, "password")
+
+            # Sync with the user's token to mark the user as active.
+            request, channel = self.make_request(
+                "GET", "/sync?timeout=0", access_token=tok,
+            )
+            self.render(request)
+
+            # Also retrieves the list of invites for this user. We don't care about that
+            # one except if we're processing the last user, which should have received an
+            # invite to a room with a server notice about the MAU limit being reached.
+            # We could also pick another user and sync with it, which would return an
+            # invite to a system notices room, but it doesn't matter which user we're
+            # using so we use the last one because it saves us an extra sync.
+            invites = channel.json_body["rooms"]["invite"]
+
+        # Make sure we have an invite to process.
+        self.assertEqual(len(invites), 1, invites)
+
+        # Join the room.
+        room_id = list(invites.keys())[0]
+        self.helper.join(room=room_id, user=user_id, tok=tok)
+
+        return user_id, tok, room_id