When is pad added for the demuxer in gst-rtsp-server?

[Howling wong]

Hi

    I have been working with gst-rtsp-server libraries to demux an udpsrc to audio and video gchar *pipeline. Have modelled after the example in test_mp4.c
    "https://github.com/GStreamer/gst-rtsp-server/blob/master/examples/test-mp4.c<https://github.com/GStreamer/gst-rtsp-server/blob/master/examples/test-mp4.c>"

Code:
   g_strdup_printf ("( udpsrc port=%d name=src0 "
                  "! queue ! rtpmp2tdepay ! video/mpegts, systemstream=true, packetsize=188  ! aiurdemux  name=dm "
                  " dm.video_0 ! queue ! h264parse name=video !  rtph264pay config-interval=1 name=pay0 pt=96 "
                  " dm.audio_0  !  queue ! aacparse name=audio ! rtpmp4gpay name=pay1 pt=97 )",
                   5678);
    gst_rtsp_media_factory_set_launch (factory,pipeline );

    // to configure pipeline
    g_signal_connect (factory, "media-configure",
                      (GCallback) media_configure_cb, NULL);

And I have added a callback when pad is added for the demuxer at media_configure_cb function (much like in normal gstreamer),

    void
             l_media_configure_cb (GstRTSPMediaFactory * factory,
                                                 GstRTSPMedia * media,
                                                NULL)
   {
         //  other code to configure
        pipeline    = gst_rtsp_media_get_element (media);
         GstElement* demuxer  = gst_bin_get_by_name_recurse_up (GST_BIN (pipeline ), "dm");
        g_signal_connect (demuxer, "pad-added", G_CALLBACK (on_rtsp_pad_added),rtsp_app.pipeline);

    }

The problem I have is that the demuxer only has pad added inconsistently (ie sometimes adding them and sometimes not adding them). Just wondering the following question.

       1. When is pad-added for this case? the example seem to suggest pad adding is taken care of by system
       2. Should I instead watch for the signal media_prepared ( I believed it is too late?) or other signal ?
       3. What should I do otherwise... checking the bus and take appropriate action

I am using Gstreamer 1.14

Regards
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