#
# This file is licensed under the Affero General Public License (AGPL) version 3.
#
# Copyright 2019 The Matrix.org Foundation C.I.C.
# Copyright 2014-2016 OpenMarket Ltd
# Copyright (C) 2023 New Vector, Ltd
#
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as
# published by the Free Software Foundation, either version 3 of the
# License, or (at your option) any later version.
#
# See the GNU Affero General Public License for more details:
# <https://www.gnu.org/licenses/agpl-3.0.html>.
#
# Originally licensed under the Apache License, Version 2.0:
# <http://www.apache.org/licenses/LICENSE-2.0>.
#
# [This file includes modifications made by New Vector Limited]
#
#

import itertools
import logging
from collections import deque
from typing import (
    TYPE_CHECKING,
    AbstractSet,
    Any,
    Awaitable,
    Callable,
    ClassVar,
    Collection,
    Generator,
    Generic,
    Iterable,
    TypeVar,
    cast,
)

import attr
from prometheus_client import Counter, Histogram

from twisted.internet import defer

from synapse.api.constants import EventTypes, Membership
from synapse.api.errors import SynapseError
from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
from synapse.events import EventBase, FrozenEventVMSC4242, event_exists_in_state_dag
from synapse.events.snapshot import EventContext, EventPersistencePair
from synapse.handlers.worker_lock import NEW_EVENT_DURING_PURGE_LOCK_NAME
from synapse.logging.context import PreserveLoggingContext, make_deferred_yieldable
from synapse.logging.opentracing import (
    SynapseTags,
    active_span,
    set_tag,
    start_active_span_follows_from,
    trace,
)
from synapse.metrics import SERVER_NAME_LABEL
from synapse.storage.controllers.state import StateStorageController
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.types.state import StateFilter
from synapse.util.async_helpers import ObservableDeferred, yieldable_gather_results
from synapse.util.metrics import Measure
from synapse.util.stringutils import shortstr

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", "", labelnames=[SERVER_NAME_LABEL]
)

# 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",
    "",
    labelnames=[SERVER_NAME_LABEL],
)

# 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", "", labelnames=[SERVER_NAME_LABEL]
)

# 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",
    labelnames=[SERVER_NAME_LABEL],
    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",
    labelnames=[SERVER_NAME_LABEL],
    buckets=(0, 1, 2, 3, 5, 7, 10, 15, 20, 50, 100, 200, 500, "+Inf"),
)

# The number of forward extremities for each new event.
msc4242_state_dag_forward_extremities_counter = Histogram(
    "synapse_storage_msc4242_state_dag_forward_extremities_persisted",
    "Number of forward extremities for each new event in the state DAG",
    labelnames=[SERVER_NAME_LABEL],
    buckets=(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",
    labelnames=[SERVER_NAME_LABEL],
)

potential_times_prune_extremities = Counter(
    "synapse_storage_events_potential_times_prune_extremities",
    "Number of times we might be able to prune extremities",
    labelnames=[SERVER_NAME_LABEL],
)

times_pruned_extremities = Counter(
    "synapse_storage_events_times_pruned_extremities",
    "Number of times we were actually be able to prune extremities",
    labelnames=[SERVER_NAME_LABEL],
)


@attr.s(auto_attribs=True, slots=True)
class _PersistEventsTask:
    """A batch of events to persist."""

    name: ClassVar[str] = "persist_event_batch"  # used for opentracing

    events_and_contexts: list[EventPersistencePair]
    backfilled: bool

    def try_merge(self, task: "_EventPersistQueueTask") -> bool:
        """Batches events with the same backfilled option together."""
        if (
            not isinstance(task, _PersistEventsTask)
            or self.backfilled != task.backfilled
        ):
            return False

        self.events_and_contexts.extend(task.events_and_contexts)
        return True


@attr.s(auto_attribs=True, slots=True)
class _UpdateCurrentStateTask:
    """A room whose current state needs recalculating."""

    name: ClassVar[str] = "update_current_state"  # used for opentracing

    def try_merge(self, task: "_EventPersistQueueTask") -> bool:
        """Deduplicates consecutive recalculations of current state."""
        return isinstance(task, _UpdateCurrentStateTask)


_EventPersistQueueTask = _PersistEventsTask | _UpdateCurrentStateTask
_PersistResult = TypeVar("_PersistResult")


@attr.s(auto_attribs=True, slots=True)
class _EventPersistQueueItem(Generic[_PersistResult]):
    task: _EventPersistQueueTask
    deferred: ObservableDeferred[_PersistResult]

    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"""


class _EventPeristenceQueue(Generic[_PersistResult]):
    """Queues up tasks so that they can be processed with only one concurrent
    transaction per room.

    Tasks can be bulk persistence of events or recalculation of a room's current state.
    """

    def __init__(
        self,
        hs: "HomeServer",
        server_name: str,
        per_item_callback: Callable[
            [str, _EventPersistQueueTask],
            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.server_name = server_name
        self.hs = hs
        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,
        task: _EventPersistQueueTask,
    ) -> _PersistResult:
        """Add a task to the queue.

        If we are not already processing tasks in this room, starts off a background
        process to to so, calling the per_item_callback for each item.

        Args:
            room_id:
            task: A _PersistEventsTask or _UpdateCurrentStateTask to process.

        Returns:
            the result returned by the `_per_item_callback` passed to
            `__init__`.
        """
        queue = self._event_persist_queues.setdefault(room_id, deque())

        if queue and queue[-1].task.try_merge(task):
            # the new task has been merged into the last task in the queue
            end_item = queue[-1]
        else:
            deferred: ObservableDeferred[_PersistResult] = ObservableDeferred(
                defer.Deferred(), consumeErrors=True
            )

            end_item = _EventPersistQueueItem(
                task=task,
                deferred=deferred,
            )
            queue.append(end_item)

        # also add our active opentracing span to the item so that we get a link back
        span = 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 start_active_span_follows_from(
            f"{task.name}_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 start_active_span_follows_from(
                            item.task.name,
                            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(room_id, item.task)
                    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
        self.hs.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,
        state_controller: StateStorageController,
    ):
        # 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
        self._state_deletion_store = stores.state_deletion

        assert stores.persist_events
        self.persist_events_store = stores.persist_events

        self.server_name = hs.hostname
        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(
            hs, self.server_name, self._process_event_persist_queue_task
        )
        self._state_resolution_handler = hs.get_state_resolution_handler()
        self._state_controller = state_controller
        self.hs = hs

    async def _process_event_persist_queue_task(
        self,
        room_id: str,
        task: _EventPersistQueueTask,
    ) -> dict[str, str]:
        """Callback for the _event_persist_queue

        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.
        """

        # Ensure that the room can't be deleted while we're persisting events to
        # it. We might already have taken out the lock, but since this is just a
        # "read" lock its inherently reentrant.
        async with self.hs.get_worker_locks_handler().acquire_read_write_lock(
            NEW_EVENT_DURING_PURGE_LOCK_NAME, room_id, write=False
        ):
            if isinstance(task, _PersistEventsTask):
                return await self._persist_event_batch(room_id, task)
            elif isinstance(task, _UpdateCurrentStateTask):
                await self._update_current_state(room_id, task)
                return {}
            else:
                raise AssertionError(
                    f"Found an unexpected task type in event persistence queue: {task}"
                )

    @trace
    async def persist_events(
        self,
        events_and_contexts: Iterable[EventPersistencePair],
        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.

        Raises:
            PartialStateConflictError: if attempting to persist a partial state event in
                a room that has been un-partial stated.
        """
        event_ids: list[str] = []
        partitioned: dict[str, list[EventPersistencePair]] = {}
        for event, ctx in events_and_contexts:
            partitioned.setdefault(event.room_id, []).append((event, ctx))
            event_ids.append(event.event_id)

        set_tag(
            SynapseTags.FUNC_ARG_PREFIX + "event_ids",
            str(event_ids),
        )
        set_tag(
            SynapseTags.FUNC_ARG_PREFIX + "event_ids.length",
            str(len(event_ids)),
        )
        set_tag(SynapseTags.FUNC_ARG_PREFIX + "backfilled", str(backfilled))

        async def enqueue(
            item: tuple[str, list[EventPersistencePair]],
        ) -> dict[str, str]:
            room_id, evs_ctxs = item
            return await self._event_persist_queue.add_to_queue(
                room_id,
                _PersistEventsTask(events_and_contexts=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)

        persisted_events = []
        for event, _ in events_and_contexts:
            existing_event_id = replaced_events.get(event.event_id)
            if existing_event_id:
                persisted_events.append(
                    await self.main_store.get_event(existing_event_id)
                )
            else:
                persisted_events.append(event)

        return (
            persisted_events,
            self.main_store.get_room_max_token(),
        )

    @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.

        Raises:
            PartialStateConflictError: if attempting to persist a partial state event in
                a room that has been un-partial stated.
        """
        # 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,
            _PersistEventsTask(
                events_and_contexts=[(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"""
        await self._event_persist_queue.add_to_queue(
            room_id,
            _UpdateCurrentStateTask(),
        )

    async def _update_current_state(
        self, room_id: str, _task: _UpdateCurrentStateTask
    ) -> None:
        """Callback for the _event_persist_queue

        Recalculates the current state for a room, and persists it.
        """
        state = await self._calculate_current_state(room_id)
        delta = await self._calculate_state_delta(room_id, state)
        sliding_sync_table_changes = (
            await self.persist_events_store._calculate_sliding_sync_table_changes(
                room_id, [], delta
            )
        )

        await self.persist_events_store.update_current_state(
            room_id, delta, sliding_sync_table_changes
        )

    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
        """
        room_version = await self.main_store.get_room_version_id(room_id)
        room_version_obj = KNOWN_ROOM_VERSIONS[room_version]
        if room_version_obj.msc4242_state_dags:
            latest_event_ids = await self.main_store.get_state_dag_extremities(room_id)
        else:
            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

        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, self._state_deletion_store
            ),
        )

        return await res.get_state(self._state_controller, StateFilter.all())

    async def _persist_event_batch(
        self, room_id: str, task: _PersistEventsTask
    ) -> 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.

        Assumes that we are only persisting events for one room at a time.

        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.

        Raises:
            PartialStateConflictError: if attempting to persist a partial state event in
                a room that has been un-partial stated.
        """
        events_and_contexts = task.events_and_contexts
        backfilled = task.backfilled

        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)
        ]

        # Get the room version for the first event. This room version is the same for all events
        # as events_and_contexts is all for one room.
        assert len(events_and_contexts) > 0
        room_version = events_and_contexts[0][0].room_version

        for chunk in chunks:
            # We can't easily parallelize these since different chunks
            # might contain the same event. :(

            new_forward_extremities = None
            state_delta_for_room = None
            new_state_dag_extrems = None

            if not backfilled:
                if room_version.msc4242_state_dags:
                    with Measure(
                        self._clock,
                        name="_process_state_dag_forward_extremities_and_state_delta",
                        server_name=self.server_name,
                    ):
                        assert all(
                            isinstance(ev, FrozenEventVMSC4242) for ev, _ in chunk
                        )
                        (
                            new_forward_extremities,  # for prev_events
                            state_delta_for_room,  # for state groups
                            new_state_dag_extrems,  # for prev_state_events
                        ) = await self._process_state_dag_forward_extremities_and_state_delta(
                            room_id,
                            cast(list[tuple[FrozenEventVMSC4242, EventContext]], chunk),
                        )
                else:
                    with Measure(
                        self._clock,
                        name="_calculate_state_and_extrem",
                        server_name=self.server_name,
                    ):
                        # Work out the new "current state" for the room.
                        # We do this by working out what the new extremities are and then
                        # calculating the state from that.
                        (
                            new_forward_extremities,
                            state_delta_for_room,
                        ) = await self._calculate_new_forward_extremities_and_state_delta(
                            room_id, chunk
                        )

            with Measure(
                self._clock,
                name="calculate_chain_cover_index_for_events",
                server_name=self.server_name,
            ):
                # We now calculate chain ID/sequence numbers for any state events we're
                # persisting. We ignore out of band memberships as we're not in the room
                # and won't have their auth chain (we'll fix it up later if we join the
                # room).
                #
                # See: docs/auth_chain_difference_algorithm.md
                new_event_links = await self.persist_events_store.calculate_chain_cover_index_for_events(
                    room_id, [e for e, _ in chunk]
                )

            # Stop the state groups from being deleted while we're persisting
            # them.
            async with self._state_deletion_store.persisting_state_group_references(
                events_and_contexts
            ):
                await self.persist_events_store._persist_events_and_state_updates(
                    room_id,
                    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,
                    new_event_links=new_event_links,
                    new_state_dag_forward_extremities=new_state_dag_extrems,
                )

        return replaced_events

    async def _calculate_new_forward_extremities_and_state_delta(
        self, room_id: str, ev_ctx_rm: list[EventPersistencePair]
    ) -> tuple[set[str] | None, DeltaState | None]:
        """Calculates the new forward extremities and state delta for a room
        given events to persist.

        Assumes that we are only persisting events for one room at a time.

        Returns:
            A tuple of:
                A set of str giving the new forward extremities the room

                The state delta for the room.
        """

        latest_event_ids = 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
            return (None, None)

        # 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 = 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:
                return (new_forward_extremities, None)

        state_delta_counter.labels(**{SERVER_NAME_LABEL: self.server_name}).inc()
        if len(new_latest_event_ids) == 1:
            state_delta_single_event_counter.labels(
                **{SERVER_NAME_LABEL: self.server_name}
            ).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.labels(
                        **{SERVER_NAME_LABEL: self.server_name}
                    ).inc()
                    break

        logger.debug("Calculating state delta for room %s", room_id)
        with Measure(
            self._clock,
            name="persist_events.get_new_state_after_events",
            server_name=self.server_name,
        ):
            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 = 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,
                name="persist_events.calculate_state_delta",
                server_name=self.server_name,
            ):
                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,
            )
            if not is_still_joined:
                logger.info("Server no longer in room %s", room_id)
                delta.no_longer_in_room = True

        return (new_forward_extremities, delta)

    async def _process_state_dag_forward_extremities_and_state_delta(
        self,
        room_id: str,
        event_contexts: list[tuple[FrozenEventVMSC4242, EventContext]],
    ) -> tuple[set[str] | None, DeltaState | None, set[str] | None]:
        """Process the forwards extremities for state DAG rooms.
        Returns:
         - the new room dag extremities which should be written when these events are persisted.
         - the state delta for the room, if applicable.
         - the new state dag extremities which should be written when these events are persisted.

        NB: this does not write them because if it did, new events may see them _before_ the events
        get persisted, causing failures in retrieving state groups.
        """
        # Update forward extremities
        # ...for the state DAG
        existing_state_dag_fwd_extrems = (
            await self.main_store.get_state_dag_extremities(room_id)
        )
        new_state_dag_fwd_extrems = await self._calculate_new_state_dag_extremities(
            room_id,
            existing_state_dag_fwd_extrems,
            event_contexts,
        )
        # ...and the room DAG
        existing_room_dag_fwd_extrems = (
            await self.main_store.get_latest_event_ids_in_room(room_id)
        )
        new_room_dag_fwd_extrems = await self._calculate_new_extremities(
            room_id,
            cast(list[EventPersistencePair], event_contexts),
            existing_room_dag_fwd_extrems,
        )
        assert new_room_dag_fwd_extrems, (
            f"No room dag forward extremities left in room {room_id}!"
        )

        # See if we need to calculate a state delta
        if new_state_dag_fwd_extrems == existing_state_dag_fwd_extrems:
            # No change in state extremities, so no new state to calculate
            return new_room_dag_fwd_extrems, None, new_state_dag_fwd_extrems

        with Measure(
            self._clock,
            name="persist_events.state_dag.get_new_state_after_events",
            server_name=self.server_name,
        ):
            (current_state, delta_ids, _) = await self._get_new_state_after_events(
                room_id,
                cast(list[EventPersistencePair], event_contexts),
                existing_state_dag_fwd_extrems,
                new_state_dag_fwd_extrems,
                # do not prune forward extremities in the state DAG
                # else we lose eventual delivery
                should_prune=False,
            )

        # Following logic cargoculted from _calculate_new_forward_extremities_and_state_delta
        # 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,
                name="persist_events.calculate_state_delta",
                server_name=self.server_name,
            ):
                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,
                cast(list[EventPersistencePair], event_contexts),
                delta,
            )
            if not is_still_joined:
                logger.info("Server no longer in room %s", room_id)
                delta.no_longer_in_room = True

        return new_room_dag_fwd_extrems, delta, new_state_dag_fwd_extrems

    async def _calculate_new_state_dag_extremities(
        self,
        room_id: str,
        existing_fwd_extrems: frozenset[str],
        event_contexts: list[tuple[FrozenEventVMSC4242, EventContext]],
    ) -> set[str]:
        """Calculate the new state dag forward extremities. Modifies existing_fwd_extrems.

        Assumes that event_contexts are only state events which should be in the state DAG.

        Raises:
            SynapseError: if the new events include unknown prev_state_events
            AssertionError: if there are no state DAG forward extremities remaining in the room
        """
        # Events are always processed in causal order without any gaps in the DAG
        # (prev_state_events are always known), guaranteeing that processed events have a path to the
        # create event. This is an emergent property of state DAGs as asserting that there is a path
        # to the create event every time we insert an event would be prohibitively expensive.
        # This is similar to how doubly-linked lists can potentially not refer to previous items correctly
        # without verifying the list's integrity, but doing it on every insert is too expensive.

        # filter out events which don't belong in the state dag.
        new_state_events_contexts = [
            (e, ctx) for e, ctx in event_contexts if event_exists_in_state_dag(e)
        ]
        if len(new_state_events_contexts) == 0:
            # if there are no state events being persisted, then the fwd extremities of the state dag
            # do not change.
            return set(existing_fwd_extrems)

        # This logic is very similar to _calculate_new_extremities with a few key differences:
        #  - We do not "Remove any events which are prev_events of any existing events." because the
        #    state DAG mandates that events are processed in causal order, so there MUST NOT be any
        #    existing, processed events which have the to-be-persisted events as prev_state_events.
        #  - We don't care if they are an "outlier" in the main room dag, so long as they AREN'T
        #    an outlier on the state dag, which this function checks, so we don't check outlier-ness.
        #  - We allow *soft-failed* events to become forward extremities, as per the MSC. We do not
        #    allow *rejected* events to become forward extremities though.

        rejected_events = [ev for ev, ctx in new_state_events_contexts if ctx.rejected]
        new_state_events = [
            ev for ev, ctx in new_state_events_contexts if not ctx.rejected
        ]
        # We want to check that we are not missing any prev_state_events.
        # To do this, we include rejected events in this check because other events may point to them.
        # If we didn't include them, we might incorrectly say we are missing events when we are not.
        all_new_state_events = set(rejected_events + new_state_events)

        # First, verify that we know all prev_state_events. If we fail this check then we don't have
        # a complete DAG and that is bad, so bail out.

        # Start with them all missing.
        missing_prev_state_events = {
            e_id for event in all_new_state_events for e_id in event.prev_state_events
        }

        # remove prev events which appear in all_events
        missing_prev_state_events.difference_update(
            event.event_id for event in all_new_state_events
        )
        # the rest of these events should be present in the DB. Some of them may be forward extremities,
        # some may not be, that's ok.
        seen_events = await self.main_store.have_seen_events(
            room_id,
            missing_prev_state_events,
        )
        missing_prev_state_events.difference_update(seen_events)

        if len(missing_prev_state_events) > 0:
            logger.error(
                "_calculate_new_state_dag_extremities: missing the following prev_state_events in room %s : %s",
                room_id,
                missing_prev_state_events,
            )
            logger.error(
                "_calculate_new_state_dag_extremities: was handling %s",
                shortstr([ev.event_id for ev in all_new_state_events]),
            )
            raise SynapseError(
                code=500,
                msg=f"missing {len(missing_prev_state_events)} prev_state_events in room {room_id}",
            )

        # Now calculate the forward extremities.

        # start with the existing forward extremities
        result = set(existing_fwd_extrems)

        # add all the new events to the list
        result.update(event.event_id for event in new_state_events)

        # Now remove all events which are prev_state_events of any of the new events
        result.difference_update(
            e_id for event in new_state_events for e_id in event.prev_state_events
        )

        # Finally handle the case where the new events have rejected/soft-failed `prev_state_events`.
        # If they do we need to remove them and their `prev_state_events`,
        # otherwise we end up with dangling extremities.
        # Specifically, this handles the case where (F=fwd extrem, SF=soft-failed, N=new event)
        # F <-- SF <-- SF <-- N
        # where we want to remove F as a forward extremity and replace with N.
        existing_prevs = await self.persist_events_store._get_prevs_before_rejected(
            (e_id for event in new_state_events for e_id in event.prev_state_events),
            include_soft_failed=False,
        )
        result.difference_update(existing_prevs)

        # We only update metrics for events that change forward extremities
        if result != existing_fwd_extrems:
            msc4242_state_dag_forward_extremities_counter.labels(
                **{SERVER_NAME_LABEL: self.server_name}
            ).observe(len(result))

        # There should always be at least one forward extremity.
        assert result, f"No state dag forward extremities left in room {room_id}!"
        return result

    async def _calculate_new_extremities(
        self,
        room_id: str,
        event_contexts: list[EventPersistencePair],
        latest_event_ids: AbstractSet[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()
        ]

        # 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.labels(
                **{SERVER_NAME_LABEL: self.server_name}
            ).observe(len(result))
            stale = latest_event_ids & result
            stale_forward_extremities_counter.labels(
                **{SERVER_NAME_LABEL: self.server_name}
            ).observe(len(stale))

        return result

    async def _get_new_state_after_events(
        self,
        room_id: str,
        events_context: list[EventPersistencePair],
        old_latest_event_ids: AbstractSet[str],
        new_latest_event_ids: set[str],
        should_prune: bool = True,
    ) -> tuple[StateMap[str] | None, StateMap[str] | None, 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.

            should_prune:
                if true, attempt to prune the forward extremities.
                Pruning means we will not communicate some new events to other servers,
                which can compromise eventual delivery, so graphs which are fully synchronised
                e.g. state DAGs should not prune.

        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.state_group_deltas:
                state_group_deltas.update(ctx.state_group_deltas)

        # 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, self._state_deletion_store
            ),
        )

        state_resolutions_during_persistence.labels(
            **{SERVER_NAME_LABEL: self.server_name}
        ).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 should_prune and 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,
            )

        full_state = await res.get_state(self._state_controller)
        return full_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[EventPersistencePair],
    ) -> set[str]:
        """See if we can prune any of the extremities after calculating the
        resolved state.
        """
        potential_times_prune_extremities.labels(
            **{SERVER_NAME_LABEL: self.server_name}
        ).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.labels(**{SERVER_NAME_LABEL: self.server_name}).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_partial_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[EventPersistencePair],
        delta: DeltaState,
    ) -> 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.
        """

        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

        return False
