Use ijson to parse the response to /send_join, reducing memory usage. (#9958)

Instead of parsing the full response to `/send_join` into Python objects (which can be huge for large rooms) and *then* parsing that into events, we instead use ijson to stream parse the response directly into `EventBase` objects.

synapse/federation/federation_client.py

@@ -55,6 +55,7 @@ from synapse.api.room_versions import (
 )
 from synapse.events import EventBase, builder
 from synapse.federation.federation_base import FederationBase, event_from_pdu_json
+from synapse.federation.transport.client import SendJoinResponse
 from synapse.logging.context import make_deferred_yieldable, preserve_fn
 from synapse.logging.utils import log_function
 from synapse.types import JsonDict, get_domain_from_id
@@ -665,19 +666,10 @@ class FederationClient(FederationBase):
         """
 
         async def send_request(destination) -> Dict[str, Any]:
-            content = await self._do_send_join(destination, pdu)
+            response = await self._do_send_join(room_version, destination, pdu)
 
-            logger.debug("Got content: %s", content)
-
-            state = [
-                event_from_pdu_json(p, room_version, outlier=True)
-                for p in content.get("state", [])
-            ]
-
-            auth_chain = [
-                event_from_pdu_json(p, room_version, outlier=True)
-                for p in content.get("auth_chain", [])
-            ]
+            state = response.state
+            auth_chain = response.auth_events
 
             pdus = {p.event_id: p for p in itertools.chain(state, auth_chain)}
 
@@ -752,11 +744,14 @@ class FederationClient(FederationBase):
 
         return await self._try_destination_list("send_join", destinations, send_request)
 
-    async def _do_send_join(self, destination: str, pdu: EventBase) -> JsonDict:
+    async def _do_send_join(
+        self, room_version: RoomVersion, destination: str, pdu: EventBase
+    ) -> SendJoinResponse:
         time_now = self._clock.time_msec()
 
         try:
             return await self.transport_layer.send_join_v2(
+                room_version=room_version,
                 destination=destination,
                 room_id=pdu.room_id,
                 event_id=pdu.event_id,
@@ -771,17 +766,14 @@ class FederationClient(FederationBase):
 
         logger.debug("Couldn't send_join with the v2 API, falling back to the v1 API")
 
-        resp = await self.transport_layer.send_join_v1(
+        return await self.transport_layer.send_join_v1(
+            room_version=room_version,
             destination=destination,
             room_id=pdu.room_id,
             event_id=pdu.event_id,
             content=pdu.get_pdu_json(time_now),
         )
 
-        # We expect the v1 API to respond with [200, content], so we only return the
-        # content.
-        return resp[1]
-
     async def send_invite(
         self,
         destination: str,