Option to suppress resource exceeded alerting (#6173)

The expected use case is to suppress MAU limiting on small instances

8 files changed, 161 insertions, 47 deletions

diff --git a/changelog.d/6173.feature b/changelog.d/6173.feature
new file mode 100644
index 0000000000..b1cabc322b
--- /dev/null
+++ b/changelog.d/6173.feature
@@ -0,0 +1 @@
+Add config option to suppress client side resource limit alerting.
diff --git a/docs/sample_config.yaml b/docs/sample_config.yaml
index b4dd146f06..6c81c0db75 100644
--- a/docs/sample_config.yaml
+++ b/docs/sample_config.yaml
@@ -241,7 +241,6 @@ listeners:
 #
 #hs_disabled: false
 #hs_disabled_message: 'Human readable reason for why the HS is blocked'
-#hs_disabled_limit_type: 'error code(str), to help clients decode reason'
 
 # Monthly Active User Blocking
 #
@@ -261,9 +260,16 @@ listeners:
 # sign up in a short space of time never to return after their initial
 # session.
 #
+# 'mau_limit_alerting' is a means of limiting client side alerting
+# should the mau limit be reached. This is useful for small instances
+# where the admin has 5 mau seats (say) for 5 specific people and no
+# interest increasing the mau limit further. Defaults to True, which
+# means that alerting is enabled
+#
 #limit_usage_by_mau: false
 #max_mau_value: 50
 #mau_trial_days: 2
+#mau_limit_alerting: false
 
 # If enabled, the metrics for the number of monthly active users will
 # be populated, however no one will be limited. If limit_usage_by_mau
diff --git a/synapse/api/auth.py b/synapse/api/auth.py
index cd347fbe1b..53f3bb0fa8 100644
--- a/synapse/api/auth.py
+++ b/synapse/api/auth.py
@@ -25,7 +25,13 @@ from twisted.internet import defer
 import synapse.logging.opentracing as opentracing
 import synapse.types
 from synapse import event_auth
-from synapse.api.constants import EventTypes, JoinRules, Membership, UserTypes
+from synapse.api.constants import (
+    EventTypes,
+    JoinRules,
+    LimitBlockingTypes,
+    Membership,
+    UserTypes,
+)
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -726,7 +732,7 @@ class Auth(object):
                 self.hs.config.hs_disabled_message,
                 errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
                 admin_contact=self.hs.config.admin_contact,
-                limit_type=self.hs.config.hs_disabled_limit_type,
+                limit_type=LimitBlockingTypes.HS_DISABLED,
             )
         if self.hs.config.limit_usage_by_mau is True:
             assert not (user_id and threepid)
@@ -759,5 +765,5 @@ class Auth(object):
                     "Monthly Active User Limit Exceeded",
                     admin_contact=self.hs.config.admin_contact,
                     errcode=Codes.RESOURCE_LIMIT_EXCEEDED,
-                    limit_type="monthly_active_user",
+                    limit_type=LimitBlockingTypes.MONTHLY_ACTIVE_USER,
                 )
diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index 60e99e4663..312196675e 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -131,3 +131,10 @@ class RelationTypes(object):
     ANNOTATION = "m.annotation"
     REPLACE = "m.replace"
     REFERENCE = "m.reference"
+
+
+class LimitBlockingTypes(object):
+    """Reasons that a server may be blocked"""
+
+    MONTHLY_ACTIVE_USER = "monthly_active_user"
+    HS_DISABLED = "hs_disabled"
diff --git a/synapse/config/server.py b/synapse/config/server.py
index c942841578..d556df308d 100644
--- a/synapse/config/server.py
+++ b/synapse/config/server.py
@@ -171,6 +171,7 @@ class ServerConfig(Config):
         )
 
         self.mau_trial_days = config.get("mau_trial_days", 0)
+        self.mau_limit_alerting = config.get("mau_limit_alerting", True)
 
         # How long to keep redacted events in the database in unredacted form
         # before redacting them.
@@ -192,7 +193,6 @@ class ServerConfig(Config):
         # Options to disable HS
         self.hs_disabled = config.get("hs_disabled", False)
         self.hs_disabled_message = config.get("hs_disabled_message", "")
-        self.hs_disabled_limit_type = config.get("hs_disabled_limit_type", "")
 
         # Admin uri to direct users at should their instance become blocked
         # due to resource constraints
@@ -675,7 +675,6 @@ class ServerConfig(Config):
         #
         #hs_disabled: false
         #hs_disabled_message: 'Human readable reason for why the HS is blocked'
-        #hs_disabled_limit_type: 'error code(str), to help clients decode reason'
 
         # Monthly Active User Blocking
         #
@@ -695,9 +694,16 @@ class ServerConfig(Config):
         # sign up in a short space of time never to return after their initial
         # session.
         #
+        # 'mau_limit_alerting' is a means of limiting client side alerting
+        # should the mau limit be reached. This is useful for small instances
+        # where the admin has 5 mau seats (say) for 5 specific people and no
+        # interest increasing the mau limit further. Defaults to True, which
+        # means that alerting is enabled
+        #
         #limit_usage_by_mau: false
         #max_mau_value: 50
         #mau_trial_days: 2
+        #mau_limit_alerting: false
 
         # If enabled, the metrics for the number of monthly active users will
         # be populated, however no one will be limited. If limit_usage_by_mau
diff --git a/synapse/server_notices/resource_limits_server_notices.py b/synapse/server_notices/resource_limits_server_notices.py
index 81c4aff496..c0e7f475c9 100644
--- a/synapse/server_notices/resource_limits_server_notices.py
+++ b/synapse/server_notices/resource_limits_server_notices.py
@@ -20,6 +20,7 @@ from twisted.internet import defer
 
 from synapse.api.constants import (
     EventTypes,
+    LimitBlockingTypes,
     ServerNoticeLimitReached,
     ServerNoticeMsgType,
 )
@@ -70,7 +71,7 @@ class ResourceLimitsServerNotices(object):
             return
 
         if not self._server_notices_manager.is_enabled():
-            # Don't try and send server notices unles they've been enabled
+            # Don't try and send server notices unless they've been enabled
             return
 
         timestamp = yield self._store.user_last_seen_monthly_active(user_id)
@@ -79,8 +80,6 @@ class ResourceLimitsServerNotices(object):
             # In practice, not sure we can ever get here
             return
 
-        # Determine current state of room
-
         room_id = yield self._server_notices_manager.get_notice_room_for_user(user_id)
 
         if not room_id:
@@ -88,51 +87,86 @@ class ResourceLimitsServerNotices(object):
             return
 
         yield self._check_and_set_tags(user_id, room_id)
+
+        # Determine current state of room
         currently_blocked, ref_events = yield self._is_room_currently_blocked(room_id)
 
+        limit_msg = None
+        limit_type = None
         try:
-            # Normally should always pass in user_id if you have it, but in
-            # this case are checking what would happen to other users if they
-            # were to arrive.
-            try:
-                yield self._auth.check_auth_blocking()
-                is_auth_blocking = False
-            except ResourceLimitError as e:
-                is_auth_blocking = True
-                event_content = e.msg
-                event_limit_type = e.limit_type
-
-            if currently_blocked and not is_auth_blocking:
-                # Room is notifying of a block, when it ought not to be.
-                # Remove block notification
-                content = {"pinned": ref_events}
-                yield self._server_notices_manager.send_notice(
-                    user_id, content, EventTypes.Pinned, ""
-                )
+            # Normally should always pass in user_id to check_auth_blocking
+            # if you have it, but in this case are checking what would happen
+            # to other users if they were to arrive.
+            yield self._auth.check_auth_blocking()
+        except ResourceLimitError as e:
+            limit_msg = e.msg
+            limit_type = e.limit_type
 
-            elif not currently_blocked and is_auth_blocking:
+        try:
+            if (
+                limit_type == LimitBlockingTypes.MONTHLY_ACTIVE_USER
+                and not self._config.mau_limit_alerting
+            ):
+                # We have hit the MAU limit, but MAU alerting is disabled:
+                # reset room if necessary and return
+                if currently_blocked:
+                    self._remove_limit_block_notification(user_id, ref_events)
+                return
+
+            if currently_blocked and not limit_msg:
+                # Room is notifying of a block, when it ought not to be.
+                yield self._remove_limit_block_notification(user_id, ref_events)
+            elif not currently_blocked and limit_msg:
                 # Room is not notifying of a block, when it ought to be.
-                # Add block notification
-                content = {
-                    "body": event_content,
-                    "msgtype": ServerNoticeMsgType,
-                    "server_notice_type": ServerNoticeLimitReached,
-                    "admin_contact": self._config.admin_contact,
-                    "limit_type": event_limit_type,
-                }
-                event = yield self._server_notices_manager.send_notice(
-                    user_id, content, EventTypes.Message
+                yield self._apply_limit_block_notification(
+                    user_id, limit_msg, limit_type
                 )
-
-                content = {"pinned": [event.event_id]}
-                yield self._server_notices_manager.send_notice(
-                    user_id, content, EventTypes.Pinned, ""
-                )
-
         except SynapseError as e:
             logger.error("Error sending resource limits server notice: %s", e)
 
     @defer.inlineCallbacks
+    def _remove_limit_block_notification(self, user_id, ref_events):
+        """Utility method to remove limit block notifications from the server
+        notices room.
+
+        Args:
+            user_id (str): user to notify
+            ref_events (list[str]): The event_ids of pinned events that are unrelated to
+            limit blocking and need to be preserved.
+        """
+        content = {"pinned": ref_events}
+        yield self._server_notices_manager.send_notice(
+            user_id, content, EventTypes.Pinned, ""
+        )
+
+    @defer.inlineCallbacks
+    def _apply_limit_block_notification(self, user_id, event_body, event_limit_type):
+        """Utility method to apply limit block notifications in the server
+        notices room.
+
+        Args:
+            user_id (str): user to notify
+            event_body(str): The human readable text that describes the block.
+            event_limit_type(str): Specifies the type of block e.g. monthly active user
+            limit has been exceeded.
+        """
+        content = {
+            "body": event_body,
+            "msgtype": ServerNoticeMsgType,
+            "server_notice_type": ServerNoticeLimitReached,
+            "admin_contact": self._config.admin_contact,
+            "limit_type": event_limit_type,
+        }
+        event = yield self._server_notices_manager.send_notice(
+            user_id, content, EventTypes.Message
+        )
+
+        content = {"pinned": [event.event_id]}
+        yield self._server_notices_manager.send_notice(
+            user_id, content, EventTypes.Pinned, ""
+        )
+
+    @defer.inlineCallbacks
     def _check_and_set_tags(self, user_id, room_id):
         """
         Since server notices rooms were originally not with tags,
diff --git a/tests/server_notices/test_resource_limits_server_notices.py b/tests/server_notices/test_resource_limits_server_notices.py
index cdf89e3383..eb540e34f6 100644
--- a/tests/server_notices/test_resource_limits_server_notices.py
+++ b/tests/server_notices/test_resource_limits_server_notices.py
@@ -17,7 +17,7 @@ from mock import Mock
 
 from twisted.internet import defer
 
-from synapse.api.constants import EventTypes, ServerNoticeMsgType
+from synapse.api.constants import EventTypes, LimitBlockingTypes, ServerNoticeMsgType
 from synapse.api.errors import ResourceLimitError
 from synapse.server_notices.resource_limits_server_notices import (
     ResourceLimitsServerNotices,
@@ -133,7 +133,7 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
         self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
 
         # Would be better to check contents, but 2 calls == set blocking event
-        self.assertTrue(self._send_notice.call_count == 2)
+        self.assertEqual(self._send_notice.call_count, 2)
 
     def test_maybe_send_server_notice_to_user_add_blocked_notice_noop(self):
         """
@@ -158,6 +158,61 @@ class TestResourceLimitsServerNotices(unittest.HomeserverTestCase):
 
         self._send_notice.assert_not_called()
 
+    def test_maybe_send_server_notice_when_alerting_suppressed_room_unblocked(self):
+        """
+        Test that when server is over MAU limit and alerting is suppressed, then
+        an alert message is not sent into the room
+        """
+        self.hs.config.mau_limit_alerting = False
+        self._rlsn._auth.check_auth_blocking = Mock(
+            side_effect=ResourceLimitError(
+                403, "foo", limit_type=LimitBlockingTypes.MONTHLY_ACTIVE_USER
+            )
+        )
+        self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
+
+        self.assertTrue(self._send_notice.call_count == 0)
+
+    def test_check_hs_disabled_unaffected_by_mau_alert_suppression(self):
+        """
+        Test that when a server is disabled, that MAU limit alerting is ignored.
+        """
+        self.hs.config.mau_limit_alerting = False
+        self._rlsn._auth.check_auth_blocking = Mock(
+            side_effect=ResourceLimitError(
+                403, "foo", limit_type=LimitBlockingTypes.HS_DISABLED
+            )
+        )
+        self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
+
+        # Would be better to check contents, but 2 calls == set blocking event
+        self.assertEqual(self._send_notice.call_count, 2)
+
+    def test_maybe_send_server_notice_when_alerting_suppressed_room_blocked(self):
+        """
+        When the room is already in a blocked state, test that when alerting
+        is suppressed that the room is returned to an unblocked state.
+        """
+        self.hs.config.mau_limit_alerting = False
+        self._rlsn._auth.check_auth_blocking = Mock(
+            side_effect=ResourceLimitError(
+                403, "foo", limit_type=LimitBlockingTypes.MONTHLY_ACTIVE_USER
+            )
+        )
+        self._rlsn._server_notices_manager.__is_room_currently_blocked = Mock(
+            return_value=defer.succeed((True, []))
+        )
+
+        mock_event = Mock(
+            type=EventTypes.Message, content={"msgtype": ServerNoticeMsgType}
+        )
+        self._rlsn._store.get_events = Mock(
+            return_value=defer.succeed({"123": mock_event})
+        )
+        self.get_success(self._rlsn.maybe_send_server_notice_to_user(self.user_id))
+
+        self._send_notice.assert_called_once()
+
 
 class TestResourceLimitsServerNoticesWithRealRooms(unittest.HomeserverTestCase):
     def prepare(self, reactor, clock, hs):
diff --git a/tests/utils.py b/tests/utils.py
index 0a64f75d04..8cced4b7e8 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -137,7 +137,6 @@ def default_config(name, parse=False):
         "limit_usage_by_mau": False,
         "hs_disabled": False,
         "hs_disabled_message": "",
-        "hs_disabled_limit_type": "",
         "max_mau_value": 50,
         "mau_trial_days": 0,
         "mau_stats_only": False,