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Add some assertions about outliers (#10773)
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I think I have finally teased apart the codepaths which handle outliers, and those that handle non-outliers. 
Let's add some assertions to demonstrate my newfound knowledge.
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@richvdh
richvdh authored Sep 8, 2021
1 parent 5724883 commit aacdce8
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1 change: 1 addition & 0 deletions changelog.d/10773.misc
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Clean up some of the federation event authentication code for clarity.
148 changes: 77 additions & 71 deletions synapse/handlers/federation_event.py
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Original file line number Diff line number Diff line change
Expand Up @@ -173,6 +173,9 @@ async def on_receive_pdu(self, origin: str, pdu: EventBase) -> None:
pdu: received PDU
"""

# We should never see any outliers here.
assert not pdu.internal_metadata.outlier

room_id = pdu.room_id
event_id = pdu.event_id

Expand Down Expand Up @@ -232,77 +235,71 @@ async def on_receive_pdu(self, origin: str, pdu: EventBase) -> None:
)
return None

# Check that the event passes auth based on the state at the event. This is
# done for events that are to be added to the timeline (non-outliers).
#
# Get missing pdus if necessary:
# - 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():
prevs = set(pdu.prev_event_ids())
seen = await self._store.have_events_in_timeline(prevs)
missing_prevs = prevs - seen
# Try to fetch any missing prev events to fill in gaps in the graph
prevs = set(pdu.prev_event_ids())
seen = await self._store.have_events_in_timeline(prevs)
missing_prevs = prevs - seen

if missing_prevs:
# 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 missing_prevs:
# 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.
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(
"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)):
logger.info(
"Acquiring room lock to fetch %d missing prev_events: %s",
"Acquired room lock to fetch %d missing prev_events",
len(missing_prevs),
shortstr(missing_prevs),
)
with (await self._room_pdu_linearizer.queue(pdu.room_id)):
logger.info(
"Acquired room lock to fetch %d missing prev_events",
len(missing_prevs),

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

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,
)
# 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,
)

await self._process_received_pdu(origin, pdu, state=None)

Expand Down Expand Up @@ -885,8 +882,13 @@ async def _process_received_pdu(
state: Optional[Iterable[EventBase]],
backfilled: bool = False,
) -> None:
"""Called when we have a new pdu. We need to do auth checks and put it
through the StateHandler.
"""Called when we have a new non-outlier event.
This is called when we have a new event to add to the room DAG - either directly
via a /send request, retrieved via get_missing_events after a /send request, or
backfilled after a client request.
We need to do auth checks and put it through the StateHandler.
Args:
origin: server sending the event
Expand All @@ -901,6 +903,7 @@ async def _process_received_pdu(
notification to clients, and validation of device keys.)
"""
logger.debug("Processing event: %s", event)
assert not event.internal_metadata.outlier

try:
context = await self._state_handler.compute_event_context(
Expand Down Expand Up @@ -1263,11 +1266,13 @@ async def _auth_and_persist_event(
Possibly incomplete, and possibly including events that are not yet
persisted, or authed, or in the right room.
Only populated where we may not already have persisted these events -
for example, when populating outliers.
Only populated when populating outliers.
backfilled: True if the event was backfilled.
"""
# claimed_auth_event_map should be given iff the event is an outlier
assert bool(claimed_auth_event_map) == event.internal_metadata.outlier

context = await self._check_event_auth(
origin,
event,
Expand Down Expand Up @@ -1306,15 +1311,16 @@ async def _check_event_auth(
Possibly incomplete, and possibly including events that are not yet
persisted, or authed, or in the right room.
Only populated where we may not already have persisted these events -
for example, when populating outliers, or the state for a backwards
extremity.
Only populated when populating outliers.
backfilled: True if the event was backfilled.
Returns:
The updated context object.
"""
# claimed_auth_event_map should be given iff the event is an outlier
assert bool(claimed_auth_event_map) == event.internal_metadata.outlier

room_version = await self._store.get_room_version_id(event.room_id)
room_version_obj = KNOWN_ROOM_VERSIONS[room_version]

Expand Down

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