rtc: broadcast an unlabeled data message from a source identity (#5112)

* rtc: broadcast an unlabeled data message from a source identity

BroadcastDataMessageForRoomFrom takes the source ID and identity instead
of a LocalParticipant, so a caller can stamp the sender of a message
whose source is not on this node. BroadcastDataMessageForRoom keeps its
signature and calls it.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

* rtc: say what sourceID does in the BroadcastDataMessageForRoomFrom comment

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

pkg/rtc/room.go

@@ -2188,8 +2188,20 @@ func BroadcastDataMessageForRoom(r types.Room, source types.LocalParticipant, da
 		sourceIdentity = source.Identity()
 	}
 
+	BroadcastDataMessageForRoomFrom(r, sourceID, sourceIdentity, data, logger)
+}
+
+// BroadcastDataMessageForRoomFrom sends an unlabeled data message to every local participant, stamping
+// sourceIdentity as the sender and skipping the local participant with sourceID, if there is one.
+func BroadcastDataMessageForRoomFrom(
+	r types.Room,
+	sourceID livekit.ParticipantID,
+	sourceIdentity livekit.ParticipantIdentity,
+	data []byte,
+	logger logger.Logger,
+) {
 	utils.ParallelExec(r.GetLocalParticipants(), dataForwardLoadBalanceThreshold, 1, func(op types.LocalParticipant) {
-		if source != nil && op.ID() == sourceID {
+		if sourceID != "" && op.ID() == sourceID {
 			return
 		}
 

pkg/rtc/room_test.go

@@ -788,6 +788,38 @@ func TestDataChannel(t *testing.T) {
 		}
 	})
 
+	t.Run("unlabeled broadcast from a remote identity", func(t *testing.T) {
+		rm := newRoomWithParticipants(t, testRoomOpts{num: 2})
+		defer rm.Close(types.RoomCloseReasonUnknown)
+
+		BroadcastDataMessageForRoomFrom(rm, "", "remote", []byte("relayed"), rm.logger)
+
+		for _, op := range rm.GetParticipants() {
+			fp := op.(*typesfakes.FakeLocalParticipant)
+			require.Equal(t, 1, fp.SendDataMessageUnlabeledCallCount())
+			got, _, sender := fp.SendDataMessageUnlabeledArgsForCall(0)
+			require.Equal(t, []byte("relayed"), got)
+			require.Equal(t, livekit.ParticipantIdentity("remote"), sender)
+		}
+	})
+
+	t.Run("unlabeled broadcast from a local id skips the source", func(t *testing.T) {
+		rm := newRoomWithParticipants(t, testRoomOpts{num: 2})
+		defer rm.Close(types.RoomCloseReasonUnknown)
+
+		participants := rm.GetParticipants()
+		p := participants[0].(*typesfakes.FakeLocalParticipant)
+		BroadcastDataMessageForRoomFrom(rm, p.ID(), p.Identity(), []byte("local"), rm.logger)
+
+		require.Zero(t, p.SendDataMessageUnlabeledCallCount())
+		for _, op := range participants[1:] {
+			fp := op.(*typesfakes.FakeLocalParticipant)
+			require.Equal(t, 1, fp.SendDataMessageUnlabeledCallCount())
+			_, _, sender := fp.SendDataMessageUnlabeledArgsForCall(0)
+			require.Equal(t, p.Identity(), sender)
+		}
+	})
+
 	t.Run("publishing disallowed", func(t *testing.T) {
 		rm := newRoomWithParticipants(t, testRoomOpts{num: 2})
 		defer rm.Close(types.RoomCloseReasonUnknown)