Merge branch 'develop' of github.com:matrix-org/synapse into t3chguy/default_inviter_display_name_3pid

1 files changed, 403 insertions, 188 deletions

diff --git a/synapse/state.py b/synapse/state.py
index 390799fbd5..033f55d967 100644
--- a/synapse/state.py
+++ b/synapse/state.py
@@ -14,23 +14,25 @@
 # limitations under the License.
 
 
+import hashlib
+import logging
+from collections import namedtuple
+
+from six import iteritems, iterkeys, itervalues
+
+from frozendict import frozendict
+
 from twisted.internet import defer
 
 from synapse import event_auth
-from synapse.util.logutils import log_function
-from synapse.util.caches.expiringcache import ExpiringCache
-from synapse.util.metrics import Measure
 from synapse.api.constants import EventTypes
 from synapse.api.errors import AuthError
 from synapse.events.snapshot import EventContext
 from synapse.util.async import Linearizer
 from synapse.util.caches import CACHE_SIZE_FACTOR
-
-from collections import namedtuple
-from frozendict import frozendict
-
-import logging
-import hashlib
+from synapse.util.caches.expiringcache import ExpiringCache
+from synapse.util.logutils import log_function
+from synapse.util.metrics import Measure
 
 logger = logging.getLogger(__name__)
 
@@ -58,7 +60,11 @@ class _StateCacheEntry(object):
     __slots__ = ["state", "state_group", "state_id", "prev_group", "delta_ids"]
 
     def __init__(self, state, state_group, prev_group=None, delta_ids=None):
+        # dict[(str, str), str] map  from (type, state_key) to event_id
         self.state = frozendict(state)
+
+        # the ID of a state group if one and only one is involved.
+        # otherwise, None otherwise?
         self.state_group = state_group
 
         self.prev_group = prev_group
@@ -81,31 +87,19 @@ class _StateCacheEntry(object):
 
 
 class StateHandler(object):
-    """ Responsible for doing state conflict resolution.
+    """Fetches bits of state from the stores, and does state resolution
+    where necessary
     """
 
     def __init__(self, hs):
         self.clock = hs.get_clock()
         self.store = hs.get_datastore()
         self.hs = hs
-
-        # dict of set of event_ids -> _StateCacheEntry.
-        self._state_cache = None
-        self.resolve_linearizer = Linearizer(name="state_resolve_lock")
+        self._state_resolution_handler = hs.get_state_resolution_handler()
 
     def start_caching(self):
-        logger.debug("start_caching")
-
-        self._state_cache = ExpiringCache(
-            cache_name="state_cache",
-            clock=self.clock,
-            max_len=SIZE_OF_CACHE,
-            expiry_ms=EVICTION_TIMEOUT_SECONDS * 1000,
-            iterable=True,
-            reset_expiry_on_get=True,
-        )
-
-        self._state_cache.start()
+        # TODO: remove this shim
+        self._state_resolution_handler.start_caching()
 
     @defer.inlineCallbacks
     def get_current_state(self, room_id, event_type=None, state_key="",
@@ -127,7 +121,7 @@ class StateHandler(object):
             latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
 
         logger.debug("calling resolve_state_groups from get_current_state")
-        ret = yield self.resolve_state_groups(room_id, latest_event_ids)
+        ret = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
         state = ret.state
 
         if event_type:
@@ -138,27 +132,36 @@ class StateHandler(object):
             defer.returnValue(event)
             return
 
-        state_map = yield self.store.get_events(state.values(), get_prev_content=False)
+        state_map = yield self.store.get_events(list(state.values()),
+                                                get_prev_content=False)
         state = {
-            key: state_map[e_id] for key, e_id in state.items() if e_id in state_map
+            key: state_map[e_id] for key, e_id in iteritems(state) if e_id in state_map
         }
 
         defer.returnValue(state)
 
     @defer.inlineCallbacks
-    def get_current_state_ids(self, room_id, event_type=None, state_key="",
-                              latest_event_ids=None):
+    def get_current_state_ids(self, room_id, latest_event_ids=None):
+        """Get the current state, or the state at a set of events, for a room
+
+        Args:
+            room_id (str):
+
+            latest_event_ids (iterable[str]|None): if given, the forward
+                extremities to resolve. If None, we look them up from the
+                database (via a cache)
+
+        Returns:
+            Deferred[dict[(str, str), str)]]: the state dict, mapping from
+                (event_type, state_key) -> event_id
+        """
         if not latest_event_ids:
             latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
 
         logger.debug("calling resolve_state_groups from get_current_state_ids")
-        ret = yield self.resolve_state_groups(room_id, latest_event_ids)
+        ret = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
         state = ret.state
 
-        if event_type:
-            defer.returnValue(state.get((event_type, state_key)))
-            return
-
         defer.returnValue(state)
 
     @defer.inlineCallbacks
@@ -166,7 +169,7 @@ class StateHandler(object):
         if not latest_event_ids:
             latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
         logger.debug("calling resolve_state_groups from get_current_user_in_room")
-        entry = yield self.resolve_state_groups(room_id, latest_event_ids)
+        entry = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
         joined_users = yield self.store.get_joined_users_from_state(room_id, entry)
         defer.returnValue(joined_users)
 
@@ -175,7 +178,7 @@ class StateHandler(object):
         if not latest_event_ids:
             latest_event_ids = yield self.store.get_latest_event_ids_in_room(room_id)
         logger.debug("calling resolve_state_groups from get_current_hosts_in_room")
-        entry = yield self.resolve_state_groups(room_id, latest_event_ids)
+        entry = yield self.resolve_state_groups_for_events(room_id, latest_event_ids)
         joined_hosts = yield self.store.get_joined_hosts(room_id, entry)
         defer.returnValue(joined_hosts)
 
@@ -183,8 +186,15 @@ class StateHandler(object):
     def compute_event_context(self, event, old_state=None):
         """Build an EventContext structure for the event.
 
+        This works out what the current state should be for the event, and
+        generates a new state group if necessary.
+
         Args:
             event (synapse.events.EventBase):
+            old_state (dict|None): The state at the event if it can't be
+                calculated from existing events. This is normally only specified
+                when receiving an event from federation where we don't have the
+                prev events for, e.g. when backfilling.
         Returns:
             synapse.events.snapshot.EventContext:
         """
@@ -193,113 +203,158 @@ class StateHandler(object):
             # If this is an outlier, then we know it shouldn't have any current
             # state. Certainly store.get_current_state won't return any, and
             # persisting the event won't store the state group.
-            context = EventContext()
             if old_state:
-                context.prev_state_ids = {
+                prev_state_ids = {
                     (s.type, s.state_key): s.event_id for s in old_state
                 }
                 if event.is_state():
-                    context.current_state_ids = dict(context.prev_state_ids)
+                    current_state_ids = dict(prev_state_ids)
                     key = (event.type, event.state_key)
-                    context.current_state_ids[key] = event.event_id
+                    current_state_ids[key] = event.event_id
                 else:
-                    context.current_state_ids = context.prev_state_ids
+                    current_state_ids = prev_state_ids
             else:
-                context.current_state_ids = {}
-                context.prev_state_ids = {}
-            context.prev_state_events = []
-            context.state_group = self.store.get_next_state_group()
+                current_state_ids = {}
+                prev_state_ids = {}
+
+            # We don't store state for outliers, so we don't generate a state
+            # group for it.
+            context = EventContext.with_state(
+                state_group=None,
+                current_state_ids=current_state_ids,
+                prev_state_ids=prev_state_ids,
+            )
+
             defer.returnValue(context)
 
         if old_state:
-            context = EventContext()
-            context.prev_state_ids = {
+            # We already have the state, so we don't need to calculate it.
+            # Let's just correctly fill out the context and create a
+            # new state group for it.
+
+            prev_state_ids = {
                 (s.type, s.state_key): s.event_id for s in old_state
             }
-            context.state_group = self.store.get_next_state_group()
 
             if event.is_state():
                 key = (event.type, event.state_key)
-                if key in context.prev_state_ids:
-                    replaces = context.prev_state_ids[key]
+                if key in prev_state_ids:
+                    replaces = prev_state_ids[key]
                     if replaces != event.event_id:  # Paranoia check
                         event.unsigned["replaces_state"] = replaces
-                context.current_state_ids = dict(context.prev_state_ids)
-                context.current_state_ids[key] = event.event_id
+                current_state_ids = dict(prev_state_ids)
+                current_state_ids[key] = event.event_id
             else:
-                context.current_state_ids = context.prev_state_ids
+                current_state_ids = prev_state_ids
+
+            state_group = yield self.store.store_state_group(
+                event.event_id,
+                event.room_id,
+                prev_group=None,
+                delta_ids=None,
+                current_state_ids=current_state_ids,
+            )
+
+            context = EventContext.with_state(
+                state_group=state_group,
+                current_state_ids=current_state_ids,
+                prev_state_ids=prev_state_ids,
+            )
 
-            context.prev_state_events = []
             defer.returnValue(context)
 
         logger.debug("calling resolve_state_groups from compute_event_context")
-        entry = yield self.resolve_state_groups(
+        entry = yield self.resolve_state_groups_for_events(
             event.room_id, [e for e, _ in event.prev_events],
         )
 
-        curr_state = entry.state
+        prev_state_ids = entry.state
+        prev_group = None
+        delta_ids = None
 
-        context = EventContext()
-        context.prev_state_ids = curr_state
         if event.is_state():
-            context.state_group = self.store.get_next_state_group()
+            # If this is a state event then we need to create a new state
+            # group for the state after this event.
 
             key = (event.type, event.state_key)
-            if key in context.prev_state_ids:
-                replaces = context.prev_state_ids[key]
+            if key in prev_state_ids:
+                replaces = prev_state_ids[key]
                 event.unsigned["replaces_state"] = replaces
 
-            context.current_state_ids = dict(context.prev_state_ids)
-            context.current_state_ids[key] = event.event_id
+            current_state_ids = dict(prev_state_ids)
+            current_state_ids[key] = event.event_id
 
             if entry.state_group:
-                context.prev_group = entry.state_group
-                context.delta_ids = {
+                # If the state at the event has a state group assigned then
+                # we can use that as the prev group
+                prev_group = entry.state_group
+                delta_ids = {
                     key: event.event_id
                 }
             elif entry.prev_group:
-                context.prev_group = entry.prev_group
-                context.delta_ids = dict(entry.delta_ids)
-                context.delta_ids[key] = event.event_id
+                # If the state at the event only has a prev group, then we can
+                # use that as a prev group too.
+                prev_group = entry.prev_group
+                delta_ids = dict(entry.delta_ids)
+                delta_ids[key] = event.event_id
+
+            state_group = yield self.store.store_state_group(
+                event.event_id,
+                event.room_id,
+                prev_group=prev_group,
+                delta_ids=delta_ids,
+                current_state_ids=current_state_ids,
+            )
         else:
+            current_state_ids = prev_state_ids
+            prev_group = entry.prev_group
+            delta_ids = entry.delta_ids
+
             if entry.state_group is None:
-                entry.state_group = self.store.get_next_state_group()
+                entry.state_group = yield self.store.store_state_group(
+                    event.event_id,
+                    event.room_id,
+                    prev_group=entry.prev_group,
+                    delta_ids=entry.delta_ids,
+                    current_state_ids=current_state_ids,
+                )
                 entry.state_id = entry.state_group
 
-            context.state_group = entry.state_group
-            context.current_state_ids = context.prev_state_ids
-            context.prev_group = entry.prev_group
-            context.delta_ids = entry.delta_ids
+            state_group = entry.state_group
+
+        context = EventContext.with_state(
+            state_group=state_group,
+            current_state_ids=current_state_ids,
+            prev_state_ids=prev_state_ids,
+            prev_group=prev_group,
+            delta_ids=delta_ids,
+        )
 
-        context.prev_state_events = []
         defer.returnValue(context)
 
     @defer.inlineCallbacks
-    @log_function
-    def resolve_state_groups(self, room_id, event_ids):
+    def resolve_state_groups_for_events(self, room_id, event_ids):
         """ Given a list of event_ids this method fetches the state at each
         event, resolves conflicts between them and returns them.
 
+        Args:
+            room_id (str):
+            event_ids (list[str]):
+
         Returns:
-            a Deferred tuple of (`state_group`, `state`, `prev_state`).
-            `state_group` is the name of a state group if one and only one is
-            involved. `state` is a map from (type, state_key) to event, and
-            `prev_state` is a list of event ids.
+            Deferred[_StateCacheEntry]: resolved state
         """
         logger.debug("resolve_state_groups event_ids %s", event_ids)
 
+        # map from state group id to the state in that state group (where
+        # 'state' is a map from state key to event id)
+        # dict[int, dict[(str, str), str]]
         state_groups_ids = yield self.store.get_state_groups_ids(
             room_id, event_ids
         )
 
-        logger.debug(
-            "resolve_state_groups state_groups %s",
-            state_groups_ids.keys()
-        )
-
-        group_names = frozenset(state_groups_ids.keys())
-        if len(group_names) == 1:
-            name, state_list = state_groups_ids.items().pop()
+        if 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)
 
@@ -310,6 +365,102 @@ class StateHandler(object):
                 delta_ids=delta_ids,
             ))
 
+        result = yield self._state_resolution_handler.resolve_state_groups(
+            room_id, state_groups_ids, None, self._state_map_factory,
+        )
+        defer.returnValue(result)
+
+    def _state_map_factory(self, ev_ids):
+        return self.store.get_events(
+            ev_ids, get_prev_content=False, check_redacted=False,
+        )
+
+    def resolve_events(self, state_sets, event):
+        logger.info(
+            "Resolving state for %s with %d groups", event.room_id, len(state_sets)
+        )
+        state_set_ids = [{
+            (ev.type, ev.state_key): ev.event_id
+            for ev in st
+        } for st in state_sets]
+
+        state_map = {
+            ev.event_id: ev
+            for st in state_sets
+            for ev in st
+        }
+
+        with Measure(self.clock, "state._resolve_events"):
+            new_state = resolve_events_with_state_map(state_set_ids, state_map)
+
+        new_state = {
+            key: state_map[ev_id] for key, ev_id in iteritems(new_state)
+        }
+
+        return new_state
+
+
+class StateResolutionHandler(object):
+    """Responsible for doing state conflict resolution.
+
+    Note that the storage layer depends on this handler, so all functions must
+    be storage-independent.
+    """
+    def __init__(self, hs):
+        self.clock = hs.get_clock()
+
+        # dict of set of event_ids -> _StateCacheEntry.
+        self._state_cache = None
+        self.resolve_linearizer = Linearizer(name="state_resolve_lock")
+
+    def start_caching(self):
+        logger.debug("start_caching")
+
+        self._state_cache = ExpiringCache(
+            cache_name="state_cache",
+            clock=self.clock,
+            max_len=SIZE_OF_CACHE,
+            expiry_ms=EVICTION_TIMEOUT_SECONDS * 1000,
+            iterable=True,
+            reset_expiry_on_get=True,
+        )
+
+        self._state_cache.start()
+
+    @defer.inlineCallbacks
+    @log_function
+    def resolve_state_groups(
+        self, room_id, state_groups_ids, event_map, state_map_factory,
+    ):
+        """Resolves conflicts between a set of state groups
+
+        Always generates a new state group (unless we hit the cache), so should
+        not be called for a single state group
+
+        Args:
+            room_id (str): room we are resolving for (used for logging)
+            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)
+
+            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.
+
+        Returns:
+            Deferred[_StateCacheEntry]: resolved state
+        """
+        logger.debug(
+            "resolve_state_groups state_groups %s",
+            state_groups_ids.keys()
+        )
+
+        group_names = frozenset(state_groups_ids.keys())
+
         with (yield self.resolve_linearizer.queue(group_names)):
             if self._state_cache is not None:
                 cache = self._state_cache.get(group_names, None)
@@ -320,112 +471,128 @@ class StateHandler(object):
                 "Resolving state for %s with %d groups", room_id, len(state_groups_ids)
             )
 
-            state = {}
-            for st in state_groups_ids.values():
-                for key, e_id in st.items():
-                    state.setdefault(key, set()).add(e_id)
-
-            conflicted_state = {
-                k: list(v)
-                for k, v in state.items()
-                if len(v) > 1
-            }
+            # start by assuming we won't have any conflicted state, and build up the new
+            # state map by iterating through the state groups. If we discover a conflict,
+            # we give up and instead use `resolve_events_with_factory`.
+            #
+            # XXX: is this actually worthwhile, or should we just let
+            # resolve_events_with_factory do it?
+            new_state = {}
+            conflicted_state = False
+            for st in itervalues(state_groups_ids):
+                for key, e_id in iteritems(st):
+                    if key in new_state:
+                        conflicted_state = True
+                        break
+                    new_state[key] = e_id
+                if conflicted_state:
+                    break
 
             if conflicted_state:
                 logger.info("Resolving conflicted state for %r", room_id)
                 with Measure(self.clock, "state._resolve_events"):
-                    new_state = yield resolve_events(
-                        state_groups_ids.values(),
-                        state_map_factory=lambda ev_ids: self.store.get_events(
-                            ev_ids, get_prev_content=False, check_redacted=False,
-                        ),
+                    new_state = yield resolve_events_with_factory(
+                        list(itervalues(state_groups_ids)),
+                        event_map=event_map,
+                        state_map_factory=state_map_factory,
                     )
-            else:
-                new_state = {
-                    key: e_ids.pop() for key, e_ids in state.items()
-                }
 
-            state_group = None
-            new_state_event_ids = frozenset(new_state.values())
-            for sg, events in state_groups_ids.items():
-                if new_state_event_ids == frozenset(e_id for e_id in events):
-                    state_group = sg
-                    break
+            # if the new state matches any of the input state groups, we can
+            # use that state group again. Otherwise we will generate a state_id
+            # which will be used as a cache key for future resolutions, but
+            # not get persisted.
 
-            # TODO: We want to create a state group for this set of events, to
-            # increase cache hits, but we need to make sure that it doesn't
-            # end up as a prev_group without being added to the database
-
-            prev_group = None
-            delta_ids = None
-            for old_group, old_ids in state_groups_ids.iteritems():
-                if not set(new_state) - set(old_ids):
-                    n_delta_ids = {
-                        k: v
-                        for k, v in new_state.iteritems()
-                        if old_ids.get(k) != v
-                    }
-                    if not delta_ids or len(n_delta_ids) < len(delta_ids):
-                        prev_group = old_group
-                        delta_ids = n_delta_ids
-
-            cache = _StateCacheEntry(
-                state=new_state,
-                state_group=state_group,
-                prev_group=prev_group,
-                delta_ids=delta_ids,
-            )
+            with Measure(self.clock, "state.create_group_ids"):
+                cache = _make_state_cache_entry(new_state, state_groups_ids)
 
             if self._state_cache is not None:
                 self._state_cache[group_names] = cache
 
             defer.returnValue(cache)
 
-    def resolve_events(self, state_sets, event):
-        logger.info(
-            "Resolving state for %s with %d groups", event.room_id, len(state_sets)
-        )
-        state_set_ids = [{
-            (ev.type, ev.state_key): ev.event_id
-            for ev in st
-        } for st in state_sets]
 
-        state_map = {
-            ev.event_id: ev
-            for st in state_sets
-            for ev in st
-        }
+def _make_state_cache_entry(
+    new_state,
+    state_groups_ids,
+):
+    """Given a resolved state, and a set of input state groups, pick one to base
+    a new state group on (if any), and return an appropriately-constructed
+    _StateCacheEntry.
 
-        with Measure(self.clock, "state._resolve_events"):
-            new_state = resolve_events(state_set_ids, state_map)
+    Args:
+        new_state (dict[(str, str), str]): resolved state map (mapping from
+           (type, state_key) to event_id)
 
-        new_state = {
-            key: state_map[ev_id] for key, ev_id in new_state.items()
-        }
+        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)
 
-        return new_state
+    Returns:
+        _StateCacheEntry
+    """
+    # if the new state matches any of the input state groups, we can
+    # use that state group again. Otherwise we will generate a state_id
+    # which will be used as a cache key for future resolutions, but
+    # not get persisted.
+
+    # first look for exact matches
+    new_state_event_ids = set(itervalues(new_state))
+    for sg, state in iteritems(state_groups_ids):
+        if len(new_state_event_ids) != len(state):
+            continue
+
+        old_state_event_ids = set(itervalues(state))
+        if new_state_event_ids == old_state_event_ids:
+            # got an exact match.
+            return _StateCacheEntry(
+                state=new_state,
+                state_group=sg,
+            )
+
+    # TODO: We want to create a state group for this set of events, to
+    # increase cache hits, but we need to make sure that it doesn't
+    # end up as a prev_group without being added to the database
+
+    # failing that, look for the closest match.
+    prev_group = None
+    delta_ids = None
+
+    for old_group, old_state in iteritems(state_groups_ids):
+        n_delta_ids = {
+            k: v
+            for k, v in iteritems(new_state)
+            if old_state.get(k) != v
+        }
+        if not delta_ids or len(n_delta_ids) < len(delta_ids):
+            prev_group = old_group
+            delta_ids = n_delta_ids
+
+    return _StateCacheEntry(
+        state=new_state,
+        state_group=None,
+        prev_group=prev_group,
+        delta_ids=delta_ids,
+    )
 
 
 def _ordered_events(events):
     def key_func(e):
-        return -int(e.depth), hashlib.sha1(e.event_id).hexdigest()
+        return -int(e.depth), hashlib.sha1(e.event_id.encode()).hexdigest()
 
     return sorted(events, key=key_func)
 
 
-def resolve_events(state_sets, state_map_factory):
+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_factory(dict|callable): If callable, then 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. Otherwise, should be
-            a dict from event_id to event of all events in state_sets.
+        state_map(dict): a dict from event_id to event, for all events in
+            state_sets.
 
     Returns
-        dict[(str, str), synapse.events.FrozenEvent] is a map from
-        (type, state_key) to event.
+        dict[(str, str), str]:
+            a map from (type, state_key) to event_id.
     """
     if len(state_sets) == 1:
         return state_sets[0]
@@ -434,13 +601,6 @@ def resolve_events(state_sets, state_map_factory):
         state_sets,
     )
 
-    if callable(state_map_factory):
-        return _resolve_with_state_fac(
-            unconflicted_state, conflicted_state, state_map_factory
-        )
-
-    state_map = state_map_factory
-
     auth_events = _create_auth_events_from_maps(
         unconflicted_state, conflicted_state, state_map
     )
@@ -454,12 +614,28 @@ 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.
     """
-    unconflicted_state = dict(state_sets[0])
+    state_set_iterator = iter(state_sets)
+    unconflicted_state = dict(next(state_set_iterator))
     conflicted_state = {}
 
-    for state_set in state_sets[1:]:
-        for key, value in state_set.iteritems():
+    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:
@@ -484,24 +660,63 @@ def _seperate(state_sets):
 
 
 @defer.inlineCallbacks
-def _resolve_with_state_fac(unconflicted_state, conflicted_state,
-                            state_map_factory):
+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 conflicted_state.itervalues()
+        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(auth_events.itervalues())
+    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))
 
@@ -515,7 +730,7 @@ def _resolve_with_state_fac(unconflicted_state, conflicted_state,
 
 def _create_auth_events_from_maps(unconflicted_state, conflicted_state, state_map):
     auth_events = {}
-    for event_ids in conflicted_state.itervalues():
+    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])
@@ -527,10 +742,10 @@ def _create_auth_events_from_maps(unconflicted_state, conflicted_state, state_ma
     return auth_events
 
 
-def _resolve_with_state(unconflicted_state_ids, conflicted_state_ds, auth_event_ids,
+def _resolve_with_state(unconflicted_state_ids, conflicted_state_ids, auth_event_ids,
                         state_map):
     conflicted_state = {}
-    for key, event_ids in conflicted_state_ds.iteritems():
+    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
@@ -539,7 +754,7 @@ def _resolve_with_state(unconflicted_state_ids, conflicted_state_ds, auth_event_
 
     auth_events = {
         key: state_map[ev_id]
-        for key, ev_id in auth_event_ids.items()
+        for key, ev_id in iteritems(auth_event_ids)
         if ev_id in state_map
     }
 
@@ -547,12 +762,12 @@ def _resolve_with_state(unconflicted_state_ids, conflicted_state_ds, auth_event_
         resolved_state = _resolve_state_events(
             conflicted_state, auth_events
         )
-    except:
+    except Exception:
         logger.exception("Failed to resolve state")
         raise
 
     new_state = unconflicted_state_ids
-    for key, event in resolved_state.iteritems():
+    for key, event in iteritems(resolved_state):
         new_state[key] = event.event_id
 
     return new_state
@@ -577,7 +792,7 @@ def _resolve_state_events(conflicted_state, auth_events):
 
     auth_events.update(resolved_state)
 
-    for key, events in conflicted_state.items():
+    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(
@@ -587,7 +802,7 @@ def _resolve_state_events(conflicted_state, auth_events):
 
     auth_events.update(resolved_state)
 
-    for key, events in conflicted_state.items():
+    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(
@@ -597,7 +812,7 @@ def _resolve_state_events(conflicted_state, auth_events):
 
     auth_events.update(resolved_state)
 
-    for key, events in conflicted_state.items():
+    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(