MatrixSynapse/synapse/storage/state.py

# -*- coding: utf-8 -*-
# Copyright 2014-2016 OpenMarket Ltd
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.

import logging
from collections import namedtuple

from six import iteritems, itervalues
from six.moves import range

import attr

from twisted.internet import defer

from synapse.api.constants import EventTypes
from synapse.api.errors import NotFoundError
from synapse.storage._base import SQLBaseStore
from synapse.storage.background_updates import BackgroundUpdateStore
from synapse.storage.engines import PostgresEngine
from synapse.storage.events_worker import EventsWorkerStore
from synapse.util.caches import get_cache_factor_for, intern_string
from synapse.util.caches.descriptors import cached, cachedList
from synapse.util.caches.dictionary_cache import DictionaryCache
from synapse.util.stringutils import to_ascii

logger = logging.getLogger(__name__)


MAX_STATE_DELTA_HOPS = 100


class _GetStateGroupDelta(namedtuple("_GetStateGroupDelta", ("prev_group", "delta_ids"))):
    """Return type of get_state_group_delta that implements __len__, which lets
    us use the itrable flag when caching
    """
    __slots__ = []

    def __len__(self):
        return len(self.delta_ids) if self.delta_ids else 0


@attr.s(slots=True)
class StateFilter(object):
    """A filter used when querying for state.

    Attributes:
        types (dict[str, set[str]|None]): Map from type to set of state keys (or
            None). This specifies which state_keys for the given type to fetch
            from the DB. If None then all events with that type are fetched. If
            the set is empty then no events with that type are fetched.
        include_others (bool): Whether to fetch events with types that do not
            appear in `types`.
    """

    types = attr.ib()
    include_others = attr.ib(default=False)

    def __attrs_post_init__(self):
        # If `include_others` is set we canonicalise the filter by removing
        # wildcards from the types dictionary
        if self.include_others:
            self.types = {
                k: v for k, v in iteritems(self.types)
                if v is not None
            }

    @staticmethod
    def all():
        """Creates a filter that fetches everything.

        Returns:
            StateFilter
        """
        return StateFilter(types={}, include_others=True)

    @staticmethod
    def none():
        """Creates a filter that fetches nothing.

        Returns:
            StateFilter
        """
        return StateFilter(types={}, include_others=False)

    @staticmethod
    def from_types(types):
        """Creates a filter that only fetches the given types

        Args:
            types (Iterable[tuple[str, str|None]]): A list of type and state
                keys to fetch. A state_key of None fetches everything for
                that type

        Returns:
            StateFilter
        """
        type_dict = {}
        for typ, s in types:
            if typ in type_dict:
                if type_dict[typ] is None:
                    continue

            if s is None:
                type_dict[typ] = None
                continue

            type_dict.setdefault(typ, set()).add(s)

        return StateFilter(types=type_dict)

    @staticmethod
    def from_lazy_load_member_list(members):
        """Creates a filter that returns all non-member events, plus the member
        events for the given users

        Args:
            members (iterable[str]): Set of user IDs

        Returns:
            StateFilter
        """
        return StateFilter(
            types={EventTypes.Member: set(members)},
            include_others=True,
        )

    def return_expanded(self):
        """Creates a new StateFilter where type wild cards have been removed
        (except for memberships). The returned filter is a superset of the
        current one, i.e. anything that passes the current filter will pass
        the returned filter.

        This helps the caching as the DictionaryCache knows if it has *all* the
        state, but does not know if it has all of the keys of a particular type,
        which makes wildcard lookups expensive unless we have a complete cache.
        Hence, if we are doing a wildcard lookup, populate the cache fully so
        that we can do an efficient lookup next time.

        Note that since we have two caches, one for membership events and one for
        other events, we can be a bit more clever than simply returning
        `StateFilter.all()` if `has_wildcards()` is True.

        We return a StateFilter where:
            1. the list of membership events to return is the same
            2. if there is a wildcard that matches non-member events we
               return all non-member events

        Returns:
            StateFilter
        """

        if self.is_full():
            # If we're going to return everything then there's nothing to do
            return self

        if not self.has_wildcards():
            # If there are no wild cards, there's nothing to do
            return self

        if EventTypes.Member in self.types:
            get_all_members = self.types[EventTypes.Member] is None
        else:
            get_all_members = self.include_others

        has_non_member_wildcard = self.include_others or any(
            state_keys is None
            for t, state_keys in iteritems(self.types)
            if t != EventTypes.Member
        )

        if not has_non_member_wildcard:
            # If there are no non-member wild cards we can just return ourselves
            return self

        if get_all_members:
            # We want to return everything.
            return StateFilter.all()
        else:
            # We want to return all non-members, but only particular
            # memberships
            return StateFilter(
                types={EventTypes.Member: self.types[EventTypes.Member]},
                include_others=True,
            )

    def make_sql_filter_clause(self):
        """Converts the filter to an SQL clause.

        For example:

            f = StateFilter.from_types([("m.room.create", "")])
            clause, args = f.make_sql_filter_clause()
            clause == "(type = ? AND state_key = ?)"
            args == ['m.room.create', '']


        Returns:
            tuple[str, list]: The SQL string (may be empty) and arguments. An
            empty SQL string is returned when the filter matches everything
            (i.e. is "full").
        """

        where_clause = ""
        where_args = []

        if self.is_full():
            return where_clause, where_args

        if not self.include_others and not self.types:
            # i.e. this is an empty filter, so we need to return a clause that
            # will match nothing
            return "1 = 2", []

        # First we build up a lost of clauses for each type/state_key combo
        clauses = []
        for etype, state_keys in iteritems(self.types):
            if state_keys is None:
                clauses.append("(type = ?)")
                where_args.append(etype)
                continue

            for state_key in state_keys:
                clauses.append("(type = ? AND state_key = ?)")
                where_args.extend((etype, state_key))

        # This will match anything that appears in `self.types`
        where_clause = " OR ".join(clauses)

        # If we want to include stuff that's not in the types dict then we add
        # a `OR type NOT IN (...)` clause to the end.
        if self.include_others:
            if where_clause:
                where_clause += " OR "

            where_clause += "type NOT IN (%s)" % (
                ",".join(["?"] * len(self.types)),
            )
            where_args.extend(self.types)

        return where_clause, where_args

    def max_entries_returned(self):
        """Returns the maximum number of entries this filter will return if
        known, otherwise returns None.

        For example a simple state filter asking for `("m.room.create", "")`
        will return 1, whereas the default state filter will return None.

        This is used to bail out early if the right number of entries have been
        fetched.
        """
        if self.has_wildcards():
            return None

        return len(self.concrete_types())

    def filter_state(self, state_dict):
        """Returns the state filtered with by this StateFilter

        Args:
            state (dict[tuple[str, str], Any]): The state map to filter

        Returns:
            dict[tuple[str, str], Any]: The filtered state map
        """
        if self.is_full():
            return dict(state_dict)

        filtered_state = {}
        for k, v in iteritems(state_dict):
            typ, state_key = k
            if typ in self.types:
                state_keys = self.types[typ]
                if state_keys is None or state_key in state_keys:
                    filtered_state[k] = v
            elif self.include_others:
                filtered_state[k] = v

        return filtered_state

    def is_full(self):
        """Whether this filter fetches everything or not

        Returns:
            bool
        """
        return self.include_others and not self.types

    def has_wildcards(self):
        """Whether the filter includes wildcards or is attempting to fetch
        specific state.

        Returns:
            bool
        """

        return (
            self.include_others
            or any(
                state_keys is None
                for state_keys in itervalues(self.types)
            )
        )

    def concrete_types(self):
        """Returns a list of concrete type/state_keys (i.e. not None) that
        will be fetched. This will be a complete list if `has_wildcards`
        returns False, but otherwise will be a subset (or even empty).

        Returns:
            list[tuple[str,str]]
        """
        return [
            (t, s)
            for t, state_keys in iteritems(self.types)
            if state_keys is not None
            for s in state_keys
        ]

    def get_member_split(self):
        """Return the filter split into two: one which assumes it's exclusively
        matching against member state, and one which assumes it's matching
        against non member state.

        This is useful due to the returned filters giving correct results for
        `is_full()`, `has_wildcards()`, etc, when operating against maps that
        either exclusively contain member events or only contain non-member
        events. (Which is the case when dealing with the member vs non-member
        state caches).

        Returns:
            tuple[StateFilter, StateFilter]: The member and non member filters
        """

        if EventTypes.Member in self.types:
            state_keys = self.types[EventTypes.Member]
            if state_keys is None:
                member_filter = StateFilter.all()
            else:
                member_filter = StateFilter({EventTypes.Member: state_keys})
        elif self.include_others:
            member_filter = StateFilter.all()
        else:
            member_filter = StateFilter.none()

        non_member_filter = StateFilter(
            types={k: v for k, v in iteritems(self.types) if k != EventTypes.Member},
            include_others=self.include_others,
        )

        return member_filter, non_member_filter


# this inherits from EventsWorkerStore because it calls self.get_events
class StateGroupWorkerStore(EventsWorkerStore, SQLBaseStore):
    """The parts of StateGroupStore that can be called from workers.
    """

    STATE_GROUP_DEDUPLICATION_UPDATE_NAME = "state_group_state_deduplication"
    STATE_GROUP_INDEX_UPDATE_NAME = "state_group_state_type_index"
    CURRENT_STATE_INDEX_UPDATE_NAME = "current_state_members_idx"

    def __init__(self, db_conn, hs):
        super(StateGroupWorkerStore, self).__init__(db_conn, hs)

        # Originally the state store used a single DictionaryCache to cache the
        # event IDs for the state types in a given state group to avoid hammering
        # on the state_group* tables.
        #
        # The point of using a DictionaryCache is that it can cache a subset
        # of the state events for a given state group (i.e. a subset of the keys for a
        # given dict which is an entry in the cache for a given state group ID).
        #
        # However, this poses problems when performing complicated queries
        # on the store - for instance: "give me all the state for this group, but
        # limit members to this subset of users", as DictionaryCache's API isn't
        # rich enough to say "please cache any of these fields, apart from this subset".
        # This is problematic when lazy loading members, which requires this behaviour,
        # as without it the cache has no choice but to speculatively load all
        # state events for the group, which negates the efficiency being sought.
        #
        # Rather than overcomplicating DictionaryCache's API, we instead split the
        # state_group_cache into two halves - one for tracking non-member events,
        # and the other for tracking member_events.  This means that lazy loading
        # queries can be made in a cache-friendly manner by querying both caches
        # separately and then merging the result.  So for the example above, you
        # would query the members cache for a specific subset of state keys
        # (which DictionaryCache will handle efficiently and fine) and the non-members
        # cache for all state (which DictionaryCache will similarly handle fine)
        # and then just merge the results together.
        #
        # We size the non-members cache to be smaller than the members cache as the
        # vast majority of state in Matrix (today) is member events.

        self._state_group_cache = DictionaryCache(
            "*stateGroupCache*",
            # TODO: this hasn't been tuned yet
            50000 * get_cache_factor_for("stateGroupCache")
        )
        self._state_group_members_cache = DictionaryCache(
            "*stateGroupMembersCache*",
            500000 * get_cache_factor_for("stateGroupMembersCache")
        )

    @defer.inlineCallbacks
    def get_room_version(self, room_id):
        """Get the room_version of a given room

        Args:
            room_id (str)

        Returns:
            Deferred[str]

        Raises:
            NotFoundError if the room is unknown
        """
        # for now we do this by looking at the create event. We may want to cache this
        # more intelligently in future.

        # Retrieve the room's create event
        create_event = yield self.get_create_event_for_room(room_id)
        defer.returnValue(create_event.content.get("room_version", "1"))

    @defer.inlineCallbacks
    def get_room_predecessor(self, room_id):
        """Get the predecessor room of an upgraded room if one exists.
        Otherwise return None.

        Args:
            room_id (str)

        Returns:
            Deferred[unicode|None]: predecessor room id

        Raises:
            NotFoundError if the room is unknown
        """
        # Retrieve the room's create event
        create_event = yield self.get_create_event_for_room(room_id)

        # Return predecessor if present
        defer.returnValue(create_event.content.get("predecessor", None))

    @defer.inlineCallbacks
    def get_create_event_for_room(self, room_id):
        """Get the create state event for a room.

        Args:
            room_id (str)

        Returns:
            Deferred[EventBase]: The room creation event.

        Raises:
            NotFoundError if the room is unknown
        """
        state_ids = yield self.get_current_state_ids(room_id)
        create_id = state_ids.get((EventTypes.Create, ""))

        # If we can't find the create event, assume we've hit a dead end
        if not create_id:
            raise NotFoundError("Unknown room %s" % (room_id))

        # Retrieve the room's create event and return
        create_event = yield self.get_event(create_id)
        defer.returnValue(create_event)

    @cached(max_entries=100000, iterable=True)
    def get_current_state_ids(self, room_id):
        """Get the current state event ids for a room based on the
        current_state_events table.

        Args:
            room_id (str)

        Returns:
            deferred: dict of (type, state_key) -> event_id
        """
        def _get_current_state_ids_txn(txn):
            txn.execute(
                """SELECT type, state_key, event_id FROM current_state_events
                WHERE room_id = ?
                """,
                (room_id,)
            )

            return {
                (intern_string(r[0]), intern_string(r[1])): to_ascii(r[2]) for r in txn
            }

        return self.runInteraction(
            "get_current_state_ids",
            _get_current_state_ids_txn,
        )

    # FIXME: how should this be cached?
    def get_filtered_current_state_ids(self, room_id, state_filter=StateFilter.all()):
        """Get the current state event of a given type for a room based on the
        current_state_events table.  This may not be as up-to-date as the result
        of doing a fresh state resolution as per state_handler.get_current_state

        Args:
            room_id (str)
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            Deferred[dict[tuple[str, str], str]]: Map from type/state_key to
            event ID.
        """

        def _get_filtered_current_state_ids_txn(txn):
            results = {}
            sql = """
                SELECT type, state_key, event_id FROM current_state_events
                WHERE room_id = ?
            """

            where_clause, where_args = state_filter.make_sql_filter_clause()

            if where_clause:
                sql += " AND (%s)" % (where_clause,)

            args = [room_id]
            args.extend(where_args)
            txn.execute(sql, args)
            for row in txn:
                typ, state_key, event_id = row
                key = (intern_string(typ), intern_string(state_key))
                results[key] = event_id

            return results

        return self.runInteraction(
            "get_filtered_current_state_ids",
            _get_filtered_current_state_ids_txn,
        )

    @defer.inlineCallbacks
    def get_canonical_alias_for_room(self, room_id):
        """Get canonical alias for room, if any

        Args:
            room_id (str)

        Returns:
            Deferred[str|None]: The canonical alias, if any
        """

        state = yield self.get_filtered_current_state_ids(room_id, StateFilter.from_types(
            [(EventTypes.CanonicalAlias, "")]
        ))

        event_id = state.get((EventTypes.CanonicalAlias, ""))
        if not event_id:
            return

        event = yield self.get_event(event_id, allow_none=True)
        if not event:
            return

        defer.returnValue(event.content.get("canonical_alias"))

    @cached(max_entries=10000, iterable=True)
    def get_state_group_delta(self, state_group):
        """Given a state group try to return a previous group and a delta between
        the old and the new.

        Returns:
            (prev_group, delta_ids), where both may be None.
        """
        def _get_state_group_delta_txn(txn):
            prev_group = self._simple_select_one_onecol_txn(
                txn,
                table="state_group_edges",
                keyvalues={
                    "state_group": state_group,
                },
                retcol="prev_state_group",
                allow_none=True,
            )

            if not prev_group:
                return _GetStateGroupDelta(None, None)

            delta_ids = self._simple_select_list_txn(
                txn,
                table="state_groups_state",
                keyvalues={
                    "state_group": state_group,
                },
                retcols=("type", "state_key", "event_id",)
            )

            return _GetStateGroupDelta(prev_group, {
                (row["type"], row["state_key"]): row["event_id"]
                for row in delta_ids
            })
        return self.runInteraction(
            "get_state_group_delta",
            _get_state_group_delta_txn,
        )

    @defer.inlineCallbacks
    def get_state_groups_ids(self, _room_id, event_ids):
        """Get the event IDs of all the state for the state groups for the given events

        Args:
            _room_id (str): id of the room for these events
            event_ids (iterable[str]): ids of the events

        Returns:
            Deferred[dict[int, dict[tuple[str, str], str]]]:
                dict of state_group_id -> (dict of (type, state_key) -> event id)
        """
        if not event_ids:
            defer.returnValue({})

        event_to_groups = yield self._get_state_group_for_events(
            event_ids,
        )

        groups = set(itervalues(event_to_groups))
        group_to_state = yield self._get_state_for_groups(groups)

        defer.returnValue(group_to_state)

    @defer.inlineCallbacks
    def get_state_ids_for_group(self, state_group):
        """Get the event IDs of all the state in the given state group

        Args:
            state_group (int)

        Returns:
            Deferred[dict]: Resolves to a map of (type, state_key) -> event_id
        """
        group_to_state = yield self._get_state_for_groups((state_group,))

        defer.returnValue(group_to_state[state_group])

    @defer.inlineCallbacks
    def get_state_groups(self, room_id, event_ids):
        """ Get the state groups for the given list of event_ids

        Returns:
            Deferred[dict[int, list[EventBase]]]:
                dict of state_group_id -> list of state events.
        """
        if not event_ids:
            defer.returnValue({})

        group_to_ids = yield self.get_state_groups_ids(room_id, event_ids)

        state_event_map = yield self.get_events(
            [
                ev_id for group_ids in itervalues(group_to_ids)
                for ev_id in itervalues(group_ids)
            ],
            get_prev_content=False
        )

        defer.returnValue({
            group: [
                state_event_map[v] for v in itervalues(event_id_map)
                if v in state_event_map
            ]
            for group, event_id_map in iteritems(group_to_ids)
        })

    @defer.inlineCallbacks
    def _get_state_groups_from_groups(self, groups, state_filter):
        """Returns the state groups for a given set of groups, filtering on
        types of state events.

        Args:
            groups(list[int]): list of state group IDs to query
            state_filter (StateFilter): The state filter used to fetch state
                from the database.
        Returns:
            Deferred[dict[int, dict[tuple[str, str], str]]]:
                dict of state_group_id -> (dict of (type, state_key) -> event id)
        """
        results = {}

        chunks = [groups[i:i + 100] for i in range(0, len(groups), 100)]
        for chunk in chunks:
            res = yield self.runInteraction(
                "_get_state_groups_from_groups",
                self._get_state_groups_from_groups_txn, chunk, state_filter,
            )
            results.update(res)

        defer.returnValue(results)

    def _get_state_groups_from_groups_txn(
        self, txn, groups, state_filter=StateFilter.all(),
    ):
        results = {group: {} for group in groups}

        where_clause, where_args = state_filter.make_sql_filter_clause()

        # Unless the filter clause is empty, we're going to append it after an
        # existing where clause
        if where_clause:
            where_clause = " AND (%s)" % (where_clause,)

        if isinstance(self.database_engine, PostgresEngine):
            # Temporarily disable sequential scans in this transaction. This is
            # a temporary hack until we can add the right indices in
            txn.execute("SET LOCAL enable_seqscan=off")

            # The below query walks the state_group tree so that the "state"
            # table includes all state_groups in the tree. It then joins
            # against `state_groups_state` to fetch the latest state.
            # It assumes that previous state groups are always numerically
            # lesser.
            # The PARTITION is used to get the event_id in the greatest state
            # group for the given type, state_key.
            # This may return multiple rows per (type, state_key), but last_value
            # should be the same.
            sql = """
                WITH RECURSIVE state(state_group) AS (
                    VALUES(?::bigint)
                    UNION ALL
                    SELECT prev_state_group FROM state_group_edges e, state s
                    WHERE s.state_group = e.state_group
                )
                SELECT DISTINCT type, state_key, last_value(event_id) OVER (
                    PARTITION BY type, state_key ORDER BY state_group ASC
                    ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
                ) AS event_id FROM state_groups_state
                WHERE state_group IN (
                    SELECT state_group FROM state
                )
            """

            for group in groups:
                args = [group]
                args.extend(where_args)

                txn.execute(sql + where_clause, args)
                for row in txn:
                    typ, state_key, event_id = row
                    key = (typ, state_key)
                    results[group][key] = event_id
        else:
            max_entries_returned = state_filter.max_entries_returned()

            # We don't use WITH RECURSIVE on sqlite3 as there are distributions
            # that ship with an sqlite3 version that doesn't support it (e.g. wheezy)
            for group in groups:
                next_group = group

                while next_group:
                    # We did this before by getting the list of group ids, and
                    # then passing that list to sqlite to get latest event for
                    # each (type, state_key). However, that was terribly slow
                    # without the right indices (which we can't add until
                    # after we finish deduping state, which requires this func)
                    args = [next_group]
                    args.extend(where_args)

                    txn.execute(
                        "SELECT type, state_key, event_id FROM state_groups_state"
                        " WHERE state_group = ? " + where_clause,
                        args
                    )
                    results[group].update(
                        ((typ, state_key), event_id)
                        for typ, state_key, event_id in txn
                        if (typ, state_key) not in results[group]
                    )

                    # If the number of entries in the (type,state_key)->event_id dict
                    # matches the number of (type,state_keys) types we were searching
                    # for, then we must have found them all, so no need to go walk
                    # further down the tree... UNLESS our types filter contained
                    # wildcards (i.e. Nones) in which case we have to do an exhaustive
                    # search
                    if (
                        max_entries_returned is not None and
                        len(results[group]) == max_entries_returned
                    ):
                        break

                    next_group = self._simple_select_one_onecol_txn(
                        txn,
                        table="state_group_edges",
                        keyvalues={"state_group": next_group},
                        retcol="prev_state_group",
                        allow_none=True,
                    )

        return results

    @defer.inlineCallbacks
    def get_state_for_events(self, event_ids, state_filter=StateFilter.all()):
        """Given a list of event_ids and type tuples, return a list of state
        dicts for each event.

        Args:
            event_ids (list[string])
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            deferred: A dict of (event_id) -> (type, state_key) -> [state_events]
        """
        event_to_groups = yield self._get_state_group_for_events(
            event_ids,
        )

        groups = set(itervalues(event_to_groups))
        group_to_state = yield self._get_state_for_groups(groups, state_filter)

        state_event_map = yield self.get_events(
            [ev_id for sd in itervalues(group_to_state) for ev_id in itervalues(sd)],
            get_prev_content=False
        )

        event_to_state = {
            event_id: {
                k: state_event_map[v]
                for k, v in iteritems(group_to_state[group])
                if v in state_event_map
            }
            for event_id, group in iteritems(event_to_groups)
        }

        defer.returnValue({event: event_to_state[event] for event in event_ids})

    @defer.inlineCallbacks
    def get_state_ids_for_events(self, event_ids, state_filter=StateFilter.all()):
        """
        Get the state dicts corresponding to a list of events, containing the event_ids
        of the state events (as opposed to the events themselves)

        Args:
            event_ids(list(str)): events whose state should be returned
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            A deferred dict from event_id -> (type, state_key) -> event_id
        """
        event_to_groups = yield self._get_state_group_for_events(
            event_ids,
        )

        groups = set(itervalues(event_to_groups))
        group_to_state = yield self._get_state_for_groups(groups, state_filter)

        event_to_state = {
            event_id: group_to_state[group]
            for event_id, group in iteritems(event_to_groups)
        }

        defer.returnValue({event: event_to_state[event] for event in event_ids})

    @defer.inlineCallbacks
    def get_state_for_event(self, event_id, state_filter=StateFilter.all()):
        """
        Get the state dict corresponding to a particular event

        Args:
            event_id(str): event whose state should be returned
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            A deferred dict from (type, state_key) -> state_event
        """
        state_map = yield self.get_state_for_events([event_id], state_filter)
        defer.returnValue(state_map[event_id])

    @defer.inlineCallbacks
    def get_state_ids_for_event(self, event_id, state_filter=StateFilter.all()):
        """
        Get the state dict corresponding to a particular event

        Args:
            event_id(str): event whose state should be returned
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            A deferred dict from (type, state_key) -> state_event
        """
        state_map = yield self.get_state_ids_for_events([event_id], state_filter)
        defer.returnValue(state_map[event_id])

    @cached(max_entries=50000)
    def _get_state_group_for_event(self, event_id):
        return self._simple_select_one_onecol(
            table="event_to_state_groups",
            keyvalues={
                "event_id": event_id,
            },
            retcol="state_group",
            allow_none=True,
            desc="_get_state_group_for_event",
        )

    @cachedList(cached_method_name="_get_state_group_for_event",
                list_name="event_ids", num_args=1, inlineCallbacks=True)
    def _get_state_group_for_events(self, event_ids):
        """Returns mapping event_id -> state_group
        """
        rows = yield self._simple_select_many_batch(
            table="event_to_state_groups",
            column="event_id",
            iterable=event_ids,
            keyvalues={},
            retcols=("event_id", "state_group",),
            desc="_get_state_group_for_events",
        )

        defer.returnValue({row["event_id"]: row["state_group"] for row in rows})

    def _get_state_for_group_using_cache(self, cache, group, state_filter):
        """Checks if group is in cache. See `_get_state_for_groups`

        Args:
            cache(DictionaryCache): the state group cache to use
            group(int): The state group to lookup
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns 2-tuple (`state_dict`, `got_all`).
        `got_all` is a bool indicating if we successfully retrieved all
        requests state from the cache, if False we need to query the DB for the
        missing state.
        """
        is_all, known_absent, state_dict_ids = cache.get(group)

        if is_all or state_filter.is_full():
            # Either we have everything or want everything, either way
            # `is_all` tells us whether we've gotten everything.
            return state_filter.filter_state(state_dict_ids), is_all

        # tracks whether any of our requested types are missing from the cache
        missing_types = False

        if state_filter.has_wildcards():
            # We don't know if we fetched all the state keys for the types in
            # the filter that are wildcards, so we have to assume that we may
            # have missed some.
            missing_types = True
        else:
            # There aren't any wild cards, so `concrete_types()` returns the
            # complete list of event types we're wanting.
            for key in state_filter.concrete_types():
                if key not in state_dict_ids and key not in known_absent:
                    missing_types = True
                    break

        return state_filter.filter_state(state_dict_ids), not missing_types

    @defer.inlineCallbacks
    def _get_state_for_groups(self, groups, state_filter=StateFilter.all()):
        """Gets the state at each of a list of state groups, optionally
        filtering by type/state_key

        Args:
            groups (iterable[int]): list of state groups for which we want
                to get the state.
            state_filter (StateFilter): The state filter used to fetch state
                from the database.
        Returns:
            Deferred[dict[int, dict[tuple[str, str], str]]]:
                dict of state_group_id -> (dict of (type, state_key) -> event id)
        """

        member_filter, non_member_filter = state_filter.get_member_split()

        # Now we look them up in the member and non-member caches
        non_member_state, incomplete_groups_nm, = (
            yield self._get_state_for_groups_using_cache(
                groups, self._state_group_cache,
                state_filter=non_member_filter,
            )
        )

        member_state, incomplete_groups_m, = (
            yield self._get_state_for_groups_using_cache(
                groups, self._state_group_members_cache,
                state_filter=member_filter,
            )
        )

        state = dict(non_member_state)
        for group in groups:
            state[group].update(member_state[group])

        # Now fetch any missing groups from the database

        incomplete_groups = incomplete_groups_m | incomplete_groups_nm

        if not incomplete_groups:
            defer.returnValue(state)

        cache_sequence_nm = self._state_group_cache.sequence
        cache_sequence_m = self._state_group_members_cache.sequence

        # Help the cache hit ratio by expanding the filter a bit
        db_state_filter = state_filter.return_expanded()

        group_to_state_dict = yield self._get_state_groups_from_groups(
            list(incomplete_groups),
            state_filter=db_state_filter,
        )

        # Now lets update the caches
        self._insert_into_cache(
            group_to_state_dict,
            db_state_filter,
            cache_seq_num_members=cache_sequence_m,
            cache_seq_num_non_members=cache_sequence_nm,
        )

        # And finally update the result dict, by filtering out any extra
        # stuff we pulled out of the database.
        for group, group_state_dict in iteritems(group_to_state_dict):
            # We just replace any existing entries, as we will have loaded
            # everything we need from the database anyway.
            state[group] = state_filter.filter_state(group_state_dict)

        defer.returnValue(state)

    def _get_state_for_groups_using_cache(
        self, groups, cache, state_filter,
    ):
        """Gets the state at each of a list of state groups, optionally
        filtering by type/state_key, querying from a specific cache.

        Args:
            groups (iterable[int]): list of state groups for which we want
                to get the state.
            cache (DictionaryCache): the cache of group ids to state dicts which
                we will pass through - either the normal state cache or the specific
                members state cache.
            state_filter (StateFilter): The state filter used to fetch state
                from the database.

        Returns:
            tuple[dict[int, dict[tuple[str, str], str]], set[int]]: Tuple of
            dict of state_group_id -> (dict of (type, state_key) -> event id)
            of entries in the cache, and the state group ids either missing
            from the cache or incomplete.
        """
        results = {}
        incomplete_groups = set()
        for group in set(groups):
            state_dict_ids, got_all = self._get_state_for_group_using_cache(
                cache, group, state_filter
            )
            results[group] = state_dict_ids

            if not got_all:
                incomplete_groups.add(group)

        return results, incomplete_groups

    def _insert_into_cache(self, group_to_state_dict, state_filter,
                           cache_seq_num_members, cache_seq_num_non_members):
        """Inserts results from querying the database into the relevant cache.

        Args:
            group_to_state_dict (dict): The new entries pulled from database.
                Map from state group to state dict
            state_filter (StateFilter): The state filter used to fetch state
                from the database.
            cache_seq_num_members (int): Sequence number of member cache since
                last lookup in cache
            cache_seq_num_non_members (int): Sequence number of member cache since
                last lookup in cache
        """

        # We need to work out which types we've fetched from the DB for the
        # member vs non-member caches. This should be as accurate as possible,
        # but can be an underestimate (e.g. when we have wild cards)

        member_filter, non_member_filter = state_filter.get_member_split()
        if member_filter.is_full():
            # We fetched all member events
            member_types = None
        else:
            # `concrete_types()` will only return a subset when there are wild
            # cards in the filter, but that's fine.
            member_types = member_filter.concrete_types()

        if non_member_filter.is_full():
            # We fetched all non member events
            non_member_types = None
        else:
            non_member_types = non_member_filter.concrete_types()

        for group, group_state_dict in iteritems(group_to_state_dict):
            state_dict_members = {}
            state_dict_non_members = {}

            for k, v in iteritems(group_state_dict):
                if k[0] == EventTypes.Member:
                    state_dict_members[k] = v
                else:
                    state_dict_non_members[k] = v

            self._state_group_members_cache.update(
                cache_seq_num_members,
                key=group,
                value=state_dict_members,
                fetched_keys=member_types,
            )

            self._state_group_cache.update(
                cache_seq_num_non_members,
                key=group,
                value=state_dict_non_members,
                fetched_keys=non_member_types,
            )

    def store_state_group(self, event_id, room_id, prev_group, delta_ids,
                          current_state_ids):
        """Store a new set of state, returning a newly assigned state group.

        Args:
            event_id (str): The event ID for which the state was calculated
            room_id (str)
            prev_group (int|None): A previous state group for the room, optional.
            delta_ids (dict|None): The delta between state at `prev_group` and
                `current_state_ids`, if `prev_group` was given. Same format as
                `current_state_ids`.
            current_state_ids (dict): The state to store. Map of (type, state_key)
                to event_id.

        Returns:
            Deferred[int]: The state group ID
        """
        def _store_state_group_txn(txn):
            if current_state_ids is None:
                # AFAIK, this can never happen
                raise Exception("current_state_ids cannot be None")

            state_group = self.database_engine.get_next_state_group_id(txn)

            self._simple_insert_txn(
                txn,
                table="state_groups",
                values={
                    "id": state_group,
                    "room_id": room_id,
                    "event_id": event_id,
                },
            )

            # We persist as a delta if we can, while also ensuring the chain
            # of deltas isn't tooo long, as otherwise read performance degrades.
            if prev_group:
                is_in_db = self._simple_select_one_onecol_txn(
                    txn,
                    table="state_groups",
                    keyvalues={"id": prev_group},
                    retcol="id",
                    allow_none=True,
                )
                if not is_in_db:
                    raise Exception(
                        "Trying to persist state with unpersisted prev_group: %r"
                        % (prev_group,)
                    )

                potential_hops = self._count_state_group_hops_txn(
                    txn, prev_group
                )
            if prev_group and potential_hops < MAX_STATE_DELTA_HOPS:
                self._simple_insert_txn(
                    txn,
                    table="state_group_edges",
                    values={
                        "state_group": state_group,
                        "prev_state_group": prev_group,
                    },
                )

                self._simple_insert_many_txn(
                    txn,
                    table="state_groups_state",
                    values=[
                        {
                            "state_group": state_group,
                            "room_id": room_id,
                            "type": key[0],
                            "state_key": key[1],
                            "event_id": state_id,
                        }
                        for key, state_id in iteritems(delta_ids)
                    ],
                )
            else:
                self._simple_insert_many_txn(
                    txn,
                    table="state_groups_state",
                    values=[
                        {
                            "state_group": state_group,
                            "room_id": room_id,
                            "type": key[0],
                            "state_key": key[1],
                            "event_id": state_id,
                        }
                        for key, state_id in iteritems(current_state_ids)
                    ],
                )

            # Prefill the state group caches with this group.
            # It's fine to use the sequence like this as the state group map
            # is immutable. (If the map wasn't immutable then this prefill could
            # race with another update)

            current_member_state_ids = {
                s: ev
                for (s, ev) in iteritems(current_state_ids)
                if s[0] == EventTypes.Member
            }
            txn.call_after(
                self._state_group_members_cache.update,
                self._state_group_members_cache.sequence,
                key=state_group,
                value=dict(current_member_state_ids),
            )

            current_non_member_state_ids = {
                s: ev
                for (s, ev) in iteritems(current_state_ids)
                if s[0] != EventTypes.Member
            }
            txn.call_after(
                self._state_group_cache.update,
                self._state_group_cache.sequence,
                key=state_group,
                value=dict(current_non_member_state_ids),
            )

            return state_group

        return self.runInteraction("store_state_group", _store_state_group_txn)

    def _count_state_group_hops_txn(self, txn, state_group):
        """Given a state group, count how many hops there are in the tree.

        This is used to ensure the delta chains don't get too long.
        """
        if isinstance(self.database_engine, PostgresEngine):
            sql = ("""
                WITH RECURSIVE state(state_group) AS (
                    VALUES(?::bigint)
                    UNION ALL
                    SELECT prev_state_group FROM state_group_edges e, state s
                    WHERE s.state_group = e.state_group
                )
                SELECT count(*) FROM state;
            """)

            txn.execute(sql, (state_group,))
            row = txn.fetchone()
            if row and row[0]:
                return row[0]
            else:
                return 0
        else:
            # We don't use WITH RECURSIVE on sqlite3 as there are distributions
            # that ship with an sqlite3 version that doesn't support it (e.g. wheezy)
            next_group = state_group
            count = 0

            while next_group:
                next_group = self._simple_select_one_onecol_txn(
                    txn,
                    table="state_group_edges",
                    keyvalues={"state_group": next_group},
                    retcol="prev_state_group",
                    allow_none=True,
                )
                if next_group:
                    count += 1

            return count


class StateStore(StateGroupWorkerStore, BackgroundUpdateStore):
    """ Keeps track of the state at a given event.

    This is done by the concept of `state groups`. Every event is a assigned
    a state group (identified by an arbitrary string), which references a
    collection of state events. The current state of an event is then the
    collection of state events referenced by the event's state group.

    Hence, every change in the current state causes a new state group to be
    generated. However, if no change happens (e.g., if we get a message event
    with only one parent it inherits the state group from its parent.)

    There are three tables:
      * `state_groups`: Stores group name, first event with in the group and
        room id.
      * `event_to_state_groups`: Maps events to state groups.
      * `state_groups_state`: Maps state group to state events.
    """

    STATE_GROUP_DEDUPLICATION_UPDATE_NAME = "state_group_state_deduplication"
    STATE_GROUP_INDEX_UPDATE_NAME = "state_group_state_type_index"
    CURRENT_STATE_INDEX_UPDATE_NAME = "current_state_members_idx"
    EVENT_STATE_GROUP_INDEX_UPDATE_NAME = "event_to_state_groups_sg_index"

    def __init__(self, db_conn, hs):
        super(StateStore, self).__init__(db_conn, hs)
        self.register_background_update_handler(
            self.STATE_GROUP_DEDUPLICATION_UPDATE_NAME,
            self._background_deduplicate_state,
        )
        self.register_background_update_handler(
            self.STATE_GROUP_INDEX_UPDATE_NAME,
            self._background_index_state,
        )
        self.register_background_index_update(
            self.CURRENT_STATE_INDEX_UPDATE_NAME,
            index_name="current_state_events_member_index",
            table="current_state_events",
            columns=["state_key"],
            where_clause="type='m.room.member'",
        )
        self.register_background_index_update(
            self.EVENT_STATE_GROUP_INDEX_UPDATE_NAME,
            index_name="event_to_state_groups_sg_index",
            table="event_to_state_groups",
            columns=["state_group"],
        )

    def _store_event_state_mappings_txn(self, txn, events_and_contexts):
        state_groups = {}
        for event, context in events_and_contexts:
            if event.internal_metadata.is_outlier():
                continue

            # if the event was rejected, just give it the same state as its
            # predecessor.
            if context.rejected:
                state_groups[event.event_id] = context.prev_group
                continue

            state_groups[event.event_id] = context.state_group

        self._simple_insert_many_txn(
            txn,
            table="event_to_state_groups",
            values=[
                {
                    "state_group": state_group_id,
                    "event_id": event_id,
                }
                for event_id, state_group_id in iteritems(state_groups)
            ],
        )

        for event_id, state_group_id in iteritems(state_groups):
            txn.call_after(
                self._get_state_group_for_event.prefill,
                (event_id,), state_group_id
            )

    @defer.inlineCallbacks
    def _background_deduplicate_state(self, progress, batch_size):
        """This background update will slowly deduplicate state by reencoding
        them as deltas.
        """
        last_state_group = progress.get("last_state_group", 0)
        rows_inserted = progress.get("rows_inserted", 0)
        max_group = progress.get("max_group", None)

        BATCH_SIZE_SCALE_FACTOR = 100

        batch_size = max(1, int(batch_size / BATCH_SIZE_SCALE_FACTOR))

        if max_group is None:
            rows = yield self._execute(
                "_background_deduplicate_state", None,
                "SELECT coalesce(max(id), 0) FROM state_groups",
            )
            max_group = rows[0][0]

        def reindex_txn(txn):
            new_last_state_group = last_state_group
            for count in range(batch_size):
                txn.execute(
                    "SELECT id, room_id FROM state_groups"
                    " WHERE ? < id AND id <= ?"
                    " ORDER BY id ASC"
                    " LIMIT 1",
                    (new_last_state_group, max_group,)
                )
                row = txn.fetchone()
                if row:
                    state_group, room_id = row

                if not row or not state_group:
                    return True, count

                txn.execute(
                    "SELECT state_group FROM state_group_edges"
                    " WHERE state_group = ?",
                    (state_group,)
                )

                # If we reach a point where we've already started inserting
                # edges we should stop.
                if txn.fetchall():
                    return True, count

                txn.execute(
                    "SELECT coalesce(max(id), 0) FROM state_groups"
                    " WHERE id < ? AND room_id = ?",
                    (state_group, room_id,)
                )
                prev_group, = txn.fetchone()
                new_last_state_group = state_group

                if prev_group:
                    potential_hops = self._count_state_group_hops_txn(
                        txn, prev_group
                    )
                    if potential_hops >= MAX_STATE_DELTA_HOPS:
                        # We want to ensure chains are at most this long,#
                        # otherwise read performance degrades.
                        continue

                    prev_state = self._get_state_groups_from_groups_txn(
                        txn, [prev_group],
                    )
                    prev_state = prev_state[prev_group]

                    curr_state = self._get_state_groups_from_groups_txn(
                        txn, [state_group],
                    )
                    curr_state = curr_state[state_group]

                    if not set(prev_state.keys()) - set(curr_state.keys()):
                        # We can only do a delta if the current has a strict super set
                        # of keys

                        delta_state = {
                            key: value for key, value in iteritems(curr_state)
                            if prev_state.get(key, None) != value
                        }

                        self._simple_delete_txn(
                            txn,
                            table="state_group_edges",
                            keyvalues={
                                "state_group": state_group,
                            }
                        )

                        self._simple_insert_txn(
                            txn,
                            table="state_group_edges",
                            values={
                                "state_group": state_group,
                                "prev_state_group": prev_group,
                            }
                        )

                        self._simple_delete_txn(
                            txn,
                            table="state_groups_state",
                            keyvalues={
                                "state_group": state_group,
                            }
                        )

                        self._simple_insert_many_txn(
                            txn,
                            table="state_groups_state",
                            values=[
                                {
                                    "state_group": state_group,
                                    "room_id": room_id,
                                    "type": key[0],
                                    "state_key": key[1],
                                    "event_id": state_id,
                                }
                                for key, state_id in iteritems(delta_state)
                            ],
                        )

            progress = {
                "last_state_group": state_group,
                "rows_inserted": rows_inserted + batch_size,
                "max_group": max_group,
            }

            self._background_update_progress_txn(
                txn, self.STATE_GROUP_DEDUPLICATION_UPDATE_NAME, progress
            )

            return False, batch_size

        finished, result = yield self.runInteraction(
            self.STATE_GROUP_DEDUPLICATION_UPDATE_NAME, reindex_txn
        )

        if finished:
            yield self._end_background_update(self.STATE_GROUP_DEDUPLICATION_UPDATE_NAME)

        defer.returnValue(result * BATCH_SIZE_SCALE_FACTOR)

    @defer.inlineCallbacks
    def _background_index_state(self, progress, batch_size):
        def reindex_txn(conn):
            conn.rollback()
            if isinstance(self.database_engine, PostgresEngine):
                # postgres insists on autocommit for the index
                conn.set_session(autocommit=True)
                try:
                    txn = conn.cursor()
                    txn.execute(
                        "CREATE INDEX CONCURRENTLY state_groups_state_type_idx"
                        " ON state_groups_state(state_group, type, state_key)"
                    )
                    txn.execute(
                        "DROP INDEX IF EXISTS state_groups_state_id"
                    )
                finally:
                    conn.set_session(autocommit=False)
            else:
                txn = conn.cursor()
                txn.execute(
                    "CREATE INDEX state_groups_state_type_idx"
                    " ON state_groups_state(state_group, type, state_key)"
                )
                txn.execute(
                    "DROP INDEX IF EXISTS state_groups_state_id"
                )

        yield self.runWithConnection(reindex_txn)

        yield self._end_background_update(self.STATE_GROUP_INDEX_UPDATE_NAME)

        defer.returnValue(1)