Merge pull request #3673 from matrix-org/erikj/refactor_state_handler

Refactor state module to support multiple room versions

6 files changed, 388 insertions, 271 deletions

diff --git a/synapse/api/constants.py b/synapse/api/constants.py
index b0da506f6d..912bf024bf 100644
--- a/synapse/api/constants.py
+++ b/synapse/api/constants.py
@@ -97,9 +97,14 @@ class ThirdPartyEntityKind(object):
     LOCATION = "location"
 
 
+class RoomVersions(object):
+    V1 = "1"
+    VDH_TEST = "vdh-test-version"
+
+
 # the version we will give rooms which are created on this server
-DEFAULT_ROOM_VERSION = "1"
+DEFAULT_ROOM_VERSION = RoomVersions.V1
 
 # vdh-test-version is a placeholder to get room versioning support working and tested
 # until we have a working v2.
-KNOWN_ROOM_VERSIONS = {"1", "vdh-test-version"}
+KNOWN_ROOM_VERSIONS = {RoomVersions.V1, RoomVersions.VDH_TEST}
diff --git a/synapse/handlers/federation.py b/synapse/handlers/federation.py
index 3dd107a285..0ebf0fd188 100644
--- a/synapse/handlers/federation.py
+++ b/synapse/handlers/federation.py
@@ -291,8 +291,9 @@ class FederationHandler(BaseHandler):
                             ev_ids, get_prev_content=False, check_redacted=False
                         )
 
+                    room_version = yield self.store.get_room_version(pdu.room_id)
                     state_map = yield resolve_events_with_factory(
-                        state_groups, {pdu.event_id: pdu}, fetch
+                        room_version, state_groups, {pdu.event_id: pdu}, fetch
                     )
 
                     state = (yield self.store.get_events(state_map.values())).values()
@@ -1828,7 +1829,10 @@ class FederationHandler(BaseHandler):
                     (d.type, d.state_key): d for d in different_events if d
                 })
 
+                room_version = yield self.store.get_room_version(event.room_id)
+
                 new_state = self.state_handler.resolve_events(
+                    room_version,
                     [list(local_view.values()), list(remote_view.values())],
                     event
                 )
diff --git a/synapse/handlers/room_member.py b/synapse/handlers/room_member.py
index fb94b5d7d4..f643619047 100644
--- a/synapse/handlers/room_member.py
+++ b/synapse/handlers/room_member.py
@@ -344,6 +344,7 @@ class RoomMemberHandler(object):
         latest_event_ids = (
             event_id for (event_id, _, _) in prev_events_and_hashes
         )
+
         current_state_ids = yield self.state_handler.get_current_state_ids(
             room_id, latest_event_ids=latest_event_ids,
         )
diff --git a/synapse/state.py b/synapse/state/__init__.py
index 0f2bedb694..b34970e4d1 100644
--- a/synapse/state.py
+++ b/synapse/state/__init__.py
@@ -1,5 +1,6 @@
 # -*- coding: utf-8 -*-
 # Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2018 New Vector Ltd
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -13,21 +14,18 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-
-import hashlib
 import logging
 from collections import namedtuple
 
-from six import iteritems, iterkeys, itervalues
+from six import iteritems, itervalues
 
 from frozendict import frozendict
 
 from twisted.internet import defer
 
-from synapse import event_auth
-from synapse.api.constants import EventTypes
-from synapse.api.errors import AuthError
+from synapse.api.constants import EventTypes, RoomVersions
 from synapse.events.snapshot import EventContext
+from synapse.state import v1
 from synapse.util.async_helpers import Linearizer
 from synapse.util.caches import get_cache_factor_for
 from synapse.util.caches.expiringcache import ExpiringCache
@@ -264,6 +262,7 @@ class StateHandler(object):
             defer.returnValue(context)
 
         logger.debug("calling resolve_state_groups from compute_event_context")
+
         entry = yield self.resolve_state_groups_for_events(
             event.room_id, [e for e, _ in event.prev_events],
         )
@@ -338,8 +337,11 @@ class StateHandler(object):
         event, resolves conflicts between them and returns them.
 
         Args:
-            room_id (str):
-            event_ids (list[str]):
+            room_id (str)
+            event_ids (list[str])
+            explicit_room_version (str|None): If set uses the the given room
+                version to choose the resolution algorithm. If None, then
+                checks the database for room version.
 
         Returns:
             Deferred[_StateCacheEntry]: resolved state
@@ -353,7 +355,12 @@ class StateHandler(object):
             room_id, event_ids
         )
 
-        if len(state_groups_ids) == 1:
+        if len(state_groups_ids) == 0:
+            defer.returnValue(_StateCacheEntry(
+                state={},
+                state_group=None,
+            ))
+        elif len(state_groups_ids) == 1:
             name, state_list = list(state_groups_ids.items()).pop()
 
             prev_group, delta_ids = yield self.store.get_state_group_delta(name)
@@ -365,8 +372,11 @@ class StateHandler(object):
                 delta_ids=delta_ids,
             ))
 
+        room_version = yield self.store.get_room_version(room_id)
+
         result = yield self._state_resolution_handler.resolve_state_groups(
-            room_id, state_groups_ids, None, self._state_map_factory,
+            room_id, room_version, state_groups_ids, None,
+            self._state_map_factory,
         )
         defer.returnValue(result)
 
@@ -375,7 +385,7 @@ class StateHandler(object):
             ev_ids, get_prev_content=False, check_redacted=False,
         )
 
-    def resolve_events(self, state_sets, event):
+    def resolve_events(self, room_version, state_sets, event):
         logger.info(
             "Resolving state for %s with %d groups", event.room_id, len(state_sets)
         )
@@ -391,7 +401,9 @@ class StateHandler(object):
         }
 
         with Measure(self.clock, "state._resolve_events"):
-            new_state = resolve_events_with_state_map(state_set_ids, state_map)
+            new_state = resolve_events_with_state_map(
+                room_version, state_set_ids, state_map,
+            )
 
         new_state = {
             key: state_map[ev_id] for key, ev_id in iteritems(new_state)
@@ -430,7 +442,7 @@ class StateResolutionHandler(object):
     @defer.inlineCallbacks
     @log_function
     def resolve_state_groups(
-        self, room_id, state_groups_ids, event_map, state_map_factory,
+        self, room_id, room_version, state_groups_ids, event_map, state_map_factory,
     ):
         """Resolves conflicts between a set of state groups
 
@@ -439,6 +451,7 @@ class StateResolutionHandler(object):
 
         Args:
             room_id (str): room we are resolving for (used for logging)
+            room_version (str): version of the room
             state_groups_ids (dict[int, dict[(str, str), str]]):
                  map from state group id to the state in that state group
                 (where 'state' is a map from state key to event id)
@@ -492,6 +505,7 @@ class StateResolutionHandler(object):
                 logger.info("Resolving conflicted state for %r", room_id)
                 with Measure(self.clock, "state._resolve_events"):
                     new_state = yield resolve_events_with_factory(
+                        room_version,
                         list(itervalues(state_groups_ids)),
                         event_map=event_map,
                         state_map_factory=state_map_factory,
@@ -575,16 +589,10 @@ def _make_state_cache_entry(
     )
 
 
-def _ordered_events(events):
-    def key_func(e):
-        return -int(e.depth), hashlib.sha1(e.event_id.encode('ascii')).hexdigest()
-
-    return sorted(events, key=key_func)
-
-
-def resolve_events_with_state_map(state_sets, state_map):
+def resolve_events_with_state_map(room_version, state_sets, state_map):
     """
     Args:
+        room_version(str): Version of the room
         state_sets(list): List of dicts of (type, state_key) -> event_id,
             which are the different state groups to resolve.
         state_map(dict): a dict from event_id to event, for all events in
@@ -594,75 +602,23 @@ def resolve_events_with_state_map(state_sets, state_map):
         dict[(str, str), str]:
             a map from (type, state_key) to event_id.
     """
-    if len(state_sets) == 1:
-        return state_sets[0]
-
-    unconflicted_state, conflicted_state = _seperate(
-        state_sets,
-    )
-
-    auth_events = _create_auth_events_from_maps(
-        unconflicted_state, conflicted_state, state_map
-    )
-
-    return _resolve_with_state(
-        unconflicted_state, conflicted_state, auth_events, state_map
-    )
-
-
-def _seperate(state_sets):
-    """Takes the state_sets and figures out which keys are conflicted and
-    which aren't. i.e., which have multiple different event_ids associated
-    with them in different state sets.
-
-    Args:
-        state_sets(iterable[dict[(str, str), str]]):
-            List of dicts of (type, state_key) -> event_id, which are the
-            different state groups to resolve.
-
-    Returns:
-        (dict[(str, str), str], dict[(str, str), set[str]]):
-            A tuple of (unconflicted_state, conflicted_state), where:
-
-            unconflicted_state is a dict mapping (type, state_key)->event_id
-            for unconflicted state keys.
-
-            conflicted_state is a dict mapping (type, state_key) to a set of
-            event ids for conflicted state keys.
-    """
-    state_set_iterator = iter(state_sets)
-    unconflicted_state = dict(next(state_set_iterator))
-    conflicted_state = {}
-
-    for state_set in state_set_iterator:
-        for key, value in iteritems(state_set):
-            # Check if there is an unconflicted entry for the state key.
-            unconflicted_value = unconflicted_state.get(key)
-            if unconflicted_value is None:
-                # There isn't an unconflicted entry so check if there is a
-                # conflicted entry.
-                ls = conflicted_state.get(key)
-                if ls is None:
-                    # There wasn't a conflicted entry so haven't seen this key before.
-                    # Therefore it isn't conflicted yet.
-                    unconflicted_state[key] = value
-                else:
-                    # This key is already conflicted, add our value to the conflict set.
-                    ls.add(value)
-            elif unconflicted_value != value:
-                # If the unconflicted value is not the same as our value then we
-                # have a new conflict. So move the key from the unconflicted_state
-                # to the conflicted state.
-                conflicted_state[key] = {value, unconflicted_value}
-                unconflicted_state.pop(key, None)
-
-    return unconflicted_state, conflicted_state
+    if room_version in (RoomVersions.V1, RoomVersions.VDH_TEST,):
+        return v1.resolve_events_with_state_map(
+            state_sets, state_map,
+        )
+    else:
+        # This should only happen if we added a version but forgot to add it to
+        # the list above.
+        raise Exception(
+            "No state resolution algorithm defined for version %r" % (room_version,)
+        )
 
 
-@defer.inlineCallbacks
-def resolve_events_with_factory(state_sets, event_map, state_map_factory):
+def resolve_events_with_factory(room_version, state_sets, event_map, state_map_factory):
     """
     Args:
+        room_version(str): Version of the room
+
         state_sets(list): List of dicts of (type, state_key) -> event_id,
             which are the different state groups to resolve.
 
@@ -682,185 +638,13 @@ def resolve_events_with_factory(state_sets, event_map, state_map_factory):
         Deferred[dict[(str, str), str]]:
             a map from (type, state_key) to event_id.
     """
-    if len(state_sets) == 1:
-        defer.returnValue(state_sets[0])
-
-    unconflicted_state, conflicted_state = _seperate(
-        state_sets,
-    )
-
-    needed_events = set(
-        event_id
-        for event_ids in itervalues(conflicted_state)
-        for event_id in event_ids
-    )
-    if event_map is not None:
-        needed_events -= set(iterkeys(event_map))
-
-    logger.info("Asking for %d conflicted events", len(needed_events))
-
-    # dict[str, FrozenEvent]: a map from state event id to event. Only includes
-    # the state events which are in conflict (and those in event_map)
-    state_map = yield state_map_factory(needed_events)
-    if event_map is not None:
-        state_map.update(event_map)
-
-    # get the ids of the auth events which allow us to authenticate the
-    # conflicted state, picking only from the unconflicting state.
-    #
-    # dict[(str, str), str]: a map from state key to event id
-    auth_events = _create_auth_events_from_maps(
-        unconflicted_state, conflicted_state, state_map
-    )
-
-    new_needed_events = set(itervalues(auth_events))
-    new_needed_events -= needed_events
-    if event_map is not None:
-        new_needed_events -= set(iterkeys(event_map))
-
-    logger.info("Asking for %d auth events", len(new_needed_events))
-
-    state_map_new = yield state_map_factory(new_needed_events)
-    state_map.update(state_map_new)
-
-    defer.returnValue(_resolve_with_state(
-        unconflicted_state, conflicted_state, auth_events, state_map
-    ))
-
-
-def _create_auth_events_from_maps(unconflicted_state, conflicted_state, state_map):
-    auth_events = {}
-    for event_ids in itervalues(conflicted_state):
-        for event_id in event_ids:
-            if event_id in state_map:
-                keys = event_auth.auth_types_for_event(state_map[event_id])
-                for key in keys:
-                    if key not in auth_events:
-                        event_id = unconflicted_state.get(key, None)
-                        if event_id:
-                            auth_events[key] = event_id
-    return auth_events
-
-
-def _resolve_with_state(unconflicted_state_ids, conflicted_state_ids, auth_event_ids,
-                        state_map):
-    conflicted_state = {}
-    for key, event_ids in iteritems(conflicted_state_ids):
-        events = [state_map[ev_id] for ev_id in event_ids if ev_id in state_map]
-        if len(events) > 1:
-            conflicted_state[key] = events
-        elif len(events) == 1:
-            unconflicted_state_ids[key] = events[0].event_id
-
-    auth_events = {
-        key: state_map[ev_id]
-        for key, ev_id in iteritems(auth_event_ids)
-        if ev_id in state_map
-    }
-
-    try:
-        resolved_state = _resolve_state_events(
-            conflicted_state, auth_events
+    if room_version in (RoomVersions.V1, RoomVersions.VDH_TEST,):
+        return v1.resolve_events_with_factory(
+            state_sets, event_map, state_map_factory,
+        )
+    else:
+        # This should only happen if we added a version but forgot to add it to
+        # the list above.
+        raise Exception(
+            "No state resolution algorithm defined for version %r" % (room_version,)
         )
-    except Exception:
-        logger.exception("Failed to resolve state")
-        raise
-
-    new_state = unconflicted_state_ids
-    for key, event in iteritems(resolved_state):
-        new_state[key] = event.event_id
-
-    return new_state
-
-
-def _resolve_state_events(conflicted_state, auth_events):
-    """ This is where we actually decide which of the conflicted state to
-    use.
-
-    We resolve conflicts in the following order:
-        1. power levels
-        2. join rules
-        3. memberships
-        4. other events.
-    """
-    resolved_state = {}
-    if POWER_KEY in conflicted_state:
-        events = conflicted_state[POWER_KEY]
-        logger.debug("Resolving conflicted power levels %r", events)
-        resolved_state[POWER_KEY] = _resolve_auth_events(
-            events, auth_events)
-
-    auth_events.update(resolved_state)
-
-    for key, events in iteritems(conflicted_state):
-        if key[0] == EventTypes.JoinRules:
-            logger.debug("Resolving conflicted join rules %r", events)
-            resolved_state[key] = _resolve_auth_events(
-                events,
-                auth_events
-            )
-
-    auth_events.update(resolved_state)
-
-    for key, events in iteritems(conflicted_state):
-        if key[0] == EventTypes.Member:
-            logger.debug("Resolving conflicted member lists %r", events)
-            resolved_state[key] = _resolve_auth_events(
-                events,
-                auth_events
-            )
-
-    auth_events.update(resolved_state)
-
-    for key, events in iteritems(conflicted_state):
-        if key not in resolved_state:
-            logger.debug("Resolving conflicted state %r:%r", key, events)
-            resolved_state[key] = _resolve_normal_events(
-                events, auth_events
-            )
-
-    return resolved_state
-
-
-def _resolve_auth_events(events, auth_events):
-    reverse = [i for i in reversed(_ordered_events(events))]
-
-    auth_keys = set(
-        key
-        for event in events
-        for key in event_auth.auth_types_for_event(event)
-    )
-
-    new_auth_events = {}
-    for key in auth_keys:
-        auth_event = auth_events.get(key, None)
-        if auth_event:
-            new_auth_events[key] = auth_event
-
-    auth_events = new_auth_events
-
-    prev_event = reverse[0]
-    for event in reverse[1:]:
-        auth_events[(prev_event.type, prev_event.state_key)] = prev_event
-        try:
-            # The signatures have already been checked at this point
-            event_auth.check(event, auth_events, do_sig_check=False, do_size_check=False)
-            prev_event = event
-        except AuthError:
-            return prev_event
-
-    return event
-
-
-def _resolve_normal_events(events, auth_events):
-    for event in _ordered_events(events):
-        try:
-            # The signatures have already been checked at this point
-            event_auth.check(event, auth_events, do_sig_check=False, do_size_check=False)
-            return event
-        except AuthError:
-            pass
-
-    # Use the last event (the one with the least depth) if they all fail
-    # the auth check.
-    return event
diff --git a/synapse/state/v1.py b/synapse/state/v1.py
new file mode 100644
index 0000000000..3a1f7054a1
--- /dev/null
+++ b/synapse/state/v1.py
@@ -0,0 +1,321 @@
+# -*- coding: utf-8 -*-
+# Copyright 2018 New Vector Ltd
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import hashlib
+import logging
+
+from six import iteritems, iterkeys, itervalues
+
+from twisted.internet import defer
+
+from synapse import event_auth
+from synapse.api.constants import EventTypes
+from synapse.api.errors import AuthError
+
+logger = logging.getLogger(__name__)
+
+
+POWER_KEY = (EventTypes.PowerLevels, "")
+
+
+def resolve_events_with_state_map(state_sets, state_map):
+    """
+    Args:
+        state_sets(list): List of dicts of (type, state_key) -> event_id,
+            which are the different state groups to resolve.
+        state_map(dict): a dict from event_id to event, for all events in
+            state_sets.
+
+    Returns
+        dict[(str, str), str]:
+            a map from (type, state_key) to event_id.
+    """
+    if len(state_sets) == 1:
+        return state_sets[0]
+
+    unconflicted_state, conflicted_state = _seperate(
+        state_sets,
+    )
+
+    auth_events = _create_auth_events_from_maps(
+        unconflicted_state, conflicted_state, state_map
+    )
+
+    return _resolve_with_state(
+        unconflicted_state, conflicted_state, auth_events, state_map
+    )
+
+
+@defer.inlineCallbacks
+def resolve_events_with_factory(state_sets, event_map, state_map_factory):
+    """
+    Args:
+        state_sets(list): List of dicts of (type, state_key) -> event_id,
+            which are the different state groups to resolve.
+
+        event_map(dict[str,FrozenEvent]|None):
+            a dict from event_id to event, for any events that we happen to
+            have in flight (eg, those currently being persisted). This will be
+            used as a starting point fof finding the state we need; any missing
+            events will be requested via state_map_factory.
+
+            If None, all events will be fetched via state_map_factory.
+
+        state_map_factory(func): will be called
+            with a list of event_ids that are needed, and should return with
+            a Deferred of dict of event_id to event.
+
+    Returns
+        Deferred[dict[(str, str), str]]:
+            a map from (type, state_key) to event_id.
+    """
+    if len(state_sets) == 1:
+        defer.returnValue(state_sets[0])
+
+    unconflicted_state, conflicted_state = _seperate(
+        state_sets,
+    )
+
+    needed_events = set(
+        event_id
+        for event_ids in itervalues(conflicted_state)
+        for event_id in event_ids
+    )
+    if event_map is not None:
+        needed_events -= set(iterkeys(event_map))
+
+    logger.info("Asking for %d conflicted events", len(needed_events))
+
+    # dict[str, FrozenEvent]: a map from state event id to event. Only includes
+    # the state events which are in conflict (and those in event_map)
+    state_map = yield state_map_factory(needed_events)
+    if event_map is not None:
+        state_map.update(event_map)
+
+    # get the ids of the auth events which allow us to authenticate the
+    # conflicted state, picking only from the unconflicting state.
+    #
+    # dict[(str, str), str]: a map from state key to event id
+    auth_events = _create_auth_events_from_maps(
+        unconflicted_state, conflicted_state, state_map
+    )
+
+    new_needed_events = set(itervalues(auth_events))
+    new_needed_events -= needed_events
+    if event_map is not None:
+        new_needed_events -= set(iterkeys(event_map))
+
+    logger.info("Asking for %d auth events", len(new_needed_events))
+
+    state_map_new = yield state_map_factory(new_needed_events)
+    state_map.update(state_map_new)
+
+    defer.returnValue(_resolve_with_state(
+        unconflicted_state, conflicted_state, auth_events, state_map
+    ))
+
+
+def _seperate(state_sets):
+    """Takes the state_sets and figures out which keys are conflicted and
+    which aren't. i.e., which have multiple different event_ids associated
+    with them in different state sets.
+
+    Args:
+        state_sets(iterable[dict[(str, str), str]]):
+            List of dicts of (type, state_key) -> event_id, which are the
+            different state groups to resolve.
+
+    Returns:
+        (dict[(str, str), str], dict[(str, str), set[str]]):
+            A tuple of (unconflicted_state, conflicted_state), where:
+
+            unconflicted_state is a dict mapping (type, state_key)->event_id
+            for unconflicted state keys.
+
+            conflicted_state is a dict mapping (type, state_key) to a set of
+            event ids for conflicted state keys.
+    """
+    state_set_iterator = iter(state_sets)
+    unconflicted_state = dict(next(state_set_iterator))
+    conflicted_state = {}
+
+    for state_set in state_set_iterator:
+        for key, value in iteritems(state_set):
+            # Check if there is an unconflicted entry for the state key.
+            unconflicted_value = unconflicted_state.get(key)
+            if unconflicted_value is None:
+                # There isn't an unconflicted entry so check if there is a
+                # conflicted entry.
+                ls = conflicted_state.get(key)
+                if ls is None:
+                    # There wasn't a conflicted entry so haven't seen this key before.
+                    # Therefore it isn't conflicted yet.
+                    unconflicted_state[key] = value
+                else:
+                    # This key is already conflicted, add our value to the conflict set.
+                    ls.add(value)
+            elif unconflicted_value != value:
+                # If the unconflicted value is not the same as our value then we
+                # have a new conflict. So move the key from the unconflicted_state
+                # to the conflicted state.
+                conflicted_state[key] = {value, unconflicted_value}
+                unconflicted_state.pop(key, None)
+
+    return unconflicted_state, conflicted_state
+
+
+def _create_auth_events_from_maps(unconflicted_state, conflicted_state, state_map):
+    auth_events = {}
+    for event_ids in itervalues(conflicted_state):
+        for event_id in event_ids:
+            if event_id in state_map:
+                keys = event_auth.auth_types_for_event(state_map[event_id])
+                for key in keys:
+                    if key not in auth_events:
+                        event_id = unconflicted_state.get(key, None)
+                        if event_id:
+                            auth_events[key] = event_id
+    return auth_events
+
+
+def _resolve_with_state(unconflicted_state_ids, conflicted_state_ids, auth_event_ids,
+                        state_map):
+    conflicted_state = {}
+    for key, event_ids in iteritems(conflicted_state_ids):
+        events = [state_map[ev_id] for ev_id in event_ids if ev_id in state_map]
+        if len(events) > 1:
+            conflicted_state[key] = events
+        elif len(events) == 1:
+            unconflicted_state_ids[key] = events[0].event_id
+
+    auth_events = {
+        key: state_map[ev_id]
+        for key, ev_id in iteritems(auth_event_ids)
+        if ev_id in state_map
+    }
+
+    try:
+        resolved_state = _resolve_state_events(
+            conflicted_state, auth_events
+        )
+    except Exception:
+        logger.exception("Failed to resolve state")
+        raise
+
+    new_state = unconflicted_state_ids
+    for key, event in iteritems(resolved_state):
+        new_state[key] = event.event_id
+
+    return new_state
+
+
+def _resolve_state_events(conflicted_state, auth_events):
+    """ This is where we actually decide which of the conflicted state to
+    use.
+
+    We resolve conflicts in the following order:
+        1. power levels
+        2. join rules
+        3. memberships
+        4. other events.
+    """
+    resolved_state = {}
+    if POWER_KEY in conflicted_state:
+        events = conflicted_state[POWER_KEY]
+        logger.debug("Resolving conflicted power levels %r", events)
+        resolved_state[POWER_KEY] = _resolve_auth_events(
+            events, auth_events)
+
+    auth_events.update(resolved_state)
+
+    for key, events in iteritems(conflicted_state):
+        if key[0] == EventTypes.JoinRules:
+            logger.debug("Resolving conflicted join rules %r", events)
+            resolved_state[key] = _resolve_auth_events(
+                events,
+                auth_events
+            )
+
+    auth_events.update(resolved_state)
+
+    for key, events in iteritems(conflicted_state):
+        if key[0] == EventTypes.Member:
+            logger.debug("Resolving conflicted member lists %r", events)
+            resolved_state[key] = _resolve_auth_events(
+                events,
+                auth_events
+            )
+
+    auth_events.update(resolved_state)
+
+    for key, events in iteritems(conflicted_state):
+        if key not in resolved_state:
+            logger.debug("Resolving conflicted state %r:%r", key, events)
+            resolved_state[key] = _resolve_normal_events(
+                events, auth_events
+            )
+
+    return resolved_state
+
+
+def _resolve_auth_events(events, auth_events):
+    reverse = [i for i in reversed(_ordered_events(events))]
+
+    auth_keys = set(
+        key
+        for event in events
+        for key in event_auth.auth_types_for_event(event)
+    )
+
+    new_auth_events = {}
+    for key in auth_keys:
+        auth_event = auth_events.get(key, None)
+        if auth_event:
+            new_auth_events[key] = auth_event
+
+    auth_events = new_auth_events
+
+    prev_event = reverse[0]
+    for event in reverse[1:]:
+        auth_events[(prev_event.type, prev_event.state_key)] = prev_event
+        try:
+            # The signatures have already been checked at this point
+            event_auth.check(event, auth_events, do_sig_check=False, do_size_check=False)
+            prev_event = event
+        except AuthError:
+            return prev_event
+
+    return event
+
+
+def _resolve_normal_events(events, auth_events):
+    for event in _ordered_events(events):
+        try:
+            # The signatures have already been checked at this point
+            event_auth.check(event, auth_events, do_sig_check=False, do_size_check=False)
+            return event
+        except AuthError:
+            pass
+
+    # Use the last event (the one with the least depth) if they all fail
+    # the auth check.
+    return event
+
+
+def _ordered_events(events):
+    def key_func(e):
+        return -int(e.depth), hashlib.sha1(e.event_id.encode('ascii')).hexdigest()
+
+    return sorted(events, key=key_func)
diff --git a/synapse/storage/events.py b/synapse/storage/events.py
index 025a7fb6d9..f39c8c8461 100644
--- a/synapse/storage/events.py
+++ b/synapse/storage/events.py
@@ -705,9 +705,11 @@ class EventsStore(EventFederationStore, EventsWorkerStore, BackgroundUpdateStore
         }
 
         events_map = {ev.event_id: ev for ev, _ in events_context}
+        room_version = yield self.get_room_version(room_id)
+
         logger.debug("calling resolve_state_groups from preserve_events")
         res = yield self._state_resolution_handler.resolve_state_groups(
-            room_id, state_groups, events_map, get_events
+            room_id, room_version, state_groups, events_map, get_events
         )
 
         defer.returnValue((res.state, None))