From d9e72f4fd0314cbf5fadadf1269a6b9612a39930 Mon Sep 17 00:00:00 2001
From: Juliusz Chroboczek <jch@irif.fr>
Date: Tue, 2 Feb 2021 19:06:37 +0100
Subject: [PATCH] Return an error when adding a duplicate down track.

---
 rtpconn/webclient.go | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/rtpconn/webclient.go b/rtpconn/webclient.go
index f7de7a3..0e1fc98 100644
--- a/rtpconn/webclient.go
+++ b/rtpconn/webclient.go
@@ -403,6 +403,20 @@ func addDownTrack(c *webClient, conn *rtpDownConnection, remoteTrack conn.UpTrac
 		return nil, errors.New("unexpected up track type")
 	}
 
+	conn.mu.Lock()
+	defer conn.mu.Unlock()
+
+	remoteSSRC := rt.track.SSRC()
+	for _, t := range conn.tracks {
+		tt, ok := t.remote.(*rtpUpTrack)
+		if !ok {
+			return nil, errors.New("unexpected up track type")
+		}
+		if tt.track.SSRC() == remoteSSRC {
+			return nil, os.ErrExist
+		}
+	}
+
 	local, err := webrtc.NewTrackLocalStaticRTP(
 		remoteTrack.Codec(),
 		rt.track.ID(), rt.track.StreamID(),
@@ -431,9 +445,7 @@ func addDownTrack(c *webClient, conn *rtpDownConnection, remoteTrack conn.UpTrac
 		atomics:    &downTrackAtomics{},
 	}
 
-	conn.mu.Lock()
 	conn.tracks = append(conn.tracks, track)
-	conn.mu.Unlock()
 
 	go rtcpDownListener(conn, track, sender)