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Optimise backfill calculation (#12522)
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Try to avoid an OOM by checking fewer extremities.

Generally this is a big rewrite of _maybe_backfill, to try and fix some of the TODOs and other problems in it. It's best reviewed commit-by-commit.
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@richvdh
richvdh authored Apr 26, 2022
1 parent e75c7e3 commit 17d99f7
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1 change: 1 addition & 0 deletions changelog.d/12522.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix a bug introduced in Synapse 0.99.3 which could cause Synapse to consume large amounts of RAM when back-paginating in a large room.
234 changes: 145 additions & 89 deletions synapse/handlers/federation.py
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
# Copyright 2014-2021 The Matrix.org Foundation C.I.C.
# Copyright 2014-2022 The Matrix.org Foundation C.I.C.
# Copyright 2020 Sorunome
#
# Licensed under the Apache License, Version 2.0 (the "License");
Expand All @@ -15,10 +15,14 @@

"""Contains handlers for federation events."""

import enum
import itertools
import logging
from enum import Enum
from http import HTTPStatus
from typing import TYPE_CHECKING, Dict, Iterable, List, Optional, Tuple, Union

import attr
from signedjson.key import decode_verify_key_bytes
from signedjson.sign import verify_signed_json
from unpaddedbase64 import decode_base64
Expand Down Expand Up @@ -92,6 +96,24 @@ def get_domains_from_state(state: StateMap[EventBase]) -> List[Tuple[str, int]]:
return sorted(joined_domains.items(), key=lambda d: d[1])


class _BackfillPointType(Enum):
# a regular backwards extremity (ie, an event which we don't yet have, but which
# is referred to by other events in the DAG)
BACKWARDS_EXTREMITY = enum.auto()

# an MSC2716 "insertion event"
INSERTION_PONT = enum.auto()


@attr.s(slots=True, auto_attribs=True, frozen=True)
class _BackfillPoint:
"""A potential point we might backfill from"""

event_id: str
depth: int
type: _BackfillPointType


class FederationHandler:
"""Handles general incoming federation requests
Expand Down Expand Up @@ -157,89 +179,51 @@ async def maybe_backfill(
async def _maybe_backfill_inner(
self, room_id: str, current_depth: int, limit: int
) -> bool:
oldest_events_with_depth = (
await self.store.get_oldest_event_ids_with_depth_in_room(room_id)
)
backwards_extremities = [
_BackfillPoint(event_id, depth, _BackfillPointType.BACKWARDS_EXTREMITY)
for event_id, depth in await self.store.get_oldest_event_ids_with_depth_in_room(
room_id
)
]

insertion_events_to_be_backfilled: Dict[str, int] = {}
insertion_events_to_be_backfilled: List[_BackfillPoint] = []
if self.hs.config.experimental.msc2716_enabled:
insertion_events_to_be_backfilled = (
await self.store.get_insertion_event_backward_extremities_in_room(
insertion_events_to_be_backfilled = [
_BackfillPoint(event_id, depth, _BackfillPointType.INSERTION_PONT)
for event_id, depth in await self.store.get_insertion_event_backward_extremities_in_room(
room_id
)
)
]
logger.debug(
"_maybe_backfill_inner: extremities oldest_events_with_depth=%s insertion_events_to_be_backfilled=%s",
oldest_events_with_depth,
"_maybe_backfill_inner: backwards_extremities=%s insertion_events_to_be_backfilled=%s",
backwards_extremities,
insertion_events_to_be_backfilled,
)

if not oldest_events_with_depth and not insertion_events_to_be_backfilled:
if not backwards_extremities and not insertion_events_to_be_backfilled:
logger.debug("Not backfilling as no extremeties found.")
return False

# We only want to paginate if we can actually see the events we'll get,
# as otherwise we'll just spend a lot of resources to get redacted
# events.
#
# We do this by filtering all the backwards extremities and seeing if
# any remain. Given we don't have the extremity events themselves, we
# need to actually check the events that reference them.
#
# *Note*: the spec wants us to keep backfilling until we reach the start
# of the room in case we are allowed to see some of the history. However
# in practice that causes more issues than its worth, as a) its
# relatively rare for there to be any visible history and b) even when
# there is its often sufficiently long ago that clients would stop
# attempting to paginate before backfill reached the visible history.
#
# TODO: If we do do a backfill then we should filter the backwards
# extremities to only include those that point to visible portions of
# history.
#
# TODO: Correctly handle the case where we are allowed to see the
# forward event but not the backward extremity, e.g. in the case of
# initial join of the server where we are allowed to see the join
# event but not anything before it. This would require looking at the
# state *before* the event, ignoring the special casing certain event
# types have.

forward_event_ids = await self.store.get_successor_events(
list(oldest_events_with_depth)
)

extremities_events = await self.store.get_events(
forward_event_ids,
redact_behaviour=EventRedactBehaviour.AS_IS,
get_prev_content=False,
# we now have a list of potential places to backpaginate from. We prefer to
# start with the most recent (ie, max depth), so let's sort the list.
sorted_backfill_points: List[_BackfillPoint] = sorted(
itertools.chain(
backwards_extremities,
insertion_events_to_be_backfilled,
),
key=lambda e: -int(e.depth),
)

# We set `check_history_visibility_only` as we might otherwise get false
# positives from users having been erased.
filtered_extremities = await filter_events_for_server(
self.storage,
self.server_name,
list(extremities_events.values()),
redact=False,
check_history_visibility_only=True,
)
logger.debug(
"_maybe_backfill_inner: filtered_extremities %s", filtered_extremities
"_maybe_backfill_inner: room_id: %s: current_depth: %s, limit: %s, "
"backfill points (%d): %s",
room_id,
current_depth,
limit,
len(sorted_backfill_points),
sorted_backfill_points,
)

if not filtered_extremities and not insertion_events_to_be_backfilled:
return False

extremities = {
**oldest_events_with_depth,
# TODO: insertion_events_to_be_backfilled is currently skipping the filtered_extremities checks
**insertion_events_to_be_backfilled,
}

# Check if we reached a point where we should start backfilling.
sorted_extremeties_tuple = sorted(extremities.items(), key=lambda e: -int(e[1]))
max_depth = sorted_extremeties_tuple[0][1]

# If we're approaching an extremity we trigger a backfill, otherwise we
# no-op.
#
Expand All @@ -249,6 +233,11 @@ async def _maybe_backfill_inner(
# chose more than one times the limit in case of failure, but choosing a
# much larger factor will result in triggering a backfill request much
# earlier than necessary.
#
# XXX: shouldn't we do this *after* the filter by depth below? Again, we don't
# care about events that have happened after our current position.
#
max_depth = sorted_backfill_points[0].depth
if current_depth - 2 * limit > max_depth:
logger.debug(
"Not backfilling as we don't need to. %d < %d - 2 * %d",
Expand All @@ -265,31 +254,98 @@ async def _maybe_backfill_inner(
# 2. we have likely previously tried and failed to backfill from that
# extremity, so to avoid getting "stuck" requesting the same
# backfill repeatedly we drop those extremities.
filtered_sorted_extremeties_tuple = [
t for t in sorted_extremeties_tuple if int(t[1]) <= current_depth
]

logger.debug(
"room_id: %s, backfill: current_depth: %s, limit: %s, max_depth: %s, extrems (%d): %s filtered_sorted_extremeties_tuple: %s",
room_id,
current_depth,
limit,
max_depth,
len(sorted_extremeties_tuple),
sorted_extremeties_tuple,
filtered_sorted_extremeties_tuple,
)

#
# However, we need to check that the filtered extremities are non-empty.
# If they are empty then either we can a) bail or b) still attempt to
# backfill. We opt to try backfilling anyway just in case we do get
# relevant events.
if filtered_sorted_extremeties_tuple:
sorted_extremeties_tuple = filtered_sorted_extremeties_tuple
#
filtered_sorted_backfill_points = [
t for t in sorted_backfill_points if t.depth <= current_depth
]
if filtered_sorted_backfill_points:
logger.debug(
"_maybe_backfill_inner: backfill points before current depth: %s",
filtered_sorted_backfill_points,
)
sorted_backfill_points = filtered_sorted_backfill_points
else:
logger.debug(
"_maybe_backfill_inner: all backfill points are *after* current depth. Backfilling anyway."
)

# For performance's sake, we only want to paginate from a particular extremity
# if we can actually see the events we'll get. Otherwise, we'd just spend a lot
# of resources to get redacted events. We check each extremity in turn and
# ignore those which users on our server wouldn't be able to see.
#
# Additionally, we limit ourselves to backfilling from at most 5 extremities,
# for two reasons:
#
# - The check which determines if we can see an extremity's events can be
# expensive (we load the full state for the room at each of the backfill
# points, or (worse) their successors)
# - We want to avoid the server-server API request URI becoming too long.
#
# *Note*: the spec wants us to keep backfilling until we reach the start
# of the room in case we are allowed to see some of the history. However,
# in practice that causes more issues than its worth, as (a) it's
# relatively rare for there to be any visible history and (b) even when
# there is it's often sufficiently long ago that clients would stop
# attempting to paginate before backfill reached the visible history.

# We don't want to specify too many extremities as it causes the backfill
# request URI to be too long.
extremities = dict(sorted_extremeties_tuple[:5])
extremities_to_request: List[str] = []
for bp in sorted_backfill_points:
if len(extremities_to_request) >= 5:
break

# For regular backwards extremities, we don't have the extremity events
# themselves, so we need to actually check the events that reference them -
# their "successor" events.
#
# TODO: Correctly handle the case where we are allowed to see the
# successor event but not the backward extremity, e.g. in the case of
# initial join of the server where we are allowed to see the join
# event but not anything before it. This would require looking at the
# state *before* the event, ignoring the special casing certain event
# types have.
if bp.type == _BackfillPointType.INSERTION_PONT:
event_ids_to_check = [bp.event_id]
else:
event_ids_to_check = await self.store.get_successor_events(bp.event_id)

events_to_check = await self.store.get_events_as_list(
event_ids_to_check,
redact_behaviour=EventRedactBehaviour.AS_IS,
get_prev_content=False,
)

# We set `check_history_visibility_only` as we might otherwise get false
# positives from users having been erased.
filtered_extremities = await filter_events_for_server(
self.storage,
self.server_name,
events_to_check,
redact=False,
check_history_visibility_only=True,
)
if filtered_extremities:
extremities_to_request.append(bp.event_id)
else:
logger.debug(
"_maybe_backfill_inner: skipping extremity %s as it would not be visible",
bp,
)

if not extremities_to_request:
logger.debug(
"_maybe_backfill_inner: found no extremities which would be visible"
)
return False

logger.debug(
"_maybe_backfill_inner: extremities_to_request %s", extremities_to_request
)

# Now we need to decide which hosts to hit first.

Expand All @@ -309,7 +365,7 @@ async def try_backfill(domains: List[str]) -> bool:
for dom in domains:
try:
await self._federation_event_handler.backfill(
dom, room_id, limit=100, extremities=extremities
dom, room_id, limit=100, extremities=extremities_to_request
)
# If this succeeded then we probably already have the
# appropriate stuff.
Expand Down
2 changes: 1 addition & 1 deletion synapse/handlers/room_batch.py
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Original file line number Diff line number Diff line change
Expand Up @@ -54,7 +54,7 @@ async def inherit_depth_from_prev_ids(self, prev_event_ids: List[str]) -> int:
# it has a larger `depth` but before the successor event because the `stream_ordering`
# is negative before the successor event.
successor_event_ids = await self.store.get_successor_events(
[most_recent_prev_event_id]
most_recent_prev_event_id
)

# If we can't find any successor events, then it's a forward extremity of
Expand Down
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