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.

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