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Refactor on_receive_pdu code (#10615)
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* drop room pdu linearizer sooner

No point holding onto it while we recheck the db

* move out `missing_prevs` calculation

we're going to need `missing_prevs` whatever we do, so we may as well calculate
it eagerly and just update it if it gets outdated.

* Add another `if missing_prevs` condition

this should be a no-op, since all the code inside the block already checks `if
missing_prevs`

* reorder if conditions

This shouldn't change the logic at all.

* Push down `min_depth` read

No point reading it from the database unless we're going to use it.

* Collect the sent_to_us_directly code together

Move the remaining `sent_to_us_directly` code inside the `if
sent_to_us_directly` block.

* Properly separate the `not sent_to_us_directly` branch

Since the only way this second block is now reachable is if we
*didn't* go into the `sent_to_us_directly` branch, we can replace it with a
simple `else`.

* changelog
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@richvdh
richvdh authored Aug 18, 2021
1 parent 6a5f8fb commit 964f29c
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1 change: 1 addition & 0 deletions changelog.d/10615.misc
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Clean up some of the federation event authentication code for clarity.
271 changes: 137 additions & 134 deletions synapse/handlers/federation.py
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Original file line number Diff line number Diff line change
Expand Up @@ -285,169 +285,172 @@ async def on_receive_pdu(
# - Fetching any missing prev events to fill in gaps in the graph
# - Fetching state if we have a hole in the graph
if not pdu.internal_metadata.is_outlier():
# We only backfill backwards to the min depth.
min_depth = await self.get_min_depth_for_context(pdu.room_id)

logger.debug("min_depth: %d", min_depth)

prevs = set(pdu.prev_event_ids())
seen = await self.store.have_events_in_timeline(prevs)
missing_prevs = prevs - seen

if min_depth is not None and pdu.depth > min_depth:
missing_prevs = prevs - seen
if sent_to_us_directly and missing_prevs:
# If we're missing stuff, ensure we only fetch stuff one
# at a time.
logger.info(
"Acquiring room lock to fetch %d missing prev_events: %s",
len(missing_prevs),
shortstr(missing_prevs),
)
with (await self._room_pdu_linearizer.queue(pdu.room_id)):
if missing_prevs:
if sent_to_us_directly:
# We only backfill backwards to the min depth.
min_depth = await self.get_min_depth_for_context(pdu.room_id)
logger.debug("min_depth: %d", min_depth)

if min_depth is not None and pdu.depth > min_depth:
# If we're missing stuff, ensure we only fetch stuff one
# at a time.
logger.info(
"Acquired room lock to fetch %d missing prev_events",
"Acquiring room lock to fetch %d missing prev_events: %s",
len(missing_prevs),
shortstr(missing_prevs),
)

try:
await self._get_missing_events_for_pdu(
origin, pdu, prevs, min_depth
with (await self._room_pdu_linearizer.queue(pdu.room_id)):
logger.info(
"Acquired room lock to fetch %d missing prev_events",
len(missing_prevs),
)
except Exception as e:
raise Exception(
"Error fetching missing prev_events for %s: %s"
% (event_id, e)
) from e

try:
await self._get_missing_events_for_pdu(
origin, pdu, prevs, min_depth
)
except Exception as e:
raise Exception(
"Error fetching missing prev_events for %s: %s"
% (event_id, e)
) from e

# Update the set of things we've seen after trying to
# fetch the missing stuff
seen = await self.store.have_events_in_timeline(prevs)
missing_prevs = prevs - seen

if not missing_prevs:
logger.info("Found all missing prev_events")

if missing_prevs:
# since this event was pushed to us, it is possible for it to
# become the only forward-extremity in the room, and we would then
# trust its state to be the state for the whole room. This is very
# bad. Further, if the event was pushed to us, there is no excuse
# for us not to have all the prev_events. (XXX: apart from
# min_depth?)
#
# We therefore reject any such events.
logger.warning(
"Rejecting: failed to fetch %d prev events: %s",
len(missing_prevs),
shortstr(missing_prevs),
)
raise FederationError(
"ERROR",
403,
(
"Your server isn't divulging details about prev_events "
"referenced in this event."
),
affected=pdu.event_id,
)

if not prevs - seen:
logger.info(
"Found all missing prev_events",
)

missing_prevs = prevs - seen
if missing_prevs:
# We've still not been able to get all of the prev_events for this event.
#
# In this case, we need to fall back to asking another server in the
# federation for the state at this event. That's ok provided we then
# resolve the state against other bits of the DAG before using it (which
# will ensure that you can't just take over a room by sending an event,
# withholding its prev_events, and declaring yourself to be an admin in
# the subsequent state request).
#
# Now, if we're pulling this event as a missing prev_event, then clearly
# this event is not going to become the only forward-extremity and we are
# guaranteed to resolve its state against our existing forward
# extremities, so that should be fine.
#
# On the other hand, if this event was pushed to us, it is possible for
# it to become the only forward-extremity in the room, and we would then
# trust its state to be the state for the whole room. This is very bad.
# Further, if the event was pushed to us, there is no excuse for us not to
# have all the prev_events. We therefore reject any such events.
#
# XXX this really feels like it could/should be merged with the above,
# but there is an interaction with min_depth that I'm not really
# following.

if sent_to_us_directly:
logger.warning(
"Rejecting: failed to fetch %d prev events: %s",
len(missing_prevs),
else:
# We don't have all of the prev_events for this event.
#
# In this case, we need to fall back to asking another server in the
# federation for the state at this event. That's ok provided we then
# resolve the state against other bits of the DAG before using it (which
# will ensure that you can't just take over a room by sending an event,
# withholding its prev_events, and declaring yourself to be an admin in
# the subsequent state request).
#
# Since we're pulling this event as a missing prev_event, then clearly
# this event is not going to become the only forward-extremity and we are
# guaranteed to resolve its state against our existing forward
# extremities, so that should be fine.
#
# XXX this really feels like it could/should be merged with the above,
# but there is an interaction with min_depth that I'm not really
# following.
logger.info(
"Event %s is missing prev_events %s: calculating state for a "
"backwards extremity",
event_id,
shortstr(missing_prevs),
)
raise FederationError(
"ERROR",
403,
(
"Your server isn't divulging details about prev_events "
"referenced in this event."
),
affected=pdu.event_id,
)

logger.info(
"Event %s is missing prev_events %s: calculating state for a "
"backwards extremity",
event_id,
shortstr(missing_prevs),
)
# Calculate the state after each of the previous events, and
# resolve them to find the correct state at the current event.
event_map = {event_id: pdu}
try:
# Get the state of the events we know about
ours = await self.state_store.get_state_groups_ids(
room_id, seen
)

# Calculate the state after each of the previous events, and
# resolve them to find the correct state at the current event.
event_map = {event_id: pdu}
try:
# Get the state of the events we know about
ours = await self.state_store.get_state_groups_ids(room_id, seen)

# state_maps is a list of mappings from (type, state_key) to event_id
state_maps: List[StateMap[str]] = list(ours.values())

# we don't need this any more, let's delete it.
del ours

# Ask the remote server for the states we don't
# know about
for p in missing_prevs:
logger.info("Requesting state after missing prev_event %s", p)

with nested_logging_context(p):
# note that if any of the missing prevs share missing state or
# auth events, the requests to fetch those events are deduped
# by the get_pdu_cache in federation_client.
remote_state = (
await self._get_state_after_missing_prev_event(
origin, room_id, p
)
# state_maps is a list of mappings from (type, state_key) to event_id
state_maps: List[StateMap[str]] = list(ours.values())

# we don't need this any more, let's delete it.
del ours

# Ask the remote server for the states we don't
# know about
for p in missing_prevs:
logger.info(
"Requesting state after missing prev_event %s", p
)

remote_state_map = {
(x.type, x.state_key): x.event_id for x in remote_state
}
state_maps.append(remote_state_map)
with nested_logging_context(p):
# note that if any of the missing prevs share missing state or
# auth events, the requests to fetch those events are deduped
# by the get_pdu_cache in federation_client.
remote_state = (
await self._get_state_after_missing_prev_event(
origin, room_id, p
)
)

remote_state_map = {
(x.type, x.state_key): x.event_id
for x in remote_state
}
state_maps.append(remote_state_map)

for x in remote_state:
event_map[x.event_id] = x
for x in remote_state:
event_map[x.event_id] = x

room_version = await self.store.get_room_version_id(room_id)
state_map = (
await self._state_resolution_handler.resolve_events_with_store(
room_version = await self.store.get_room_version_id(room_id)
state_map = await self._state_resolution_handler.resolve_events_with_store(
room_id,
room_version,
state_maps,
event_map,
state_res_store=StateResolutionStore(self.store),
)
)

# We need to give _process_received_pdu the actual state events
# rather than event ids, so generate that now.
# We need to give _process_received_pdu the actual state events
# rather than event ids, so generate that now.

# First though we need to fetch all the events that are in
# state_map, so we can build up the state below.
evs = await self.store.get_events(
list(state_map.values()),
get_prev_content=False,
redact_behaviour=EventRedactBehaviour.AS_IS,
)
event_map.update(evs)
# First though we need to fetch all the events that are in
# state_map, so we can build up the state below.
evs = await self.store.get_events(
list(state_map.values()),
get_prev_content=False,
redact_behaviour=EventRedactBehaviour.AS_IS,
)
event_map.update(evs)

state = [event_map[e] for e in state_map.values()]
except Exception:
logger.warning(
"Error attempting to resolve state at missing " "prev_events",
exc_info=True,
)
raise FederationError(
"ERROR",
403,
"We can't get valid state history.",
affected=event_id,
)
state = [event_map[e] for e in state_map.values()]
except Exception:
logger.warning(
"Error attempting to resolve state at missing "
"prev_events",
exc_info=True,
)
raise FederationError(
"ERROR",
403,
"We can't get valid state history.",
affected=event_id,
)

# A second round of checks for all events. Check that the event passes auth
# based on `auth_events`, this allows us to assert that the event would
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