[FFmpeg-user] How to stop input sources stalling when one of the sources is a live input?

[Jun Hao Xu]

Hi there, apologies if I am using the incorrect terminology but I am
currently facing an issue around my current FFMPEG set up.

Context is that I am taking in a live SRT input (which could have multiple
tracks on them) and sending that through to a Janus node which is expecting
us to send a signal on different ports. I use a tee to split my SRT tracks
and send them onto the different ports and happy days. Sometimes we might
not get a multi track SRT input and Janus will complain if I don't send any
signal on all the ports it needs and so substitute it by using a `anullsrc`
input and happy days.

This works fine at the beginning where the PTS starts from 0 for both
anullsrc and the live input but if for some reason the ffmpeg process dies
or I need to restart it, we would get the PTS of the `anullsrc` starting at
0 but obviously my stream will have progressed pass 0 and what I've
observed using the ashowinfo component was that the input will stall until
the PTS time of the anullsrc catches up with the live input. This isn't
ideal as then I hit an issue with No Room to Store incoming packet in my
FFMPEG process.

Is there a way to sync up the anullsrc with the live input in the FFMPEG
command or some way to prevent this stalling? I know you can use -itsoffset
but I won't have any idea how far along my live input is ahead of time.