Clean up a bit

syncapi/storage/shared/storage_sync.go

@@ -598,17 +598,24 @@ func (d *DatabaseTransaction) MaxStreamPositionForRelations(ctx context.Context)
 func (d *DatabaseTransaction) RelationsFor(ctx context.Context, roomID, eventID, relType, eventType string, from, to types.StreamPosition, limit int) (
 	clientEvents []gomatrixserverlib.ClientEvent, prevBatch, nextBatch string, err error,
 ) {
-	var eventIDs []string
+	clientEvents = []gomatrixserverlib.ClientEvent{}
+	eventIDs := []string{}
 	r := types.Range{
 		From:      from,
 		To:        to,
 		Backwards: from > to,
 	}
+
+	// First up look up any relations from the database. We add one to the limit here
+	// so that we can tell if we're overflowing, as we will only set the "next_batch"
+	// in the response if we are.
 	relations, _, err := d.Relations.SelectRelationsInRange(ctx, d.txn, roomID, eventID, relType, r, limit+1)
 	if err != nil {
 		return nil, "", "", fmt.Errorf("d.Relations.SelectRelationsInRange: %w", err)
 	}
 
+	// If we specified a relation type then just get those results, otherwise collate
+	// them from all of the returned relation types.
 	entries := []types.RelationEntry{}
 	if relType != "" {
 		entries = relations[relType]
@@ -618,10 +625,13 @@ func (d *DatabaseTransaction) RelationsFor(ctx context.Context, roomID, eventID,
 		}
 	}
 
+	// If there were no entries returned, there were no relations, so stop at this point.
 	if len(entries) == 0 {
 		return nil, "", "", nil
 	}
 
+	// Otherwise, let's try and work out what sensible prev_batch and next_batch values
+	// could be.
 	if from > 0 {
 		prevBatch = fmt.Sprintf("%d", entries[0].Position-1)
 	}
@@ -630,15 +640,18 @@ func (d *DatabaseTransaction) RelationsFor(ctx context.Context, roomID, eventID,
 		entries = entries[:len(entries)-1]
 	}
 
+	// Extract all of the event IDs from the relation entries so that we can pull the
+	// events out of the database.
 	for _, entry := range entries {
 		eventIDs = append(eventIDs, entry.EventID)
 	}
-
 	events, err := d.OutputEvents.SelectEvents(ctx, d.txn, eventIDs, nil, true)
 	if err != nil {
 		return nil, "", "", fmt.Errorf("d.OutputEvents.SelectEvents: %w", err)
 	}
 
+	// Convert the events into client events, and optionally filter based on the event
+	// type if it was specified.
 	for _, event := range events {
 		if eventType != "" && event.Type() != eventType {
 			continue