Handle inbound events from federation asynchronously (#10272)

Fixes #9490

This will break a couple of SyTest that are expecting failures to be added to the response of a federation /send, which obviously doesn't happen now that things are asynchronous.

Two drawbacks:

    Currently there is no logic to handle any events left in the staging area after restart, and so they'll only be handled on the next incoming event in that room. That can be fixed separately.
    We now only process one event per room at a time. This can be fixed up further down the line.

5 files changed, 241 insertions, 5 deletions

diff --git a/changelog.d/10272.bugfix b/changelog.d/10272.bugfix
new file mode 100644
index 0000000000..3cefa05788
--- /dev/null
+++ b/changelog.d/10272.bugfix
@@ -0,0 +1 @@
+Handle inbound events from federation asynchronously.
diff --git a/synapse/federation/federation_server.py b/synapse/federation/federation_server.py
index 2b07f18529..1d050e54e2 100644
--- a/synapse/federation/federation_server.py
+++ b/synapse/federation/federation_server.py
@@ -44,7 +44,7 @@ from synapse.api.errors import (
     SynapseError,
     UnsupportedRoomVersionError,
 )
-from synapse.api.room_versions import KNOWN_ROOM_VERSIONS
+from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
 from synapse.events import EventBase
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
 from synapse.federation.persistence import TransactionActions
@@ -57,10 +57,12 @@ from synapse.logging.context import (
 )
 from synapse.logging.opentracing import log_kv, start_active_span_from_edu, trace
 from synapse.logging.utils import log_function
+from synapse.metrics.background_process_metrics import wrap_as_background_process
 from synapse.replication.http.federation import (
     ReplicationFederationSendEduRestServlet,
     ReplicationGetQueryRestServlet,
 )
+from synapse.storage.databases.main.lock import Lock
 from synapse.types import JsonDict
 from synapse.util import glob_to_regex, json_decoder, unwrapFirstError
 from synapse.util.async_helpers import Linearizer, concurrently_execute
@@ -96,6 +98,11 @@ last_pdu_ts_metric = Gauge(
 )
 
 
+# The name of the lock to use when process events in a room received over
+# federation.
+_INBOUND_EVENT_HANDLING_LOCK_NAME = "federation_inbound_pdu"
+
+
 class FederationServer(FederationBase):
     def __init__(self, hs: "HomeServer"):
         super().__init__(hs)
@@ -834,7 +841,94 @@ class FederationServer(FederationBase):
         except SynapseError as e:
             raise FederationError("ERROR", e.code, e.msg, affected=pdu.event_id)
 
-        await self.handler.on_receive_pdu(origin, pdu, sent_to_us_directly=True)
+        # Add the event to our staging area
+        await self.store.insert_received_event_to_staging(origin, pdu)
+
+        # Try and acquire the processing lock for the room, if we get it start a
+        # background process for handling the events in the room.
+        lock = await self.store.try_acquire_lock(
+            _INBOUND_EVENT_HANDLING_LOCK_NAME, pdu.room_id
+        )
+        if lock:
+            self._process_incoming_pdus_in_room_inner(
+                pdu.room_id, room_version, lock, origin, pdu
+            )
+
+    @wrap_as_background_process("_process_incoming_pdus_in_room_inner")
+    async def _process_incoming_pdus_in_room_inner(
+        self,
+        room_id: str,
+        room_version: RoomVersion,
+        lock: Lock,
+        latest_origin: str,
+        latest_event: EventBase,
+    ) -> None:
+        """Process events in the staging area for the given room.
+
+        The latest_origin and latest_event args are the latest origin and event
+        received.
+        """
+
+        # The common path is for the event we just received be the only event in
+        # the room, so instead of pulling the event out of the DB and parsing
+        # the event we just pull out the next event ID and check if that matches.
+        next_origin, next_event_id = await self.store.get_next_staged_event_id_for_room(
+            room_id
+        )
+        if next_origin == latest_origin and next_event_id == latest_event.event_id:
+            origin = latest_origin
+            event = latest_event
+        else:
+            next = await self.store.get_next_staged_event_for_room(
+                room_id, room_version
+            )
+            if not next:
+                return
+
+            origin, event = next
+
+        # We loop round until there are no more events in the room in the
+        # staging area, or we fail to get the lock (which means another process
+        # has started processing).
+        while True:
+            async with lock:
+                try:
+                    await self.handler.on_receive_pdu(
+                        origin, event, sent_to_us_directly=True
+                    )
+                except FederationError as e:
+                    # XXX: Ideally we'd inform the remote we failed to process
+                    # the event, but we can't return an error in the transaction
+                    # response (as we've already responded).
+                    logger.warning("Error handling PDU %s: %s", event.event_id, e)
+                except Exception:
+                    f = failure.Failure()
+                    logger.error(
+                        "Failed to handle PDU %s",
+                        event.event_id,
+                        exc_info=(f.type, f.value, f.getTracebackObject()),  # type: ignore
+                    )
+
+                await self.store.remove_received_event_from_staging(
+                    origin, event.event_id
+                )
+
+            # We need to do this check outside the lock to avoid a race between
+            # a new event being inserted by another instance and it attempting
+            # to acquire the lock.
+            next = await self.store.get_next_staged_event_for_room(
+                room_id, room_version
+            )
+            if not next:
+                break
+
+            origin, event = next
+
+            lock = await self.store.try_acquire_lock(
+                _INBOUND_EVENT_HANDLING_LOCK_NAME, room_id
+            )
+            if not lock:
+                return
 
     def __str__(self) -> str:
         return "<ReplicationLayer(%s)>" % self.server_name
diff --git a/synapse/storage/databases/main/event_federation.py b/synapse/storage/databases/main/event_federation.py
index c0ea445550..f23f8c6ecf 100644
--- a/synapse/storage/databases/main/event_federation.py
+++ b/synapse/storage/databases/main/event_federation.py
@@ -14,18 +14,20 @@
 import itertools
 import logging
 from queue import Empty, PriorityQueue
-from typing import Collection, Dict, Iterable, List, Set, Tuple
+from typing import Collection, Dict, Iterable, List, Optional, Set, Tuple
 
 from synapse.api.constants import MAX_DEPTH
 from synapse.api.errors import StoreError
-from synapse.events import EventBase
+from synapse.api.room_versions import RoomVersion
+from synapse.events import EventBase, make_event_from_dict
 from synapse.metrics.background_process_metrics import wrap_as_background_process
-from synapse.storage._base import SQLBaseStore, make_in_list_sql_clause
+from synapse.storage._base import SQLBaseStore, db_to_json, make_in_list_sql_clause
 from synapse.storage.database import DatabasePool, LoggingTransaction
 from synapse.storage.databases.main.events_worker import EventsWorkerStore
 from synapse.storage.databases.main.signatures import SignatureWorkerStore
 from synapse.storage.engines import PostgresEngine
 from synapse.storage.types import Cursor
+from synapse.util import json_encoder
 from synapse.util.caches.descriptors import cached
 from synapse.util.caches.lrucache import LruCache
 from synapse.util.iterutils import batch_iter
@@ -1044,6 +1046,107 @@ class EventFederationWorkerStore(EventsWorkerStore, SignatureWorkerStore, SQLBas
             _delete_old_forward_extrem_cache_txn,
         )
 
+    async def insert_received_event_to_staging(
+        self, origin: str, event: EventBase
+    ) -> None:
+        """Insert a newly received event from federation into the staging area."""
+
+        # We use an upsert here to handle the case where we see the same event
+        # from the same server multiple times.
+        await self.db_pool.simple_upsert(
+            table="federation_inbound_events_staging",
+            keyvalues={
+                "origin": origin,
+                "event_id": event.event_id,
+            },
+            values={},
+            insertion_values={
+                "room_id": event.room_id,
+                "received_ts": self._clock.time_msec(),
+                "event_json": json_encoder.encode(event.get_dict()),
+                "internal_metadata": json_encoder.encode(
+                    event.internal_metadata.get_dict()
+                ),
+            },
+            desc="insert_received_event_to_staging",
+        )
+
+    async def remove_received_event_from_staging(
+        self,
+        origin: str,
+        event_id: str,
+    ) -> None:
+        """Remove the given event from the staging area"""
+        await self.db_pool.simple_delete(
+            table="federation_inbound_events_staging",
+            keyvalues={
+                "origin": origin,
+                "event_id": event_id,
+            },
+            desc="remove_received_event_from_staging",
+        )
+
+    async def get_next_staged_event_id_for_room(
+        self,
+        room_id: str,
+    ) -> Optional[Tuple[str, str]]:
+        """Get the next event ID in the staging area for the given room."""
+
+        def _get_next_staged_event_id_for_room_txn(txn):
+            sql = """
+                SELECT origin, event_id
+                FROM federation_inbound_events_staging
+                WHERE room_id = ?
+                ORDER BY received_ts ASC
+                LIMIT 1
+            """
+
+            txn.execute(sql, (room_id,))
+
+            return txn.fetchone()
+
+        return await self.db_pool.runInteraction(
+            "get_next_staged_event_id_for_room", _get_next_staged_event_id_for_room_txn
+        )
+
+    async def get_next_staged_event_for_room(
+        self,
+        room_id: str,
+        room_version: RoomVersion,
+    ) -> Optional[Tuple[str, EventBase]]:
+        """Get the next event in the staging area for the given room."""
+
+        def _get_next_staged_event_for_room_txn(txn):
+            sql = """
+                SELECT event_json, internal_metadata, origin
+                FROM federation_inbound_events_staging
+                WHERE room_id = ?
+                ORDER BY received_ts ASC
+                LIMIT 1
+            """
+            txn.execute(sql, (room_id,))
+
+            return txn.fetchone()
+
+        row = await self.db_pool.runInteraction(
+            "get_next_staged_event_for_room", _get_next_staged_event_for_room_txn
+        )
+
+        if not row:
+            return None
+
+        event_d = db_to_json(row[0])
+        internal_metadata_d = db_to_json(row[1])
+        origin = row[2]
+
+        event = make_event_from_dict(
+            event_dict=event_d,
+            room_version=room_version,
+            internal_metadata_dict=internal_metadata_d,
+        )
+
+        return origin, event
+
 
 class EventFederationStore(EventFederationWorkerStore):
     """Responsible for storing and serving up the various graphs associated
diff --git a/synapse/storage/schema/main/delta/59/16federation_inbound_staging.sql b/synapse/storage/schema/main/delta/59/16federation_inbound_staging.sql
new file mode 100644
index 0000000000..43bc5c025f
--- /dev/null
+++ b/synapse/storage/schema/main/delta/59/16federation_inbound_staging.sql
@@ -0,0 +1,32 @@
+/* Copyright 2021 The Matrix.org Foundation C.I.C
+ *
+ * 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.
+ */
+
+
+-- A staging area for newly received events over federation.
+--
+-- Note we may store the same event multiple times if it comes from different
+-- servers; this is to handle the case if we get a redacted and non-redacted
+-- versions of the event.
+CREATE TABLE federation_inbound_events_staging (
+    origin TEXT NOT NULL,
+    room_id TEXT NOT NULL,
+    event_id TEXT NOT NULL,
+    received_ts BIGINT NOT NULL,
+    event_json TEXT NOT NULL,
+    internal_metadata TEXT NOT NULL
+);
+
+CREATE INDEX federation_inbound_events_staging_room ON federation_inbound_events_staging(room_id, received_ts);
+CREATE UNIQUE INDEX federation_inbound_events_staging_instance_event ON federation_inbound_events_staging(origin, event_id);
diff --git a/sytest-blacklist b/sytest-blacklist
index de9986357b..89c4e828fd 100644
--- a/sytest-blacklist
+++ b/sytest-blacklist
@@ -41,3 +41,9 @@ We can't peek into rooms with invited history_visibility
 We can't peek into rooms with joined history_visibility
 Local users can peek by room alias
 Peeked rooms only turn up in the sync for the device who peeked them
+
+
+# Blacklisted due to changes made in #10272
+Outbound federation will ignore a missing event with bad JSON for room version 6
+Backfilled events whose prev_events are in a different room do not allow cross-room back-pagination
+Federation rejects inbound events where the prev_events cannot be found