sytest: Make 'Inbound federation can backfill events' pass (#1051)

* sytest: Make 'Inbound federation can backfill events' pass

This breaks 'Outbound federation can backfill events' because now
we are returning the right number of events, which the previous
test was relying on.

Previously, /messages was backfilling the membership event, causing
the test to pass. Now we are no longer backfilling the membership
event due to the change in this commit, causing the test to fail.

The test should instead be returning the membership event locally
from synacpis database, but it doesn't do it fast enough, resulting
in a no-op /sync response with a next_batch=s0_0 which will never
pick up the local membership event when it rolls in. The test
does attempt to retry, but doesn't take the new next_batch=s1_0
resulting in it missing from the /messages response.

* Linting

1 files changed, 7 insertions, 6 deletions

diff --git a/syncapi/storage/postgres/backwards_extremities_table.go b/syncapi/storage/postgres/backwards_extremities_table.go
index 3dd1b234..fa498a40 100644
--- a/syncapi/storage/postgres/backwards_extremities_table.go
+++ b/syncapi/storage/postgres/backwards_extremities_table.go
@@ -41,7 +41,7 @@ const insertBackwardExtremitySQL = "" +
 	" ON CONFLICT DO NOTHING"
 
 const selectBackwardExtremitiesForRoomSQL = "" +
-	"SELECT DISTINCT event_id FROM syncapi_backward_extremities WHERE room_id = $1"
+	"SELECT event_id, prev_event_id FROM syncapi_backward_extremities WHERE room_id = $1"
 
 const deleteBackwardExtremitySQL = "" +
 	"DELETE FROM syncapi_backward_extremities WHERE room_id = $1 AND prev_event_id = $2"
@@ -79,23 +79,24 @@ func (s *backwardExtremitiesStatements) InsertsBackwardExtremity(
 
 func (s *backwardExtremitiesStatements) SelectBackwardExtremitiesForRoom(
 	ctx context.Context, roomID string,
-) (eventIDs []string, err error) {
+) (bwExtrems map[string][]string, err error) {
 	rows, err := s.selectBackwardExtremitiesForRoomStmt.QueryContext(ctx, roomID)
 	if err != nil {
 		return
 	}
 	defer common.CloseAndLogIfError(ctx, rows, "selectBackwardExtremitiesForRoom: rows.close() failed")
 
+	bwExtrems = make(map[string][]string)
 	for rows.Next() {
 		var eID string
-		if err = rows.Scan(&eID); err != nil {
+		var prevEventID string
+		if err = rows.Scan(&eID, &prevEventID); err != nil {
 			return
 		}
-
-		eventIDs = append(eventIDs, eID)
+		bwExtrems[eID] = append(bwExtrems[eID], prevEventID)
 	}
 
-	return eventIDs, rows.Err()
+	return bwExtrems, rows.Err()
 }
 
 func (s *backwardExtremitiesStatements) DeleteBackwardExtremity(