Peeking via MSC2753 (#1370)

Initial implementation of MSC2753, as tested by https://github.com/matrix-org/sytest/pull/944.
Doesn't yet handle unpeeks, peeked EDUs, or history viz changing during a peek - these will follow.
https://github.com/matrix-org/dendrite/pull/1370 has full details.

roomserver/api/api.go

@@ -36,6 +36,12 @@ type RoomserverInternalAPI interface {
 		res *PerformLeaveResponse,
 	) error
 
+	PerformPeek(
+		ctx context.Context,
+		req *PerformPeekRequest,
+		res *PerformPeekResponse,
+	)
+
 	PerformPublish(
 		ctx context.Context,
 		req *PerformPublishRequest,

roomserver/api/api_trace.go

@@ -38,6 +38,15 @@ func (t *RoomserverInternalAPITrace) PerformInvite(
 	return t.Impl.PerformInvite(ctx, req, res)
 }
 
+func (t *RoomserverInternalAPITrace) PerformPeek(
+	ctx context.Context,
+	req *PerformPeekRequest,
+	res *PerformPeekResponse,
+) {
+	t.Impl.PerformPeek(ctx, req, res)
+	util.GetLogger(ctx).Infof("PerformPeek req=%+v res=%+v", js(req), js(res))
+}
+
 func (t *RoomserverInternalAPITrace) PerformJoin(
 	ctx context.Context,
 	req *PerformJoinRequest,

roomserver/api/output.go

@@ -46,6 +46,9 @@ const (
 	// - Redact the event and set the corresponding `unsigned` fields to indicate it as redacted.
 	// - Replace the event in the database.
 	OutputTypeRedactedEvent OutputType = "redacted_event"
+
+	// OutputTypeNewPeek indicates that the kafka event is an OutputNewPeek
+	OutputTypeNewPeek OutputType = "new_peek"
 )
 
 // An OutputEvent is an entry in the roomserver output kafka log.
@@ -59,8 +62,10 @@ type OutputEvent struct {
 	NewInviteEvent *OutputNewInviteEvent `json:"new_invite_event,omitempty"`
 	// The content of event with type OutputTypeRetireInviteEvent
 	RetireInviteEvent *OutputRetireInviteEvent `json:"retire_invite_event,omitempty"`
-	// The content of event with type  OutputTypeRedactedEvent
+	// The content of event with type OutputTypeRedactedEvent
 	RedactedEvent *OutputRedactedEvent `json:"redacted_event,omitempty"`
+	// The content of event with type OutputTypeNewPeek
+	NewPeek *OutputNewPeek `json:"new_peek,omitempty"`
 }
 
 // An OutputNewRoomEvent is written when the roomserver receives a new event.
@@ -195,3 +200,11 @@ type OutputRedactedEvent struct {
 	// The value of `unsigned.redacted_because` - the redaction event itself
 	RedactedBecause gomatrixserverlib.HeaderedEvent
 }
+
+// An OutputNewPeek is written whenever a user starts peeking into a room
+// using a given device.
+type OutputNewPeek struct {
+	RoomID   string
+	UserID   string
+	DeviceID string
+}

roomserver/api/perform.go

@@ -108,6 +108,20 @@ type PerformInviteResponse struct {
 	Error *PerformError
 }
 
+type PerformPeekRequest struct {
+	RoomIDOrAlias string                         `json:"room_id_or_alias"`
+	UserID        string                         `json:"user_id"`
+	DeviceID      string                         `json:"device_id"`
+	ServerNames   []gomatrixserverlib.ServerName `json:"server_names"`
+}
+
+type PerformPeekResponse struct {
+	// The room ID, populated on success.
+	RoomID string `json:"room_id"`
+	// If non-nil, the join request failed. Contains more information why it failed.
+	Error *PerformError
+}
+
 // PerformBackfillRequest is a request to PerformBackfill.
 type PerformBackfillRequest struct {
 	// The room to backfill