Factor out common code for persisting fetched auth events (#10896)

* Factor more stuff out of `_get_events_and_persist`

It turns out that the event-sorting algorithm in `_get_events_and_persist` is
also useful in other circumstances. Here we move the current
`_auth_and_persist_fetched_events` to `_auth_and_persist_fetched_events_inner`,
and then factor the sorting part out to `_auth_and_persist_fetched_events`.

* `_get_remote_auth_chain_for_event`: remove redundant `outlier` assignment

`get_event_auth` returns events with the outlier flag already set, so this is
redundant (though we need to update a test where `get_event_auth` is mocked).

* `_get_remote_auth_chain_for_event`: move existing-event tests earlier

Move a couple of tests outside the loop. This is a bit inefficient for now, but
a future commit will make it better. It should be functionally identical.

* `_get_remote_auth_chain_for_event`: use `_auth_and_persist_fetched_events`

We can use the same codepath for persisting the events fetched as part of an
auth chain as for those fetched individually by `_get_events_and_persist` for
building the state at a backwards extremity.

* `_get_remote_auth_chain_for_event`: use a dict for efficiency

`_auth_and_persist_fetched_events` sorts the events itself, so we no longer
need to care about maintaining the ordering from `get_event_auth` (and no
longer need to sort by depth in `get_event_auth`).

That means that we can use a map, making it easier to filter out events we
already have, etc.

* changelog

* `_auth_and_persist_fetched_events`: improve docstring

1 files changed, 6 insertions, 1 deletions

diff --git a/tests/handlers/test_federation.py b/tests/handlers/test_federation.py
index 6c67a16de9..936ebf3dde 100644
--- a/tests/handlers/test_federation.py
+++ b/tests/handlers/test_federation.py
@@ -308,7 +308,12 @@ class FederationTestCase(unittest.HomeserverTestCase):
         async def get_event_auth(
             destination: str, room_id: str, event_id: str
         ) -> List[EventBase]:
-            return auth_events
+            return [
+                event_from_pdu_json(
+                    ae.get_pdu_json(), room_version=room_version, outlier=True
+                )
+                for ae in auth_events
+            ]
 
         self.handler.federation_client.get_event_auth = get_event_auth