Event relations (#2790)

This adds support for tracking `m.relates_to`, as well as adding support
for the various `/room/{roomID}/relations/...` endpoints to the CS API.

syncapi/storage/shared/storage_sync.go

@@ -589,3 +589,84 @@ func (d *DatabaseTransaction) PresenceAfter(ctx context.Context, after types.Str
 func (d *DatabaseTransaction) MaxStreamPositionForPresence(ctx context.Context) (types.StreamPosition, error) {
 	return d.Presence.GetMaxPresenceID(ctx, d.txn)
 }
+
+func (d *DatabaseTransaction) MaxStreamPositionForRelations(ctx context.Context) (types.StreamPosition, error) {
+	id, err := d.Relations.SelectMaxRelationID(ctx, d.txn)
+	return types.StreamPosition(id), err
+}
+
+func (d *DatabaseTransaction) RelationsFor(ctx context.Context, roomID, eventID, relType, eventType string, from, to types.StreamPosition, backwards bool, limit int) (
+	events []types.StreamEvent, prevBatch, nextBatch string, err error,
+) {
+	r := types.Range{
+		From:      from,
+		To:        to,
+		Backwards: backwards,
+	}
+
+	if r.Backwards && r.From == 0 {
+		// If we're working backwards (dir=b) and there's no ?from= specified then
+		// we will automatically want to work backwards from the current position,
+		// so find out what that is.
+		if r.From, err = d.MaxStreamPositionForRelations(ctx); err != nil {
+			return nil, "", "", fmt.Errorf("d.MaxStreamPositionForRelations: %w", err)
+		}
+		// The result normally isn't inclusive of the event *at* the ?from=
+		// position, so add 1 here so that we include the most recent relation.
+		r.From++
+	} else if !r.Backwards && r.To == 0 {
+		// If we're working forwards (dir=f) and there's no ?to= specified then
+		// we will automatically want to work forwards towards the current position,
+		// so find out what that is.
+		if r.To, err = d.MaxStreamPositionForRelations(ctx); err != nil {
+			return nil, "", "", fmt.Errorf("d.MaxStreamPositionForRelations: %w", err)
+		}
+	}
+
+	// First 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, eventType, 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]
+	} else {
+		for _, e := range relations {
+			entries = append(entries, e...)
+		}
+	}
+
+	// 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. We've requested an extra event by adding one to the limit already so
+	// that we can determine whether or not to provide a "next_batch", so trim off that
+	// event off the end if needs be.
+	if len(entries) > limit {
+		entries = entries[:len(entries)-1]
+		nextBatch = fmt.Sprintf("%d", entries[len(entries)-1].Position)
+	}
+	// TODO: set prevBatch? doesn't seem to affect the tests...
+
+	// Extract all of the event IDs from the relation entries so that we can pull the
+	// events out of the database. Then go and fetch the events.
+	eventIDs := make([]string, 0, len(entries))
+	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)
+	}
+
+	return events, prevBatch, nextBatch, nil
+}