1 files changed, 150 insertions, 0 deletions
diff --git a/internal/events.go b/internal/events.go
new file mode 100644
index 00000000..89c82e03
--- /dev/null
+++ b/internal/events.go
@@ -0,0 +1,150 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package internal
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"time"
+
+	"github.com/matrix-org/dendrite/internal/config"
+	"github.com/matrix-org/dendrite/roomserver/api"
+
+	"github.com/matrix-org/gomatrixserverlib"
+)
+
+// ErrRoomNoExists is returned when trying to lookup the state of a room that
+// doesn't exist
+var ErrRoomNoExists = errors.New("Room does not exist")
+
+// BuildEvent builds a Matrix event using the event builder and roomserver query
+// API client provided. If also fills roomserver query API response (if provided)
+// in case the function calling FillBuilder needs to use it.
+// Returns ErrRoomNoExists if the state of the room could not be retrieved because
+// the room doesn't exist
+// Returns an error if something else went wrong
+func BuildEvent(
+	ctx context.Context,
+	builder *gomatrixserverlib.EventBuilder, cfg *config.Dendrite, evTime time.Time,
+	rsAPI api.RoomserverInternalAPI, queryRes *api.QueryLatestEventsAndStateResponse,
+) (*gomatrixserverlib.Event, error) {
+	if queryRes == nil {
+		queryRes = &api.QueryLatestEventsAndStateResponse{}
+	}
+
+	err := AddPrevEventsToEvent(ctx, builder, rsAPI, queryRes)
+	if err != nil {
+		// This can pass through a ErrRoomNoExists to the caller
+		return nil, err
+	}
+
+	event, err := builder.Build(
+		evTime, cfg.Matrix.ServerName, cfg.Matrix.KeyID,
+		cfg.Matrix.PrivateKey, queryRes.RoomVersion,
+	)
+	if err != nil {
+		return nil, err
+	}
+
+	return &event, nil
+}
+
+// AddPrevEventsToEvent fills out the prev_events and auth_events fields in builder
+func AddPrevEventsToEvent(
+	ctx context.Context,
+	builder *gomatrixserverlib.EventBuilder,
+	rsAPI api.RoomserverInternalAPI, queryRes *api.QueryLatestEventsAndStateResponse,
+) error {
+	eventsNeeded, err := gomatrixserverlib.StateNeededForEventBuilder(builder)
+	if err != nil {
+		return fmt.Errorf("gomatrixserverlib.StateNeededForEventBuilder: %w", err)
+	}
+
+	if len(eventsNeeded.Tuples()) == 0 {
+		return errors.New("expecting state tuples for event builder, got none")
+	}
+
+	// Ask the roomserver for information about this room
+	queryReq := api.QueryLatestEventsAndStateRequest{
+		RoomID:       builder.RoomID,
+		StateToFetch: eventsNeeded.Tuples(),
+	}
+	if err = rsAPI.QueryLatestEventsAndState(ctx, &queryReq, queryRes); err != nil {
+		return fmt.Errorf("rsAPI.QueryLatestEventsAndState: %w", err)
+	}
+
+	if !queryRes.RoomExists {
+		return ErrRoomNoExists
+	}
+
+	eventFormat, err := queryRes.RoomVersion.EventFormat()
+	if err != nil {
+		return fmt.Errorf("queryRes.RoomVersion.EventFormat: %w", err)
+	}
+
+	builder.Depth = queryRes.Depth
+
+	authEvents := gomatrixserverlib.NewAuthEvents(nil)
+
+	for i := range queryRes.StateEvents {
+		err = authEvents.AddEvent(&queryRes.StateEvents[i].Event)
+		if err != nil {
+			return fmt.Errorf("authEvents.AddEvent: %w", err)
+		}
+	}
+
+	refs, err := eventsNeeded.AuthEventReferences(&authEvents)
+	if err != nil {
+		return fmt.Errorf("eventsNeeded.AuthEventReferences: %w", err)
+	}
+
+	truncAuth, truncPrev := truncateAuthAndPrevEvents(refs, queryRes.LatestEvents)
+	switch eventFormat {
+	case gomatrixserverlib.EventFormatV1:
+		builder.AuthEvents = truncAuth
+		builder.PrevEvents = truncPrev
+	case gomatrixserverlib.EventFormatV2:
+		v2AuthRefs, v2PrevRefs := []string{}, []string{}
+		for _, ref := range truncAuth {
+			v2AuthRefs = append(v2AuthRefs, ref.EventID)
+		}
+		for _, ref := range truncPrev {
+			v2PrevRefs = append(v2PrevRefs, ref.EventID)
+		}
+		builder.AuthEvents = v2AuthRefs
+		builder.PrevEvents = v2PrevRefs
+	}
+
+	return nil
+}
+
+// truncateAuthAndPrevEvents limits the number of events we add into
+// an event as prev_events or auth_events.
+// NOTSPEC: The limits here feel a bit arbitrary but they are currently
+// here because of https://github.com/matrix-org/matrix-doc/issues/2307
+// and because Synapse will just drop events that don't comply.
+func truncateAuthAndPrevEvents(auth, prev []gomatrixserverlib.EventReference) (
+	truncAuth, truncPrev []gomatrixserverlib.EventReference,
+) {
+	truncAuth, truncPrev = auth, prev
+	if len(truncAuth) > 10 {
+		truncAuth = truncAuth[:10]
+	}
+	if len(truncPrev) > 20 {
+		truncPrev = truncPrev[:20]
+	}
+	return
+}