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.

4 files changed, 154 insertions, 22 deletions

diff --git a/changelog.d/7199.bugfix b/changelog.d/7199.bugfix
new file mode 100644
index 0000000000..b234163ea8
--- /dev/null
+++ b/changelog.d/7199.bugfix
@@ -0,0 +1 @@
+Fix a bug that could cause a user to be invited to a server notices (aka System Alerts) room without any notice being sent.
diff --git a/synapse/server_notices/resource_limits_server_notices.py b/synapse/server_notices/resource_limits_server_notices.py
index 9fae2e0afe..ce4a828894 100644
--- a/synapse/server_notices/resource_limits_server_notices.py
+++ b/synapse/server_notices/resource_limits_server_notices.py
@@ -80,7 +80,9 @@ class ResourceLimitsServerNotices(object):
             # In practice, not sure we can ever get here
             return
 
-        room_id = yield self._server_notices_manager.get_notice_room_for_user(user_id)
+        room_id = yield self._server_notices_manager.get_or_create_notice_room_for_user(
+            user_id
+        )
 
         if not room_id:
             logger.warning("Failed to get server notices room")
diff --git a/synapse/server_notices/server_notices_manager.py b/synapse/server_notices/server_notices_manager.py
index f7432c8d2f..bf0943f265 100644
--- a/synapse/server_notices/server_notices_manager.py
+++ b/synapse/server_notices/server_notices_manager.py
@@ -17,7 +17,7 @@ import logging
 from twisted.internet import defer
 
 from synapse.api.constants import EventTypes, Membership, RoomCreationPreset
-from synapse.types import create_requester
+from synapse.types import UserID, create_requester
 from synapse.util.caches.descriptors import cachedInlineCallbacks
 
 logger = logging.getLogger(__name__)
@@ -36,10 +36,12 @@ class ServerNoticesManager(object):
         self._store = hs.get_datastore()
         self._config = hs.config
         self._room_creation_handler = hs.get_room_creation_handler()
+        self._room_member_handler = hs.get_room_member_handler()
         self._event_creation_handler = hs.get_event_creation_handler()
         self._is_mine_id = hs.is_mine_id
 
         self._notifier = hs.get_notifier()
+        self.server_notices_mxid = self._config.server_notices_mxid
 
     def is_enabled(self):
         """Checks if server notices are enabled on this server.
@@ -66,7 +68,8 @@ class ServerNoticesManager(object):
         Returns:
             Deferred[FrozenEvent]
         """
-        room_id = yield self.get_notice_room_for_user(user_id)
+        room_id = yield self.get_or_create_notice_room_for_user(user_id)
+        yield self.maybe_invite_user_to_room(user_id, room_id)
 
         system_mxid = self._config.server_notices_mxid
         requester = create_requester(system_mxid)
@@ -89,10 +92,11 @@ class ServerNoticesManager(object):
         return res
 
     @cachedInlineCallbacks()
-    def get_notice_room_for_user(self, user_id):
+    def get_or_create_notice_room_for_user(self, user_id):
         """Get the room for notices for a given user
 
-        If we have not yet created a notice room for this user, create it
+        If we have not yet created a notice room for this user, create it, but don't
+        invite the user to it.
 
         Args:
             user_id (str): complete user id for the user we want a room for
@@ -108,7 +112,6 @@ class ServerNoticesManager(object):
         rooms = yield self._store.get_rooms_for_local_user_where_membership_is(
             user_id, [Membership.INVITE, Membership.JOIN]
         )
-        system_mxid = self._config.server_notices_mxid
         for room in rooms:
             # it's worth noting that there is an asymmetry here in that we
             # expect the user to be invited or joined, but the system user must
@@ -116,10 +119,14 @@ class ServerNoticesManager(object):
             # manages to invite the system user to a room, that doesn't make it
             # the server notices room.
             user_ids = yield self._store.get_users_in_room(room.room_id)
-            if system_mxid in user_ids:
+            if self.server_notices_mxid in user_ids:
                 # we found a room which our user shares with the system notice
                 # user
-                logger.info("Using room %s", room.room_id)
+                logger.info(
+                    "Using existing server notices room %s for user %s",
+                    room.room_id,
+                    user_id,
+                )
                 return room.room_id
 
         # apparently no existing notice room: create a new one
@@ -138,14 +145,13 @@ class ServerNoticesManager(object):
                 "avatar_url": self._config.server_notices_mxid_avatar_url,
             }
 
-        requester = create_requester(system_mxid)
+        requester = create_requester(self.server_notices_mxid)
         info = yield self._room_creation_handler.create_room(
             requester,
             config={
                 "preset": RoomCreationPreset.PRIVATE_CHAT,
                 "name": self._config.server_notices_room_name,
                 "power_level_content_override": {"users_default": -10},
-                "invite": (user_id,),
             },
             ratelimit=False,
             creator_join_profile=join_profile,
@@ -159,3 +165,30 @@ class ServerNoticesManager(object):
 
         logger.info("Created server notices room %s for %s", room_id, user_id)
         return room_id
+
+    @defer.inlineCallbacks
+    def maybe_invite_user_to_room(self, user_id: str, room_id: str):
+        """Invite the given user to the given server room, unless the user has already
+        joined or been invited to it.
+
+        Args:
+            user_id: The ID of the user to invite.
+            room_id: The ID of the room to invite the user to.
+        """
+        requester = create_requester(self.server_notices_mxid)
+
+        # Check whether the user has already joined or been invited to this room. If
+        # that's the case, there is no need to re-invite them.
+        joined_rooms = yield self._store.get_rooms_for_local_user_where_membership_is(
+            user_id, [Membership.INVITE, Membership.JOIN]
+        )
+        for room in joined_rooms:
+            if room.room_id == room_id:
+                return
+
+        yield self._room_member_handler.update_membership(
+            requester=requester,
+            target=UserID.from_string(user_id),
+            room_id=room_id,
+            action="invite",
+        )
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