Handle malformed values of notification.room in power level events (#14942)

* Better test for bad values in power levels events

The previous test only checked that Synapse didn't raise an exception,
but didn't check that we had correctly interpreted the value of the
dodgy power level.

It also conflated two things: bad room notification levels, and bad user
levels. There _is_ logic for converting the latter to integers, but we
should test it separately.

* Check we ignore types that don't convert to int

* Handle `None` values in `notifications.room`

* Changelog

* Also test that bad values are rejected by event auth

* Docstring

* linter scripttttttttt

synapse/push/bulk_push_rule_evaluator.py

@@ -69,6 +69,9 @@ STATE_EVENT_TYPES_TO_MARK_UNREAD = {
 }
 
 
+SENTINEL = object()
+
+
 def _should_count_as_unread(event: EventBase, context: EventContext) -> bool:
     # Exclude rejected and soft-failed events.
     if context.rejected or event.internal_metadata.is_soft_failed():
@@ -343,11 +346,21 @@ class BulkPushRuleEvaluator:
         related_events = await self._related_events(event)
 
         # It's possible that old room versions have non-integer power levels (floats or
-        # strings). Workaround this by explicitly converting to int.
+        # strings; even the occasional `null`). For old rooms, we interpret these as if
+        # they were integers. Do this here for the `@room` power level threshold.
+        # Note that this is done automatically for the sender's power level by
+        # _get_power_levels_and_sender_level in its call to get_user_power_level
+        # (even for room V10.)
         notification_levels = power_levels.get("notifications", {})
         if not event.room_version.msc3667_int_only_power_levels:
-            for user_id, level in notification_levels.items():
-                notification_levels[user_id] = int(level)
+            keys = list(notification_levels.keys())
+            for key in keys:
+                level = notification_levels.get(key, SENTINEL)
+                if level is not SENTINEL and type(level) is not int:
+                    try:
+                        notification_levels[key] = int(level)
+                    except (TypeError, ValueError):
+                        del notification_levels[key]
 
         # Pull out any user and room mentions.
         mentions = event.content.get(EventContentFields.MSC3952_MENTIONS)