best way to handle inactive sinks with eg. compositor

[Marc]

If one is working with multiple video streams of which you do not know up front if they are running/available. What would be the adviced method of handling these? 

Eg. if the 2nd tcpclientsrc is not running the whole pipe is stuck at ' Pipeline is PREROLLING ... '

gst-launch-1.0 \
    compositor name=c sink_1::xpos=0 sink_2::xpos=320 sink_3::ypos=240 sink_4::xpos=320 sink_4::ypos=240 \
    ! x264enc ! mpegtsmux name=mux ! srtsink uri=srt://:33333 \
    videotestsrc pattern=blue ! video/x-raw, format=I420, framerate=5/1, width=640, height=480 ! c. \
    tcpclientsrc port=40001 ! queue ! tsdemux ! video/x-h264 ! h264parse ! avdec_h264 ! video/x-raw ! c.sink_1 \
    tcpclientsrc port=40002 ! queue ! tsdemux ! video/x-h264 ! h264parse ! avdec_h264 ! video/x-raw ! c.sink_2 \
    videotestsrc ! c.sink_3 \
    videotestsrc ! c.sink_4