Rename storage classes (#12913)

1 files changed, 1124 insertions, 0 deletions

diff --git a/synapse/storage/controllers/persist_events.py b/synapse/storage/controllers/persist_events.py
new file mode 100644
index 0000000000..ef8c135b12
--- /dev/null
+++ b/synapse/storage/controllers/persist_events.py
@@ -0,0 +1,1124 @@
+# Copyright 2014-2016 OpenMarket Ltd
+# Copyright 2018-2019 New Vector Ltd
+# Copyright 2019 The Matrix.org Foundation C.I.C.
+#
+# 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 itertools
+import logging
+from collections import deque
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Awaitable,
+    Callable,
+    Collection,
+    Deque,
+    Dict,
+    Generator,
+    Generic,
+    Iterable,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    TypeVar,
+)
+
+import attr
+from prometheus_client import Counter, Histogram
+
+from twisted.internet import defer
+
+from synapse.api.constants import EventTypes, Membership
+from synapse.events import EventBase
+from synapse.events.snapshot import EventContext
+from synapse.logging import opentracing
+from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
+from synapse.metrics.background_process_metrics import run_as_background_process
+from synapse.storage.databases import Databases
+from synapse.storage.databases.main.events import DeltaState
+from synapse.storage.databases.main.events_worker import EventRedactBehaviour
+from synapse.types import (
+    PersistedEventPosition,
+    RoomStreamToken,
+    StateMap,
+    get_domain_from_id,
+)
+from synapse.util.async_helpers import ObservableDeferred, yieldable_gather_results
+from synapse.util.metrics import Measure
+
+if TYPE_CHECKING:
+    from synapse.server import HomeServer
+
+logger = logging.getLogger(__name__)
+
+# The number of times we are recalculating the current state
+state_delta_counter = Counter("synapse_storage_events_state_delta", "")
+
+# The number of times we are recalculating state when there is only a
+# single forward extremity
+state_delta_single_event_counter = Counter(
+    "synapse_storage_events_state_delta_single_event", ""
+)
+
+# The number of times we are reculating state when we could have resonably
+# calculated the delta when we calculated the state for an event we were
+# persisting.
+state_delta_reuse_delta_counter = Counter(
+    "synapse_storage_events_state_delta_reuse_delta", ""
+)
+
+# The number of forward extremities for each new event.
+forward_extremities_counter = Histogram(
+    "synapse_storage_events_forward_extremities_persisted",
+    "Number of forward extremities for each new event",
+    buckets=(1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"),
+)
+
+# The number of stale forward extremities for each new event. Stale extremities
+# are those that were in the previous set of extremities as well as the new.
+stale_forward_extremities_counter = Histogram(
+    "synapse_storage_events_stale_forward_extremities_persisted",
+    "Number of unchanged forward extremities for each new event",
+    buckets=(0, 1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"),
+)
+
+state_resolutions_during_persistence = Counter(
+    "synapse_storage_events_state_resolutions_during_persistence",
+    "Number of times we had to do state res to calculate new current state",
+)
+
+potential_times_prune_extremities = Counter(
+    "synapse_storage_events_potential_times_prune_extremities",
+    "Number of times we might be able to prune extremities",
+)
+
+times_pruned_extremities = Counter(
+    "synapse_storage_events_times_pruned_extremities",
+    "Number of times we were actually be able to prune extremities",
+)
+
+
+@attr.s(auto_attribs=True, slots=True)
+class _EventPersistQueueItem:
+    events_and_contexts: List[Tuple[EventBase, EventContext]]
+    backfilled: bool
+    deferred: ObservableDeferred
+
+    parent_opentracing_span_contexts: List = attr.ib(factory=list)
+    """A list of opentracing spans waiting for this batch"""
+
+    opentracing_span_context: Any = None
+    """The opentracing span under which the persistence actually happened"""
+
+
+_PersistResult = TypeVar("_PersistResult")
+
+
+class _EventPeristenceQueue(Generic[_PersistResult]):
+    """Queues up events so that they can be persisted in bulk with only one
+    concurrent transaction per room.
+    """
+
+    def __init__(
+        self,
+        per_item_callback: Callable[
+            [List[Tuple[EventBase, EventContext]], bool],
+            Awaitable[_PersistResult],
+        ],
+    ):
+        """Create a new event persistence queue
+
+        The per_item_callback will be called for each item added via add_to_queue,
+        and its result will be returned via the Deferreds returned from add_to_queue.
+        """
+        self._event_persist_queues: Dict[str, Deque[_EventPersistQueueItem]] = {}
+        self._currently_persisting_rooms: Set[str] = set()
+        self._per_item_callback = per_item_callback
+
+    async def add_to_queue(
+        self,
+        room_id: str,
+        events_and_contexts: Iterable[Tuple[EventBase, EventContext]],
+        backfilled: bool,
+    ) -> _PersistResult:
+        """Add events to the queue, with the given persist_event options.
+
+        If we are not already processing events in this room, starts off a background
+        process to to so, calling the per_item_callback for each item.
+
+        Args:
+            room_id (str):
+            events_and_contexts (list[(EventBase, EventContext)]):
+            backfilled (bool):
+
+        Returns:
+            the result returned by the `_per_item_callback` passed to
+            `__init__`.
+        """
+        queue = self._event_persist_queues.setdefault(room_id, deque())
+
+        # if the last item in the queue has the same `backfilled` setting,
+        # we can just add these new events to that item.
+        if queue and queue[-1].backfilled == backfilled:
+            end_item = queue[-1]
+        else:
+            # need to make a new queue item
+            deferred: ObservableDeferred[_PersistResult] = ObservableDeferred(
+                defer.Deferred(), consumeErrors=True
+            )
+
+            end_item = _EventPersistQueueItem(
+                events_and_contexts=[],
+                backfilled=backfilled,
+                deferred=deferred,
+            )
+            queue.append(end_item)
+
+        # add our events to the queue item
+        end_item.events_and_contexts.extend(events_and_contexts)
+
+        # also add our active opentracing span to the item so that we get a link back
+        span = opentracing.active_span()
+        if span:
+            end_item.parent_opentracing_span_contexts.append(span.context)
+
+        # start a processor for the queue, if there isn't one already
+        self._handle_queue(room_id)
+
+        # wait for the queue item to complete
+        res = await make_deferred_yieldable(end_item.deferred.observe())
+
+        # add another opentracing span which links to the persist trace.
+        with opentracing.start_active_span_follows_from(
+            "persist_event_batch_complete", (end_item.opentracing_span_context,)
+        ):
+            pass
+
+        return res
+
+    def _handle_queue(self, room_id: str) -> None:
+        """Attempts to handle the queue for a room if not already being handled.
+
+        The queue's callback will be invoked with for each item in the queue,
+        of type _EventPersistQueueItem. The per_item_callback will continuously
+        be called with new items, unless the queue becomes empty. The return
+        value of the function will be given to the deferreds waiting on the item,
+        exceptions will be passed to the deferreds as well.
+
+        This function should therefore be called whenever anything is added
+        to the queue.
+
+        If another callback is currently handling the queue then it will not be
+        invoked.
+        """
+        if room_id in self._currently_persisting_rooms:
+            return
+
+        self._currently_persisting_rooms.add(room_id)
+
+        async def handle_queue_loop() -> None:
+            try:
+                queue = self._get_drainining_queue(room_id)
+                for item in queue:
+                    try:
+                        with opentracing.start_active_span_follows_from(
+                            "persist_event_batch",
+                            item.parent_opentracing_span_contexts,
+                            inherit_force_tracing=True,
+                        ) as scope:
+                            if scope:
+                                item.opentracing_span_context = scope.span.context
+
+                            ret = await self._per_item_callback(
+                                item.events_and_contexts, item.backfilled
+                            )
+                    except Exception:
+                        with PreserveLoggingContext():
+                            item.deferred.errback()
+                    else:
+                        with PreserveLoggingContext():
+                            item.deferred.callback(ret)
+            finally:
+                remaining_queue = self._event_persist_queues.pop(room_id, None)
+                if remaining_queue:
+                    self._event_persist_queues[room_id] = remaining_queue
+                self._currently_persisting_rooms.discard(room_id)
+
+        # set handle_queue_loop off in the background
+        run_as_background_process("persist_events", handle_queue_loop)
+
+    def _get_drainining_queue(
+        self, room_id: str
+    ) -> Generator[_EventPersistQueueItem, None, None]:
+        queue = self._event_persist_queues.setdefault(room_id, deque())
+
+        try:
+            while True:
+                yield queue.popleft()
+        except IndexError:
+            # Queue has been drained.
+            pass
+
+
+class EventsPersistenceStorageController:
+    """High level interface for handling persisting newly received events.
+
+    Takes care of batching up events by room, and calculating the necessary
+    current state and forward extremity changes.
+    """
+
+    def __init__(self, hs: "HomeServer", stores: Databases):
+        # We ultimately want to split out the state store from the main store,
+        # so we use separate variables here even though they point to the same
+        # store for now.
+        self.main_store = stores.main
+        self.state_store = stores.state
+
+        assert stores.persist_events
+        self.persist_events_store = stores.persist_events
+
+        self._clock = hs.get_clock()
+        self._instance_name = hs.get_instance_name()
+        self.is_mine_id = hs.is_mine_id
+        self._event_persist_queue = _EventPeristenceQueue(self._persist_event_batch)
+        self._state_resolution_handler = hs.get_state_resolution_handler()
+
+    @opentracing.trace
+    async def persist_events(
+        self,
+        events_and_contexts: Iterable[Tuple[EventBase, EventContext]],
+        backfilled: bool = False,
+    ) -> Tuple[List[EventBase], RoomStreamToken]:
+        """
+        Write events to the database
+        Args:
+            events_and_contexts: list of tuples of (event, context)
+            backfilled: Whether the results are retrieved from federation
+                via backfill or not. Used to determine if they're "new" events
+                which might update the current state etc.
+
+        Returns:
+            List of events persisted, the current position room stream position.
+            The list of events persisted may not be the same as those passed in
+            if they were deduplicated due to an event already existing that
+            matched the transaction ID; the existing event is returned in such
+            a case.
+        """
+        partitioned: Dict[str, List[Tuple[EventBase, EventContext]]] = {}
+        for event, ctx in events_and_contexts:
+            partitioned.setdefault(event.room_id, []).append((event, ctx))
+
+        async def enqueue(
+            item: Tuple[str, List[Tuple[EventBase, EventContext]]]
+        ) -> Dict[str, str]:
+            room_id, evs_ctxs = item
+            return await self._event_persist_queue.add_to_queue(
+                room_id, evs_ctxs, backfilled=backfilled
+            )
+
+        ret_vals = await yieldable_gather_results(enqueue, partitioned.items())
+
+        # Each call to add_to_queue returns a map from event ID to existing event ID if
+        # the event was deduplicated. (The dict may also include other entries if
+        # the event was persisted in a batch with other events).
+        #
+        # Since we use `yieldable_gather_results` we need to merge the returned list
+        # of dicts into one.
+        replaced_events: Dict[str, str] = {}
+        for d in ret_vals:
+            replaced_events.update(d)
+
+        events = []
+        for event, _ in events_and_contexts:
+            existing_event_id = replaced_events.get(event.event_id)
+            if existing_event_id:
+                events.append(await self.main_store.get_event(existing_event_id))
+            else:
+                events.append(event)
+
+        return (
+            events,
+            self.main_store.get_room_max_token(),
+        )
+
+    @opentracing.trace
+    async def persist_event(
+        self, event: EventBase, context: EventContext, backfilled: bool = False
+    ) -> Tuple[EventBase, PersistedEventPosition, RoomStreamToken]:
+        """
+        Returns:
+            The event, stream ordering of `event`, and the stream ordering of the
+            latest persisted event. The returned event may not match the given
+            event if it was deduplicated due to an existing event matching the
+            transaction ID.
+        """
+        # add_to_queue returns a map from event ID to existing event ID if the
+        # event was deduplicated. (The dict may also include other entries if
+        # the event was persisted in a batch with other events.)
+        replaced_events = await self._event_persist_queue.add_to_queue(
+            event.room_id, [(event, context)], backfilled=backfilled
+        )
+        replaced_event = replaced_events.get(event.event_id)
+        if replaced_event:
+            event = await self.main_store.get_event(replaced_event)
+
+        event_stream_id = event.internal_metadata.stream_ordering
+        # stream ordering should have been assigned by now
+        assert event_stream_id
+
+        pos = PersistedEventPosition(self._instance_name, event_stream_id)
+        return event, pos, self.main_store.get_room_max_token()
+
+    async def update_current_state(self, room_id: str) -> None:
+        """Recalculate the current state for a room, and persist it"""
+        state = await self._calculate_current_state(room_id)
+        delta = await self._calculate_state_delta(room_id, state)
+
+        # TODO(faster_joins): get a real stream ordering, to make this work correctly
+        #    across workers.
+        #
+        # TODO(faster_joins): this can race against event persistence, in which case we
+        #    will end up with incorrect state. Perhaps we should make this a job we
+        #    farm out to the event persister, somehow.
+        stream_id = self.main_store.get_room_max_stream_ordering()
+        await self.persist_events_store.update_current_state(room_id, delta, stream_id)
+
+    async def _calculate_current_state(self, room_id: str) -> StateMap[str]:
+        """Calculate the current state of a room, based on the forward extremities
+
+        Args:
+            room_id: room for which to calculate current state
+
+        Returns:
+            map from (type, state_key) to event id for the  current state in the room
+        """
+        latest_event_ids = await self.main_store.get_latest_event_ids_in_room(room_id)
+        state_groups = set(
+            (
+                await self.main_store._get_state_group_for_events(latest_event_ids)
+            ).values()
+        )
+
+        state_maps_by_state_group = await self.state_store._get_state_for_groups(
+            state_groups
+        )
+
+        if len(state_groups) == 1:
+            # If there is only one state group, then we know what the current
+            # state is.
+            return state_maps_by_state_group[state_groups.pop()]
+
+        # Ok, we need to defer to the state handler to resolve our state sets.
+        logger.debug("calling resolve_state_groups from preserve_events")
+
+        # Avoid a circular import.
+        from synapse.state import StateResolutionStore
+
+        room_version = await self.main_store.get_room_version_id(room_id)
+        res = await self._state_resolution_handler.resolve_state_groups(
+            room_id,
+            room_version,
+            state_maps_by_state_group,
+            event_map=None,
+            state_res_store=StateResolutionStore(self.main_store),
+        )
+
+        return res.state
+
+    async def _persist_event_batch(
+        self,
+        events_and_contexts: List[Tuple[EventBase, EventContext]],
+        backfilled: bool = False,
+    ) -> Dict[str, str]:
+        """Callback for the _event_persist_queue
+
+        Calculates the change to current state and forward extremities, and
+        persists the given events and with those updates.
+
+        Returns:
+            A dictionary of event ID to event ID we didn't persist as we already
+            had another event persisted with the same TXN ID.
+        """
+        replaced_events: Dict[str, str] = {}
+        if not events_and_contexts:
+            return replaced_events
+
+        # Check if any of the events have a transaction ID that has already been
+        # persisted, and if so we don't persist it again.
+        #
+        # We should have checked this a long time before we get here, but it's
+        # possible that different send event requests race in such a way that
+        # they both pass the earlier checks. Checking here isn't racey as we can
+        # have only one `_persist_events` per room being called at a time.
+        replaced_events = await self.main_store.get_already_persisted_events(
+            (event for event, _ in events_and_contexts)
+        )
+
+        if replaced_events:
+            events_and_contexts = [
+                (e, ctx)
+                for e, ctx in events_and_contexts
+                if e.event_id not in replaced_events
+            ]
+
+            if not events_and_contexts:
+                return replaced_events
+
+        chunks = [
+            events_and_contexts[x : x + 100]
+            for x in range(0, len(events_and_contexts), 100)
+        ]
+
+        for chunk in chunks:
+            # We can't easily parallelize these since different chunks
+            # might contain the same event. :(
+
+            # NB: Assumes that we are only persisting events for one room
+            # at a time.
+
+            # map room_id->set[event_ids] giving the new forward
+            # extremities in each room
+            new_forward_extremities: Dict[str, Set[str]] = {}
+
+            # map room_id->(to_delete, to_insert) where to_delete is a list
+            # of type/state keys to remove from current state, and to_insert
+            # is a map (type,key)->event_id giving the state delta in each
+            # room
+            state_delta_for_room: Dict[str, DeltaState] = {}
+
+            # Set of remote users which were in rooms the server has left. We
+            # should check if we still share any rooms and if not we mark their
+            # device lists as stale.
+            potentially_left_users: Set[str] = set()
+
+            if not backfilled:
+                with Measure(self._clock, "_calculate_state_and_extrem"):
+                    # Work out the new "current state" for each room.
+                    # We do this by working out what the new extremities are and then
+                    # calculating the state from that.
+                    events_by_room: Dict[str, List[Tuple[EventBase, EventContext]]] = {}
+                    for event, context in chunk:
+                        events_by_room.setdefault(event.room_id, []).append(
+                            (event, context)
+                        )
+
+                    for room_id, ev_ctx_rm in events_by_room.items():
+                        latest_event_ids = set(
+                            await self.main_store.get_latest_event_ids_in_room(room_id)
+                        )
+                        new_latest_event_ids = await self._calculate_new_extremities(
+                            room_id, ev_ctx_rm, latest_event_ids
+                        )
+
+                        if new_latest_event_ids == latest_event_ids:
+                            # No change in extremities, so no change in state
+                            continue
+
+                        # there should always be at least one forward extremity.
+                        # (except during the initial persistence of the send_join
+                        # results, in which case there will be no existing
+                        # extremities, so we'll `continue` above and skip this bit.)
+                        assert new_latest_event_ids, "No forward extremities left!"
+
+                        new_forward_extremities[room_id] = new_latest_event_ids
+
+                        len_1 = (
+                            len(latest_event_ids) == 1
+                            and len(new_latest_event_ids) == 1
+                        )
+                        if len_1:
+                            all_single_prev_not_state = all(
+                                len(event.prev_event_ids()) == 1
+                                and not event.is_state()
+                                for event, ctx in ev_ctx_rm
+                            )
+                            # Don't bother calculating state if they're just
+                            # a long chain of single ancestor non-state events.
+                            if all_single_prev_not_state:
+                                continue
+
+                        state_delta_counter.inc()
+                        if len(new_latest_event_ids) == 1:
+                            state_delta_single_event_counter.inc()
+
+                            # This is a fairly handwavey check to see if we could
+                            # have guessed what the delta would have been when
+                            # processing one of these events.
+                            # What we're interested in is if the latest extremities
+                            # were the same when we created the event as they are
+                            # now. When this server creates a new event (as opposed
+                            # to receiving it over federation) it will use the
+                            # forward extremities as the prev_events, so we can
+                            # guess this by looking at the prev_events and checking
+                            # if they match the current forward extremities.
+                            for ev, _ in ev_ctx_rm:
+                                prev_event_ids = set(ev.prev_event_ids())
+                                if latest_event_ids == prev_event_ids:
+                                    state_delta_reuse_delta_counter.inc()
+                                    break
+
+                        logger.debug("Calculating state delta for room %s", room_id)
+                        with Measure(
+                            self._clock, "persist_events.get_new_state_after_events"
+                        ):
+                            res = await self._get_new_state_after_events(
+                                room_id,
+                                ev_ctx_rm,
+                                latest_event_ids,
+                                new_latest_event_ids,
+                            )
+                            current_state, delta_ids, new_latest_event_ids = res
+
+                            # there should always be at least one forward extremity.
+                            # (except during the initial persistence of the send_join
+                            # results, in which case there will be no existing
+                            # extremities, so we'll `continue` above and skip this bit.)
+                            assert new_latest_event_ids, "No forward extremities left!"
+
+                            new_forward_extremities[room_id] = new_latest_event_ids
+
+                        # If either are not None then there has been a change,
+                        # and we need to work out the delta (or use that
+                        # given)
+                        delta = None
+                        if delta_ids is not None:
+                            # If there is a delta we know that we've
+                            # only added or replaced state, never
+                            # removed keys entirely.
+                            delta = DeltaState([], delta_ids)
+                        elif current_state is not None:
+                            with Measure(
+                                self._clock, "persist_events.calculate_state_delta"
+                            ):
+                                delta = await self._calculate_state_delta(
+                                    room_id, current_state
+                                )
+
+                        if delta:
+                            # If we have a change of state then lets check
+                            # whether we're actually still a member of the room,
+                            # or if our last user left. If we're no longer in
+                            # the room then we delete the current state and
+                            # extremities.
+                            is_still_joined = await self._is_server_still_joined(
+                                room_id,
+                                ev_ctx_rm,
+                                delta,
+                                current_state,
+                                potentially_left_users,
+                            )
+                            if not is_still_joined:
+                                logger.info("Server no longer in room %s", room_id)
+                                latest_event_ids = set()
+                                current_state = {}
+                                delta.no_longer_in_room = True
+
+                            state_delta_for_room[room_id] = delta
+
+            await self.persist_events_store._persist_events_and_state_updates(
+                chunk,
+                state_delta_for_room=state_delta_for_room,
+                new_forward_extremities=new_forward_extremities,
+                use_negative_stream_ordering=backfilled,
+                inhibit_local_membership_updates=backfilled,
+            )
+
+            await self._handle_potentially_left_users(potentially_left_users)
+
+        return replaced_events
+
+    async def _calculate_new_extremities(
+        self,
+        room_id: str,
+        event_contexts: List[Tuple[EventBase, EventContext]],
+        latest_event_ids: Collection[str],
+    ) -> Set[str]:
+        """Calculates the new forward extremities for a room given events to
+        persist.
+
+        Assumes that we are only persisting events for one room at a time.
+        """
+
+        # we're only interested in new events which aren't outliers and which aren't
+        # being rejected.
+        new_events = [
+            event
+            for event, ctx in event_contexts
+            if not event.internal_metadata.is_outlier()
+            and not ctx.rejected
+            and not event.internal_metadata.is_soft_failed()
+        ]
+
+        latest_event_ids = set(latest_event_ids)
+
+        # start with the existing forward extremities
+        result = set(latest_event_ids)
+
+        # add all the new events to the list
+        result.update(event.event_id for event in new_events)
+
+        # Now remove all events which are prev_events of any of the new events
+        result.difference_update(
+            e_id for event in new_events for e_id in event.prev_event_ids()
+        )
+
+        # Remove any events which are prev_events of any existing events.
+        existing_prevs: Collection[
+            str
+        ] = await self.persist_events_store._get_events_which_are_prevs(result)
+        result.difference_update(existing_prevs)
+
+        # Finally handle the case where the new events have soft-failed prev
+        # events. If they do we need to remove them and their prev events,
+        # otherwise we end up with dangling extremities.
+        existing_prevs = await self.persist_events_store._get_prevs_before_rejected(
+            e_id for event in new_events for e_id in event.prev_event_ids()
+        )
+        result.difference_update(existing_prevs)
+
+        # We only update metrics for events that change forward extremities
+        # (e.g. we ignore backfill/outliers/etc)
+        if result != latest_event_ids:
+            forward_extremities_counter.observe(len(result))
+            stale = latest_event_ids & result
+            stale_forward_extremities_counter.observe(len(stale))
+
+        return result
+
+    async def _get_new_state_after_events(
+        self,
+        room_id: str,
+        events_context: List[Tuple[EventBase, EventContext]],
+        old_latest_event_ids: Set[str],
+        new_latest_event_ids: Set[str],
+    ) -> Tuple[Optional[StateMap[str]], Optional[StateMap[str]], Set[str]]:
+        """Calculate the current state dict after adding some new events to
+        a room
+
+        Args:
+            room_id:
+                room to which the events are being added. Used for logging etc
+
+            events_context:
+                events and contexts which are being added to the room
+
+            old_latest_event_ids:
+                the old forward extremities for the room.
+
+            new_latest_event_ids :
+                the new forward extremities for the room.
+
+        Returns:
+            Returns a tuple of two state maps and a set of new forward
+            extremities.
+
+            The first state map is the full new current state and the second
+            is the delta to the existing current state. If both are None then
+            there has been no change. Either or neither can be None if there
+            has been a change.
+
+            The function may prune some old entries from the set of new
+            forward extremities if it's safe to do so.
+
+            If there has been a change then we only return the delta if its
+            already been calculated. Conversely if we do know the delta then
+            the new current state is only returned if we've already calculated
+            it.
+        """
+        # Map from (prev state group, new state group) -> delta state dict
+        state_group_deltas = {}
+
+        for ev, ctx in events_context:
+            if ctx.state_group is None:
+                # This should only happen for outlier events.
+                if not ev.internal_metadata.is_outlier():
+                    raise Exception(
+                        "Context for new event %s has no state "
+                        "group" % (ev.event_id,)
+                    )
+                continue
+
+            if ctx.prev_group:
+                state_group_deltas[(ctx.prev_group, ctx.state_group)] = ctx.delta_ids
+
+        # We need to map the event_ids to their state groups. First, let's
+        # check if the event is one we're persisting, in which case we can
+        # pull the state group from its context.
+        # Otherwise we need to pull the state group from the database.
+
+        # Set of events we need to fetch groups for. (We know none of the old
+        # extremities are going to be in events_context).
+        missing_event_ids = set(old_latest_event_ids)
+
+        event_id_to_state_group = {}
+        for event_id in new_latest_event_ids:
+            # First search in the list of new events we're adding.
+            for ev, ctx in events_context:
+                if event_id == ev.event_id and ctx.state_group is not None:
+                    event_id_to_state_group[event_id] = ctx.state_group
+                    break
+            else:
+                # If we couldn't find it, then we'll need to pull
+                # the state from the database
+                missing_event_ids.add(event_id)
+
+        if missing_event_ids:
+            # Now pull out the state groups for any missing events from DB
+            event_to_groups = await self.main_store._get_state_group_for_events(
+                missing_event_ids
+            )
+            event_id_to_state_group.update(event_to_groups)
+
+        # State groups of old_latest_event_ids
+        old_state_groups = {
+            event_id_to_state_group[evid] for evid in old_latest_event_ids
+        }
+
+        # State groups of new_latest_event_ids
+        new_state_groups = {
+            event_id_to_state_group[evid] for evid in new_latest_event_ids
+        }
+
+        # If they old and new groups are the same then we don't need to do
+        # anything.
+        if old_state_groups == new_state_groups:
+            return None, None, new_latest_event_ids
+
+        if len(new_state_groups) == 1 and len(old_state_groups) == 1:
+            # If we're going from one state group to another, lets check if
+            # we have a delta for that transition. If we do then we can just
+            # return that.
+
+            new_state_group = next(iter(new_state_groups))
+            old_state_group = next(iter(old_state_groups))
+
+            delta_ids = state_group_deltas.get((old_state_group, new_state_group), None)
+            if delta_ids is not None:
+                # We have a delta from the existing to new current state,
+                # so lets just return that.
+                return None, delta_ids, new_latest_event_ids
+
+        # Now that we have calculated new_state_groups we need to get
+        # their state IDs so we can resolve to a single state set.
+        state_groups_map = await self.state_store._get_state_for_groups(
+            new_state_groups
+        )
+
+        if len(new_state_groups) == 1:
+            # If there is only one state group, then we know what the current
+            # state is.
+            return state_groups_map[new_state_groups.pop()], None, new_latest_event_ids
+
+        # Ok, we need to defer to the state handler to resolve our state sets.
+
+        state_groups = {sg: state_groups_map[sg] for sg in new_state_groups}
+
+        events_map = {ev.event_id: ev for ev, _ in events_context}
+
+        # We need to get the room version, which is in the create event.
+        # Normally that'd be in the database, but its also possible that we're
+        # currently trying to persist it.
+        room_version = None
+        for ev, _ in events_context:
+            if ev.type == EventTypes.Create and ev.state_key == "":
+                room_version = ev.content.get("room_version", "1")
+                break
+
+        if not room_version:
+            room_version = await self.main_store.get_room_version_id(room_id)
+
+        logger.debug("calling resolve_state_groups from preserve_events")
+
+        # Avoid a circular import.
+        from synapse.state import StateResolutionStore
+
+        res = await self._state_resolution_handler.resolve_state_groups(
+            room_id,
+            room_version,
+            state_groups,
+            events_map,
+            state_res_store=StateResolutionStore(self.main_store),
+        )
+
+        state_resolutions_during_persistence.inc()
+
+        # If the returned state matches the state group of one of the new
+        # forward extremities then we check if we are able to prune some state
+        # extremities.
+        if res.state_group and res.state_group in new_state_groups:
+            new_latest_event_ids = await self._prune_extremities(
+                room_id,
+                new_latest_event_ids,
+                res.state_group,
+                event_id_to_state_group,
+                events_context,
+            )
+
+        return res.state, None, new_latest_event_ids
+
+    async def _prune_extremities(
+        self,
+        room_id: str,
+        new_latest_event_ids: Set[str],
+        resolved_state_group: int,
+        event_id_to_state_group: Dict[str, int],
+        events_context: List[Tuple[EventBase, EventContext]],
+    ) -> Set[str]:
+        """See if we can prune any of the extremities after calculating the
+        resolved state.
+        """
+        potential_times_prune_extremities.inc()
+
+        # We keep all the extremities that have the same state group, and
+        # see if we can drop the others.
+        new_new_extrems = {
+            e
+            for e in new_latest_event_ids
+            if event_id_to_state_group[e] == resolved_state_group
+        }
+
+        dropped_extrems = set(new_latest_event_ids) - new_new_extrems
+
+        logger.debug("Might drop extremities: %s", dropped_extrems)
+
+        # We only drop events from the extremities list if:
+        #   1. we're not currently persisting them;
+        #   2. they're not our own events (or are dummy events); and
+        #   3. they're either:
+        #       1. over N hours old and more than N events ago (we use depth to
+        #          calculate); or
+        #       2. we are persisting an event from the same domain and more than
+        #          M events ago.
+        #
+        # The idea is that we don't want to drop events that are "legitimate"
+        # extremities (that we would want to include as prev events), only
+        # "stuck" extremities that are e.g. due to a gap in the graph.
+        #
+        # Note that we either drop all of them or none of them. If we only drop
+        # some of the events we don't know if state res would come to the same
+        # conclusion.
+
+        for ev, _ in events_context:
+            if ev.event_id in dropped_extrems:
+                logger.debug(
+                    "Not dropping extremities: %s is being persisted", ev.event_id
+                )
+                return new_latest_event_ids
+
+        dropped_events = await self.main_store.get_events(
+            dropped_extrems,
+            allow_rejected=True,
+            redact_behaviour=EventRedactBehaviour.as_is,
+        )
+
+        new_senders = {get_domain_from_id(e.sender) for e, _ in events_context}
+
+        one_day_ago = self._clock.time_msec() - 24 * 60 * 60 * 1000
+        current_depth = max(e.depth for e, _ in events_context)
+        for event in dropped_events.values():
+            # If the event is a local dummy event then we should check it
+            # doesn't reference any local events, as we want to reference those
+            # if we send any new events.
+            #
+            # Note we do this recursively to handle the case where a dummy event
+            # references a dummy event that only references remote events.
+            #
+            # Ideally we'd figure out a way of still being able to drop old
+            # dummy events that reference local events, but this is good enough
+            # as a first cut.
+            events_to_check: Collection[EventBase] = [event]
+            while events_to_check:
+                new_events: Set[str] = set()
+                for event_to_check in events_to_check:
+                    if self.is_mine_id(event_to_check.sender):
+                        if event_to_check.type != EventTypes.Dummy:
+                            logger.debug("Not dropping own event")
+                            return new_latest_event_ids
+                        new_events.update(event_to_check.prev_event_ids())
+
+                prev_events = await self.main_store.get_events(
+                    new_events,
+                    allow_rejected=True,
+                    redact_behaviour=EventRedactBehaviour.as_is,
+                )
+                events_to_check = prev_events.values()
+
+            if (
+                event.origin_server_ts < one_day_ago
+                and event.depth < current_depth - 100
+            ):
+                continue
+
+            # We can be less conservative about dropping extremities from the
+            # same domain, though we do want to wait a little bit (otherwise
+            # we'll immediately remove all extremities from a given server).
+            if (
+                get_domain_from_id(event.sender) in new_senders
+                and event.depth < current_depth - 20
+            ):
+                continue
+
+            logger.debug(
+                "Not dropping as too new and not in new_senders: %s",
+                new_senders,
+            )
+
+            return new_latest_event_ids
+
+        times_pruned_extremities.inc()
+
+        logger.info(
+            "Pruning forward extremities in room %s: from %s -> %s",
+            room_id,
+            new_latest_event_ids,
+            new_new_extrems,
+        )
+        return new_new_extrems
+
+    async def _calculate_state_delta(
+        self, room_id: str, current_state: StateMap[str]
+    ) -> DeltaState:
+        """Calculate the new state deltas for a room.
+
+        Assumes that we are only persisting events for one room at a time.
+        """
+        existing_state = await self.main_store.get_current_state_ids(room_id)
+
+        to_delete = [key for key in existing_state if key not in current_state]
+
+        to_insert = {
+            key: ev_id
+            for key, ev_id in current_state.items()
+            if ev_id != existing_state.get(key)
+        }
+
+        return DeltaState(to_delete=to_delete, to_insert=to_insert)
+
+    async def _is_server_still_joined(
+        self,
+        room_id: str,
+        ev_ctx_rm: List[Tuple[EventBase, EventContext]],
+        delta: DeltaState,
+        current_state: Optional[StateMap[str]],
+        potentially_left_users: Set[str],
+    ) -> bool:
+        """Check if the server will still be joined after the given events have
+        been persised.
+
+        Args:
+            room_id
+            ev_ctx_rm
+            delta: The delta of current state between what is in the database
+                and what the new current state will be.
+            current_state: The new current state if it already been calculated,
+                otherwise None.
+            potentially_left_users: If the server has left the room, then joined
+                remote users will be added to this set to indicate that the
+                server may no longer be sharing a room with them.
+        """
+
+        if not any(
+            self.is_mine_id(state_key)
+            for typ, state_key in itertools.chain(delta.to_delete, delta.to_insert)
+            if typ == EventTypes.Member
+        ):
+            # There have been no changes to membership of our users, so nothing
+            # has changed and we assume we're still in the room.
+            return True
+
+        # Check if any of the given events are a local join that appear in the
+        # current state
+        events_to_check = []  # Event IDs that aren't an event we're persisting
+        for (typ, state_key), event_id in delta.to_insert.items():
+            if typ != EventTypes.Member or not self.is_mine_id(state_key):
+                continue
+
+            for event, _ in ev_ctx_rm:
+                if event_id == event.event_id:
+                    if event.membership == Membership.JOIN:
+                        return True
+
+            # The event is not in `ev_ctx_rm`, so we need to pull it out of
+            # the DB.
+            events_to_check.append(event_id)
+
+        # Check if any of the changes that we don't have events for are joins.
+        if events_to_check:
+            members = await self.main_store.get_membership_from_event_ids(
+                events_to_check
+            )
+            is_still_joined = any(
+                member and member.membership == Membership.JOIN
+                for member in members.values()
+            )
+            if is_still_joined:
+                return True
+
+        # None of the new state events are local joins, so we check the database
+        # to see if there are any other local users in the room. We ignore users
+        # whose state has changed as we've already their new state above.
+        users_to_ignore = [
+            state_key
+            for typ, state_key in itertools.chain(delta.to_insert, delta.to_delete)
+            if typ == EventTypes.Member and self.is_mine_id(state_key)
+        ]
+
+        if await self.main_store.is_local_host_in_room_ignoring_users(
+            room_id, users_to_ignore
+        ):
+            return True
+
+        # The server will leave the room, so we go and find out which remote
+        # users will still be joined when we leave.
+        if current_state is None:
+            current_state = await self.main_store.get_current_state_ids(room_id)
+            current_state = dict(current_state)
+            for key in delta.to_delete:
+                current_state.pop(key, None)
+
+            current_state.update(delta.to_insert)
+
+        remote_event_ids = [
+            event_id
+            for (
+                typ,
+                state_key,
+            ), event_id in current_state.items()
+            if typ == EventTypes.Member and not self.is_mine_id(state_key)
+        ]
+        members = await self.main_store.get_membership_from_event_ids(remote_event_ids)
+        potentially_left_users.update(
+            member.user_id
+            for member in members.values()
+            if member and member.membership == Membership.JOIN
+        )
+
+        return False
+
+    async def _handle_potentially_left_users(self, user_ids: Set[str]) -> None:
+        """Given a set of remote users check if the server still shares a room with
+        them. If not then mark those users' device cache as stale.
+        """
+
+        if not user_ids:
+            return
+
+        joined_users = await self.main_store.get_users_server_still_shares_room_with(
+            user_ids
+        )
+        left_users = user_ids - joined_users
+
+        for user_id in left_users:
+            await self.main_store.mark_remote_user_device_list_as_unsubscribed(user_id)