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Created by Connor MacDonald@connor

webrtcbin: assertion failure when data channel is created before linking

I am observing a failed assertion when I attempt to create a data channel before linking the webrtcbin to a source:

ERROR:../subprojects/gst-plugins-bad/ext/webrtc/gstwebrtcbin.c:2291:sdp_media_from_transceiver: assertion failed: (trans->mline == -1 || trans->mline == media_idx)

I am encountering this issue in a situation where I am dynamically adding instances of webrtcbin to a pipeline. I believe this should be possible like it is in the JavaScript API, e.g.:

var pc = new RTCPeerConnection();
var channel = pc.createDataChannel('channel');
pc.addTrack(track);

Even if this is a known limitation, this error should be detected and reported.

I have uploaded test code that is simply a version of the basic webrtc example that is modified to reproduce the bug. If you change these lines:

g_signal_emit_by_name (webrtc1, "create-data-channel", "channel", NULL, &channel);
gst_element_link (rtp_caps_filter, webrtc1);

to these:

gst_element_link (rtp_caps_filter, webrtc1);
g_signal_emit_by_name (webrtc1, "create-data-channel", "channel", NULL, &channel);

then everything works as expected.

Here is the output with GST_DEBUG=4: debug_log_0.txt

For now, the work-around is to (obviously) defer data channel creation until after the element has been linked.

I am using version GStreamer 1.17.0.1.