Add auth events as per spec. (#6556)

Previously we tried to be clever and filter out some unnecessary event
IDs to keep the auth chain small, but that had some annoying
interactions with state res v2 so we stop doing that for now.

changelog.d/6556.bugfix

+Fix a cause of state resets in room versions 2 onwards.

synapse/api/auth.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import logging
+from typing import Dict, Tuple
 
 from six import itervalues
 
@@ -25,13 +26,7 @@ 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,
-    LimitBlockingTypes,
-    Membership,
-    UserTypes,
-)
+from synapse.api.constants import EventTypes, LimitBlockingTypes, Membership, UserTypes
 from synapse.api.errors import (
     AuthError,
     Codes,
@@ -513,71 +508,43 @@ class Auth(object):
         """
         return self.store.is_server_admin(user)
 
-    @defer.inlineCallbacks
-    def compute_auth_events(self, event, current_state_ids, for_verification=False):
-        if event.type == EventTypes.Create:
-            return []
-
-        auth_ids = []
-
-        key = (EventTypes.PowerLevels, "")
-        power_level_event_id = current_state_ids.get(key)
-
-        if power_level_event_id:
-            auth_ids.append(power_level_event_id)
-
-        key = (EventTypes.JoinRules, "")
-        join_rule_event_id = current_state_ids.get(key)
+    def compute_auth_events(
+        self,
+        event,
+        current_state_ids: Dict[Tuple[str, str], str],
+        for_verification: bool = False,
+    ):
+        """Given an event and current state return the list of event IDs used
+        to auth an event.
 
-        key = (EventTypes.Member, event.sender)
-        member_event_id = current_state_ids.get(key)
+        If `for_verification` is False then only return auth events that
+        should be added to the event's `auth_events`.
 
-        key = (EventTypes.Create, "")
-        create_event_id = current_state_ids.get(key)
-        if create_event_id:
-            auth_ids.append(create_event_id)
+        Returns:
+            defer.Deferred(list[str]): List of event IDs.
+        """
 
-        if join_rule_event_id:
-            join_rule_event = yield self.store.get_event(join_rule_event_id)
-            join_rule = join_rule_event.content.get("join_rule")
-            is_public = join_rule == JoinRules.PUBLIC if join_rule else False
-        else:
-            is_public = False
+        if event.type == EventTypes.Create:
+            return defer.succeed([])
+
+        # Currently we ignore the `for_verification` flag even though there are
+        # some situations where we can drop particular auth events when adding
+        # to the event's `auth_events` (e.g. joins pointing to previous joins
+        # when room is publically joinable). Dropping event IDs has the
+        # advantage that the auth chain for the room grows slower, but we use
+        # the auth chain in state resolution v2 to order events, which means
+        # care must be taken if dropping events to ensure that it doesn't
+        # introduce undesirable "state reset" behaviour.
+        #
+        # All of which sounds a bit tricky so we don't bother for now.
 
-        if event.type == EventTypes.Member:
-            e_type = event.content["membership"]
-            if e_type in [Membership.JOIN, Membership.INVITE]:
-                if join_rule_event_id:
-                    auth_ids.append(join_rule_event_id)
+        auth_ids = []
+        for etype, state_key in event_auth.auth_types_for_event(event):
+            auth_ev_id = current_state_ids.get((etype, state_key))
+            if auth_ev_id:
+                auth_ids.append(auth_ev_id)
 
-            if e_type == Membership.JOIN:
-                if member_event_id and not is_public:
-                    auth_ids.append(member_event_id)
-            else:
-                if member_event_id:
-                    auth_ids.append(member_event_id)
-
-                if for_verification:
-                    key = (EventTypes.Member, event.state_key)
-                    existing_event_id = current_state_ids.get(key)
-                    if existing_event_id:
-                        auth_ids.append(existing_event_id)
-
-            if e_type == Membership.INVITE:
-                if "third_party_invite" in event.content:
-                    key = (
-                        EventTypes.ThirdPartyInvite,
-                        event.content["third_party_invite"]["signed"]["token"],
-                    )
-                    third_party_invite_id = current_state_ids.get(key)
-                    if third_party_invite_id:
-                        auth_ids.append(third_party_invite_id)
-        elif member_event_id:
-            member_event = yield self.store.get_event(member_event_id)
-            if member_event.content["membership"] == Membership.JOIN:
-                auth_ids.append(member_event.event_id)
-
-        return auth_ids
+        return defer.succeed(auth_ids)
 
     @defer.inlineCallbacks
     def check_can_change_room_list(self, room_id, user):