Optimise checking other servers allowed to see events (#2596)

* Try optimising checking if server is allowed to see event

* Fix error

* Handle case where snapshot NID is 0

* Fix query

* Update SQL

* Clean up `CheckServerAllowedToSeeEvent`

* Not supported on SQLite

* Maybe placate the unit tests

* Review comments

1 files changed, 49 insertions, 2 deletions

diff --git a/roomserver/storage/postgres/state_snapshot_table.go b/roomserver/storage/postgres/state_snapshot_table.go
index a24b7f3f..99c76bef 100644
--- a/roomserver/storage/postgres/state_snapshot_table.go
+++ b/roomserver/storage/postgres/state_snapshot_table.go
@@ -72,9 +72,35 @@ const bulkSelectStateBlockNIDsSQL = "" +
 	"SELECT state_snapshot_nid, state_block_nids FROM roomserver_state_snapshots" +
 	" WHERE state_snapshot_nid = ANY($1) ORDER BY state_snapshot_nid ASC"
 
+// Looks up both the history visibility event and relevant membership events from
+// a given domain name from a given state snapshot. This is used to optimise the
+// helpers.CheckServerAllowedToSeeEvent function.
+// TODO: There's a sequence scan here because of the hash join strategy, which is
+// probably O(n) on state key entries, so there must be a way to avoid that somehow.
+// Event type NIDs are:
+// - 5: m.room.member as per https://github.com/matrix-org/dendrite/blob/c7f7aec4d07d59120d37d5b16a900f6d608a75c4/roomserver/storage/postgres/event_types_table.go#L40
+// - 7: m.room.history_visibility as per https://github.com/matrix-org/dendrite/blob/c7f7aec4d07d59120d37d5b16a900f6d608a75c4/roomserver/storage/postgres/event_types_table.go#L42
+const bulkSelectStateForHistoryVisibilitySQL = `
+	SELECT event_nid FROM (
+	  SELECT event_nid, event_type_nid, event_state_key_nid FROM roomserver_events
+	  WHERE (event_type_nid = 5 OR event_type_nid = 7)
+	  AND event_nid = ANY(
+	    SELECT UNNEST(event_nids) FROM roomserver_state_block
+	    WHERE state_block_nid = ANY(
+	      SELECT UNNEST(state_block_nids) FROM roomserver_state_snapshots
+	      WHERE state_snapshot_nid = $1
+	    )
+	  )
+	) AS roomserver_events
+	INNER JOIN roomserver_event_state_keys
+	  ON roomserver_events.event_state_key_nid = roomserver_event_state_keys.event_state_key_nid
+	  AND (event_type_nid = 7 OR event_state_key LIKE '%:' || $2);
+`
+
 type stateSnapshotStatements struct {
-	insertStateStmt              *sql.Stmt
-	bulkSelectStateBlockNIDsStmt *sql.Stmt
+	insertStateStmt                         *sql.Stmt
+	bulkSelectStateBlockNIDsStmt            *sql.Stmt
+	bulkSelectStateForHistoryVisibilityStmt *sql.Stmt
 }
 
 func CreateStateSnapshotTable(db *sql.DB) error {
@@ -88,6 +114,7 @@ func PrepareStateSnapshotTable(db *sql.DB) (tables.StateSnapshot, error) {
 	return s, sqlutil.StatementList{
 		{&s.insertStateStmt, insertStateSQL},
 		{&s.bulkSelectStateBlockNIDsStmt, bulkSelectStateBlockNIDsSQL},
+		{&s.bulkSelectStateForHistoryVisibilityStmt, bulkSelectStateForHistoryVisibilitySQL},
 	}.Prepare(db)
 }
 
@@ -136,3 +163,23 @@ func (s *stateSnapshotStatements) BulkSelectStateBlockNIDs(
 	}
 	return results, nil
 }
+
+func (s *stateSnapshotStatements) BulkSelectStateForHistoryVisibility(
+	ctx context.Context, txn *sql.Tx, stateSnapshotNID types.StateSnapshotNID, domain string,
+) ([]types.EventNID, error) {
+	stmt := sqlutil.TxStmt(txn, s.bulkSelectStateForHistoryVisibilityStmt)
+	rows, err := stmt.QueryContext(ctx, stateSnapshotNID, domain)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close() // nolint: errcheck
+	results := make([]types.EventNID, 0, 16)
+	for rows.Next() {
+		var eventNID types.EventNID
+		if err = rows.Scan(&eventNID); err != nil {
+			return nil, err
+		}
+		results = append(results, eventNID)
+	}
+	return results, rows.Err()
+}