synapse/synapse/storage/databases/main/user_directory.py

# -*- coding: utf-8 -*-
# Copyright 2017 Vector Creations 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
import re
from typing import Any, Dict, Iterable, Optional, Set, Tuple

from synapse.api.constants import EventTypes, HistoryVisibility, JoinRules
from synapse.storage.database import DatabasePool
from synapse.storage.databases.main.state import StateFilter
from synapse.storage.databases.main.state_deltas import StateDeltasStore
from synapse.storage.engines import PostgresEngine, Sqlite3Engine
from synapse.types import get_domain_from_id, get_localpart_from_id
from synapse.util.caches.descriptors import cached

logger = logging.getLogger(__name__)


TEMP_TABLE = "_temp_populate_user_directory"


class UserDirectoryBackgroundUpdateStore(StateDeltasStore):

    # How many records do we calculate before sending it to
    # add_users_who_share_private_rooms?
    SHARE_PRIVATE_WORKING_SET = 500

    def __init__(self, database: DatabasePool, db_conn, hs):
        super().__init__(database, db_conn, hs)

        self.server_name = hs.hostname

        self.db_pool.updates.register_background_update_handler(
            "populate_user_directory_createtables",
            self._populate_user_directory_createtables,
        )
        self.db_pool.updates.register_background_update_handler(
            "populate_user_directory_process_rooms",
            self._populate_user_directory_process_rooms,
        )
        self.db_pool.updates.register_background_update_handler(
            "populate_user_directory_process_users",
            self._populate_user_directory_process_users,
        )
        self.db_pool.updates.register_background_update_handler(
            "populate_user_directory_cleanup", self._populate_user_directory_cleanup
        )

    async def _populate_user_directory_createtables(self, progress, batch_size):

        # Get all the rooms that we want to process.
        def _make_staging_area(txn):
            sql = (
                "CREATE TABLE IF NOT EXISTS "
                + TEMP_TABLE
                + "_rooms(room_id TEXT NOT NULL, events BIGINT NOT NULL)"
            )
            txn.execute(sql)

            sql = (
                "CREATE TABLE IF NOT EXISTS "
                + TEMP_TABLE
                + "_position(position TEXT NOT NULL)"
            )
            txn.execute(sql)

            # Get rooms we want to process from the database
            sql = """
                SELECT room_id, count(*) FROM current_state_events
                GROUP BY room_id
            """
            txn.execute(sql)
            rooms = [{"room_id": x[0], "events": x[1]} for x in txn.fetchall()]
            self.db_pool.simple_insert_many_txn(txn, TEMP_TABLE + "_rooms", rooms)
            del rooms

            # If search all users is on, get all the users we want to add.
            if self.hs.config.user_directory_search_all_users:
                sql = (
                    "CREATE TABLE IF NOT EXISTS "
                    + TEMP_TABLE
                    + "_users(user_id TEXT NOT NULL)"
                )
                txn.execute(sql)

                txn.execute("SELECT name FROM users")
                users = [{"user_id": x[0]} for x in txn.fetchall()]

                self.db_pool.simple_insert_many_txn(txn, TEMP_TABLE + "_users", users)

        new_pos = await self.get_max_stream_id_in_current_state_deltas()
        await self.db_pool.runInteraction(
            "populate_user_directory_temp_build", _make_staging_area
        )
        await self.db_pool.simple_insert(
            TEMP_TABLE + "_position", {"position": new_pos}
        )

        await self.db_pool.updates._end_background_update(
            "populate_user_directory_createtables"
        )
        return 1

    async def _populate_user_directory_cleanup(self, progress, batch_size):
        """
        Update the user directory stream position, then clean up the old tables.
        """
        position = await self.db_pool.simple_select_one_onecol(
            TEMP_TABLE + "_position", None, "position"
        )
        await self.update_user_directory_stream_pos(position)

        def _delete_staging_area(txn):
            txn.execute("DROP TABLE IF EXISTS " + TEMP_TABLE + "_rooms")
            txn.execute("DROP TABLE IF EXISTS " + TEMP_TABLE + "_users")
            txn.execute("DROP TABLE IF EXISTS " + TEMP_TABLE + "_position")

        await self.db_pool.runInteraction(
            "populate_user_directory_cleanup", _delete_staging_area
        )

        await self.db_pool.updates._end_background_update(
            "populate_user_directory_cleanup"
        )
        return 1

    async def _populate_user_directory_process_rooms(self, progress, batch_size):
        """
        Args:
            progress (dict)
            batch_size (int): Maximum number of state events to process
                per cycle.
        """
        state = self.hs.get_state_handler()

        # If we don't have progress filed, delete everything.
        if not progress:
            await self.delete_all_from_user_dir()

        def _get_next_batch(txn):
            # Only fetch 250 rooms, so we don't fetch too many at once, even
            # if those 250 rooms have less than batch_size state events.
            sql = """
                SELECT room_id, events FROM %s
                ORDER BY events DESC
                LIMIT 250
            """ % (
                TEMP_TABLE + "_rooms",
            )
            txn.execute(sql)
            rooms_to_work_on = txn.fetchall()

            if not rooms_to_work_on:
                return None

            # Get how many are left to process, so we can give status on how
            # far we are in processing
            txn.execute("SELECT COUNT(*) FROM " + TEMP_TABLE + "_rooms")
            progress["remaining"] = txn.fetchone()[0]

            return rooms_to_work_on

        rooms_to_work_on = await self.db_pool.runInteraction(
            "populate_user_directory_temp_read", _get_next_batch
        )

        # No more rooms -- complete the transaction.
        if not rooms_to_work_on:
            await self.db_pool.updates._end_background_update(
                "populate_user_directory_process_rooms"
            )
            return 1

        logger.debug(
            "Processing the next %d rooms of %d remaining"
            % (len(rooms_to_work_on), progress["remaining"])
        )

        processed_event_count = 0

        for room_id, event_count in rooms_to_work_on:
            is_in_room = await self.is_host_joined(room_id, self.server_name)

            if is_in_room:
                is_public = await self.is_room_world_readable_or_publicly_joinable(
                    room_id
                )

                users_with_profile = await state.get_current_users_in_room(room_id)
                user_ids = set(users_with_profile)

                # Update each user in the user directory.
                for user_id, profile in users_with_profile.items():
                    await self.update_profile_in_user_dir(
                        user_id, profile.display_name, profile.avatar_url
                    )

                to_insert = set()

                if is_public:
                    for user_id in user_ids:
                        if self.get_if_app_services_interested_in_user(user_id):
                            continue

                        to_insert.add(user_id)

                    if to_insert:
                        await self.add_users_in_public_rooms(room_id, to_insert)
                        to_insert.clear()
                else:
                    for user_id in user_ids:
                        if not self.hs.is_mine_id(user_id):
                            continue

                        if self.get_if_app_services_interested_in_user(user_id):
                            continue

                        for other_user_id in user_ids:
                            if user_id == other_user_id:
                                continue

                            user_set = (user_id, other_user_id)
                            to_insert.add(user_set)

                            # If it gets too big, stop and write to the database
                            # to prevent storing too much in RAM.
                            if len(to_insert) >= self.SHARE_PRIVATE_WORKING_SET:
                                await self.add_users_who_share_private_room(
                                    room_id, to_insert
                                )
                                to_insert.clear()

                    if to_insert:
                        await self.add_users_who_share_private_room(room_id, to_insert)
                        to_insert.clear()

            # We've finished a room. Delete it from the table.
            await self.db_pool.simple_delete_one(
                TEMP_TABLE + "_rooms", {"room_id": room_id}
            )
            # Update the remaining counter.
            progress["remaining"] -= 1
            await self.db_pool.runInteraction(
                "populate_user_directory",
                self.db_pool.updates._background_update_progress_txn,
                "populate_user_directory_process_rooms",
                progress,
            )

            processed_event_count += event_count

            if processed_event_count > batch_size:
                # Don't process any more rooms, we've hit our batch size.
                return processed_event_count

        return processed_event_count

    async def _populate_user_directory_process_users(self, progress, batch_size):
        """
        If search_all_users is enabled, add all of the users to the user directory.
        """
        if not self.hs.config.user_directory_search_all_users:
            await self.db_pool.updates._end_background_update(
                "populate_user_directory_process_users"
            )
            return 1

        def _get_next_batch(txn):
            sql = "SELECT user_id FROM %s LIMIT %s" % (
                TEMP_TABLE + "_users",
                str(batch_size),
            )
            txn.execute(sql)
            users_to_work_on = txn.fetchall()

            if not users_to_work_on:
                return None

            users_to_work_on = [x[0] for x in users_to_work_on]

            # Get how many are left to process, so we can give status on how
            # far we are in processing
            sql = "SELECT COUNT(*) FROM " + TEMP_TABLE + "_users"
            txn.execute(sql)
            progress["remaining"] = txn.fetchone()[0]

            return users_to_work_on

        users_to_work_on = await self.db_pool.runInteraction(
            "populate_user_directory_temp_read", _get_next_batch
        )

        # No more users -- complete the transaction.
        if not users_to_work_on:
            await self.db_pool.updates._end_background_update(
                "populate_user_directory_process_users"
            )
            return 1

        logger.debug(
            "Processing the next %d users of %d remaining"
            % (len(users_to_work_on), progress["remaining"])
        )

        for user_id in users_to_work_on:
            profile = await self.get_profileinfo(get_localpart_from_id(user_id))
            await self.update_profile_in_user_dir(
                user_id, profile.display_name, profile.avatar_url
            )

            # We've finished processing a user. Delete it from the table.
            await self.db_pool.simple_delete_one(
                TEMP_TABLE + "_users", {"user_id": user_id}
            )
            # Update the remaining counter.
            progress["remaining"] -= 1
            await self.db_pool.runInteraction(
                "populate_user_directory",
                self.db_pool.updates._background_update_progress_txn,
                "populate_user_directory_process_users",
                progress,
            )

        return len(users_to_work_on)

    async def is_room_world_readable_or_publicly_joinable(self, room_id):
        """Check if the room is either world_readable or publically joinable"""

        # Create a state filter that only queries join and history state event
        types_to_filter = (
            (EventTypes.JoinRules, ""),
            (EventTypes.RoomHistoryVisibility, ""),
        )

        current_state_ids = await self.get_filtered_current_state_ids(
            room_id, StateFilter.from_types(types_to_filter)
        )

        join_rules_id = current_state_ids.get((EventTypes.JoinRules, ""))
        if join_rules_id:
            join_rule_ev = await self.get_event(join_rules_id, allow_none=True)
            if join_rule_ev:
                if join_rule_ev.content.get("join_rule") == JoinRules.PUBLIC:
                    return True

        hist_vis_id = current_state_ids.get((EventTypes.RoomHistoryVisibility, ""))
        if hist_vis_id:
            hist_vis_ev = await self.get_event(hist_vis_id, allow_none=True)
            if hist_vis_ev:
                if (
                    hist_vis_ev.content.get("history_visibility")
                    == HistoryVisibility.WORLD_READABLE
                ):
                    return True

        return False

    async def update_profile_in_user_dir(
        self, user_id: str, display_name: str, avatar_url: str
    ) -> None:
        """
        Update or add a user's profile in the user directory.
        """
        # If the display name or avatar URL are unexpected types, overwrite them.
        if not isinstance(display_name, str):
            display_name = None
        if not isinstance(avatar_url, str):
            avatar_url = None

        def _update_profile_in_user_dir_txn(txn):
            new_entry = self.db_pool.simple_upsert_txn(
                txn,
                table="user_directory",
                keyvalues={"user_id": user_id},
                values={"display_name": display_name, "avatar_url": avatar_url},
                lock=False,  # We're only inserter
            )

            if isinstance(self.database_engine, PostgresEngine):
                # We weight the localpart most highly, then display name and finally
                # server name
                if self.database_engine.can_native_upsert:
                    sql = """
                        INSERT INTO user_directory_search(user_id, vector)
                        VALUES (?,
                            setweight(to_tsvector('simple', ?), 'A')
                            || setweight(to_tsvector('simple', ?), 'D')
                            || setweight(to_tsvector('simple', COALESCE(?, '')), 'B')
                        ) ON CONFLICT (user_id) DO UPDATE SET vector=EXCLUDED.vector
                    """
                    txn.execute(
                        sql,
                        (
                            user_id,
                            get_localpart_from_id(user_id),
                            get_domain_from_id(user_id),
                            display_name,
                        ),
                    )
                else:
                    # TODO: Remove this code after we've bumped the minimum version
                    # of postgres to always support upserts, so we can get rid of
                    # `new_entry` usage
                    if new_entry is True:
                        sql = """
                            INSERT INTO user_directory_search(user_id, vector)
                            VALUES (?,
                                setweight(to_tsvector('simple', ?), 'A')
                                || setweight(to_tsvector('simple', ?), 'D')
                                || setweight(to_tsvector('simple', COALESCE(?, '')), 'B')
                            )
                        """
                        txn.execute(
                            sql,
                            (
                                user_id,
                                get_localpart_from_id(user_id),
                                get_domain_from_id(user_id),
                                display_name,
                            ),
                        )
                    elif new_entry is False:
                        sql = """
                            UPDATE user_directory_search
                            SET vector = setweight(to_tsvector('simple', ?), 'A')
                                || setweight(to_tsvector('simple', ?), 'D')
                                || setweight(to_tsvector('simple', COALESCE(?, '')), 'B')
                            WHERE user_id = ?
                        """
                        txn.execute(
                            sql,
                            (
                                get_localpart_from_id(user_id),
                                get_domain_from_id(user_id),
                                display_name,
                                user_id,
                            ),
                        )
                    else:
                        raise RuntimeError(
                            "upsert returned None when 'can_native_upsert' is False"
                        )
            elif isinstance(self.database_engine, Sqlite3Engine):
                value = "%s %s" % (user_id, display_name) if display_name else user_id
                self.db_pool.simple_upsert_txn(
                    txn,
                    table="user_directory_search",
                    keyvalues={"user_id": user_id},
                    values={"value": value},
                    lock=False,  # We're only inserter
                )
            else:
                # This should be unreachable.
                raise Exception("Unrecognized database engine")

            txn.call_after(self.get_user_in_directory.invalidate, (user_id,))

        await self.db_pool.runInteraction(
            "update_profile_in_user_dir", _update_profile_in_user_dir_txn
        )

    async def add_users_who_share_private_room(
        self, room_id: str, user_id_tuples: Iterable[Tuple[str, str]]
    ) -> None:
        """Insert entries into the users_who_share_private_rooms table. The first
        user should be a local user.

        Args:
            room_id
            user_id_tuples: iterable of 2-tuple of user IDs.
        """

        await self.db_pool.simple_upsert_many(
            table="users_who_share_private_rooms",
            key_names=["user_id", "other_user_id", "room_id"],
            key_values=[
                (user_id, other_user_id, room_id)
                for user_id, other_user_id in user_id_tuples
            ],
            value_names=(),
            value_values=None,
            desc="add_users_who_share_room",
        )

    async def add_users_in_public_rooms(
        self, room_id: str, user_ids: Iterable[str]
    ) -> None:
        """Insert entries into the users_in_public_rooms table.

        Args:
            room_id
            user_ids
        """

        await self.db_pool.simple_upsert_many(
            table="users_in_public_rooms",
            key_names=["user_id", "room_id"],
            key_values=[(user_id, room_id) for user_id in user_ids],
            value_names=(),
            value_values=None,
            desc="add_users_in_public_rooms",
        )

    async def delete_all_from_user_dir(self) -> None:
        """Delete the entire user directory"""

        def _delete_all_from_user_dir_txn(txn):
            txn.execute("DELETE FROM user_directory")
            txn.execute("DELETE FROM user_directory_search")
            txn.execute("DELETE FROM users_in_public_rooms")
            txn.execute("DELETE FROM users_who_share_private_rooms")
            txn.call_after(self.get_user_in_directory.invalidate_all)

        await self.db_pool.runInteraction(
            "delete_all_from_user_dir", _delete_all_from_user_dir_txn
        )

    @cached()
    async def get_user_in_directory(self, user_id: str) -> Optional[Dict[str, Any]]:
        return await self.db_pool.simple_select_one(
            table="user_directory",
            keyvalues={"user_id": user_id},
            retcols=("display_name", "avatar_url"),
            allow_none=True,
            desc="get_user_in_directory",
        )

    async def update_user_directory_stream_pos(self, stream_id: int) -> None:
        await self.db_pool.simple_update_one(
            table="user_directory_stream_pos",
            keyvalues={},
            updatevalues={"stream_id": stream_id},
            desc="update_user_directory_stream_pos",
        )


class UserDirectoryStore(UserDirectoryBackgroundUpdateStore):

    # How many records do we calculate before sending it to
    # add_users_who_share_private_rooms?
    SHARE_PRIVATE_WORKING_SET = 500

    def __init__(self, database: DatabasePool, db_conn, hs):
        super().__init__(database, db_conn, hs)

        self._prefer_local_users_in_search = (
            hs.config.user_directory_search_prefer_local_users
        )
        self._server_name = hs.config.server_name

    async def remove_from_user_dir(self, user_id: str) -> None:
        def _remove_from_user_dir_txn(txn):
            self.db_pool.simple_delete_txn(
                txn, table="user_directory", keyvalues={"user_id": user_id}
            )
            self.db_pool.simple_delete_txn(
                txn, table="user_directory_search", keyvalues={"user_id": user_id}
            )
            self.db_pool.simple_delete_txn(
                txn, table="users_in_public_rooms", keyvalues={"user_id": user_id}
            )
            self.db_pool.simple_delete_txn(
                txn,
                table="users_who_share_private_rooms",
                keyvalues={"user_id": user_id},
            )
            self.db_pool.simple_delete_txn(
                txn,
                table="users_who_share_private_rooms",
                keyvalues={"other_user_id": user_id},
            )
            txn.call_after(self.get_user_in_directory.invalidate, (user_id,))

        await self.db_pool.runInteraction(
            "remove_from_user_dir", _remove_from_user_dir_txn
        )

    async def get_users_in_dir_due_to_room(self, room_id):
        """Get all user_ids that are in the room directory because they're
        in the given room_id
        """
        user_ids_share_pub = await self.db_pool.simple_select_onecol(
            table="users_in_public_rooms",
            keyvalues={"room_id": room_id},
            retcol="user_id",
            desc="get_users_in_dir_due_to_room",
        )

        user_ids_share_priv = await self.db_pool.simple_select_onecol(
            table="users_who_share_private_rooms",
            keyvalues={"room_id": room_id},
            retcol="other_user_id",
            desc="get_users_in_dir_due_to_room",
        )

        user_ids = set(user_ids_share_pub)
        user_ids.update(user_ids_share_priv)

        return user_ids

    async def remove_user_who_share_room(self, user_id: str, room_id: str) -> None:
        """
        Deletes entries in the users_who_share_*_rooms table. The first
        user should be a local user.

        Args:
            user_id
            room_id
        """

        def _remove_user_who_share_room_txn(txn):
            self.db_pool.simple_delete_txn(
                txn,
                table="users_who_share_private_rooms",
                keyvalues={"user_id": user_id, "room_id": room_id},
            )
            self.db_pool.simple_delete_txn(
                txn,
                table="users_who_share_private_rooms",
                keyvalues={"other_user_id": user_id, "room_id": room_id},
            )
            self.db_pool.simple_delete_txn(
                txn,
                table="users_in_public_rooms",
                keyvalues={"user_id": user_id, "room_id": room_id},
            )

        await self.db_pool.runInteraction(
            "remove_user_who_share_room", _remove_user_who_share_room_txn
        )

    async def get_user_dir_rooms_user_is_in(self, user_id):
        """
        Returns the rooms that a user is in.

        Args:
            user_id(str): Must be a local user

        Returns:
            list: user_id
        """
        rows = await self.db_pool.simple_select_onecol(
            table="users_who_share_private_rooms",
            keyvalues={"user_id": user_id},
            retcol="room_id",
            desc="get_rooms_user_is_in",
        )

        pub_rows = await self.db_pool.simple_select_onecol(
            table="users_in_public_rooms",
            keyvalues={"user_id": user_id},
            retcol="room_id",
            desc="get_rooms_user_is_in",
        )

        users = set(pub_rows)
        users.update(rows)
        return list(users)

    async def get_shared_rooms_for_users(
        self, user_id: str, other_user_id: str
    ) -> Set[str]:
        """
        Returns the rooms that a local user shares with another local or remote user.

        Args:
            user_id: The MXID of a local user
            other_user_id: The MXID of the other user

        Returns:
            A set of room ID's that the users share.
        """

        def _get_shared_rooms_for_users_txn(txn):
            txn.execute(
                """
                SELECT p1.room_id
                FROM users_in_public_rooms as p1
                INNER JOIN users_in_public_rooms as p2
                    ON p1.room_id = p2.room_id
                    AND p1.user_id = ?
                    AND p2.user_id = ?
                UNION
                SELECT room_id
                FROM users_who_share_private_rooms
                WHERE
                    user_id = ?
                    AND other_user_id = ?
                """,
                (user_id, other_user_id, user_id, other_user_id),
            )
            rows = self.db_pool.cursor_to_dict(txn)
            return rows

        rows = await self.db_pool.runInteraction(
            "get_shared_rooms_for_users", _get_shared_rooms_for_users_txn
        )

        return {row["room_id"] for row in rows}

    async def get_user_directory_stream_pos(self) -> Optional[int]:
        """
        Get the stream ID of the user directory stream.

        Returns:
            The stream token or None if the initial background update hasn't happened yet.
        """
        return await self.db_pool.simple_select_one_onecol(
            table="user_directory_stream_pos",
            keyvalues={},
            retcol="stream_id",
            desc="get_user_directory_stream_pos",
        )

    async def search_user_dir(self, user_id, search_term, limit):
        """Searches for users in directory

        Returns:
            dict of the form::

                {
                    "limited": <bool>,  # whether there were more results or not
                    "results": [  # Ordered by best match first
                        {
                            "user_id": <user_id>,
                            "display_name": <display_name>,
                            "avatar_url": <avatar_url>
                        }
                    ]
                }
        """

        if self.hs.config.user_directory_search_all_users:
            join_args = (user_id,)
            where_clause = "user_id != ?"
        else:
            join_args = (user_id,)
            where_clause = """
                (
                    EXISTS (select 1 from users_in_public_rooms WHERE user_id = t.user_id)
                    OR EXISTS (
                        SELECT 1 FROM users_who_share_private_rooms
                        WHERE user_id = ? AND other_user_id = t.user_id
                    )
                )
            """

        # We allow manipulating the ranking algorithm by injecting statements
        # based on config options.
        additional_ordering_statements = []
        ordering_arguments = ()

        if isinstance(self.database_engine, PostgresEngine):
            full_query, exact_query, prefix_query = _parse_query_postgres(search_term)

            # If enabled, this config option will rank local users higher than those on
            # remote instances.
            if self._prefer_local_users_in_search:
                # This statement checks whether a given user's user ID contains a server name
                # that matches the local server
                statement = "* (CASE WHEN user_id LIKE ? THEN 2.0 ELSE 1.0 END)"
                additional_ordering_statements.append(statement)

                ordering_arguments += ("%:" + self._server_name,)

            # We order by rank and then if they have profile info
            # The ranking algorithm is hand tweaked for "best" results. Broadly
            # the idea is we give a higher weight to exact matches.
            # The array of numbers are the weights for the various part of the
            # search: (domain, _, display name, localpart)
            sql = """
                SELECT d.user_id AS user_id, display_name, avatar_url
                FROM user_directory_search as t
                INNER JOIN user_directory AS d USING (user_id)
                WHERE
                    %(where_clause)s
                    AND vector @@ to_tsquery('simple', ?)
                ORDER BY
                    (CASE WHEN d.user_id IS NOT NULL THEN 4.0 ELSE 1.0 END)
                    * (CASE WHEN display_name IS NOT NULL THEN 1.2 ELSE 1.0 END)
                    * (CASE WHEN avatar_url IS NOT NULL THEN 1.2 ELSE 1.0 END)
                    * (
                        3 * ts_rank_cd(
                            '{0.1, 0.1, 0.9, 1.0}',
                            vector,
                            to_tsquery('simple', ?),
                            8
                        )
                        + ts_rank_cd(
                            '{0.1, 0.1, 0.9, 1.0}',
                            vector,
                            to_tsquery('simple', ?),
                            8
                        )
                    )
                    %(order_case_statements)s
                    DESC,
                    display_name IS NULL,
                    avatar_url IS NULL
                LIMIT ?
            """ % {
                "where_clause": where_clause,
                "order_case_statements": " ".join(additional_ordering_statements),
            }
            args = (
                join_args
                + (full_query, exact_query, prefix_query)
                + ordering_arguments
                + (limit + 1,)
            )
        elif isinstance(self.database_engine, Sqlite3Engine):
            search_query = _parse_query_sqlite(search_term)

            # If enabled, this config option will rank local users higher than those on
            # remote instances.
            if self._prefer_local_users_in_search:
                # This statement checks whether a given user's user ID contains a server name
                # that matches the local server
                #
                # Note that we need to include a comma at the end for valid SQL
                statement = "user_id LIKE ? DESC,"
                additional_ordering_statements.append(statement)

                ordering_arguments += ("%:" + self._server_name,)

            sql = """
                SELECT d.user_id AS user_id, display_name, avatar_url
                FROM user_directory_search as t
                INNER JOIN user_directory AS d USING (user_id)
                WHERE
                    %(where_clause)s
                    AND value MATCH ?
                ORDER BY
                    rank(matchinfo(user_directory_search)) DESC,
                    %(order_statements)s
                    display_name IS NULL,
                    avatar_url IS NULL
                LIMIT ?
            """ % {
                "where_clause": where_clause,
                "order_statements": " ".join(additional_ordering_statements),
            }
            args = join_args + (search_query,) + ordering_arguments + (limit + 1,)
        else:
            # This should be unreachable.
            raise Exception("Unrecognized database engine")

        results = await self.db_pool.execute(
            "search_user_dir", self.db_pool.cursor_to_dict, sql, *args
        )

        limited = len(results) > limit

        return {"limited": limited, "results": results}


def _parse_query_sqlite(search_term):
    """Takes a plain unicode string from the user and converts it into a form
    that can be passed to database.
    We use this so that we can add prefix matching, which isn't something
    that is supported by default.

    We specifically add both a prefix and non prefix matching term so that
    exact matches get ranked higher.
    """

    # Pull out the individual words, discarding any non-word characters.
    results = re.findall(r"([\w\-]+)", search_term, re.UNICODE)
    return " & ".join("(%s* OR %s)" % (result, result) for result in results)


def _parse_query_postgres(search_term):
    """Takes a plain unicode string from the user and converts it into a form
    that can be passed to database.
    We use this so that we can add prefix matching, which isn't something
    that is supported by default.
    """

    # Pull out the individual words, discarding any non-word characters.
    results = re.findall(r"([\w\-]+)", search_term, re.UNICODE)

    both = " & ".join("(%s:* | %s)" % (result, result) for result in results)
    exact = " & ".join("%s" % (result,) for result in results)
    prefix = " & ".join("%s:*" % (result,) for result in results)

    return both, exact, prefix