RTMP slate fallback

This is an example on how to fallback to a "slate" stream using compositor.

In this example, we use a simple videotestsrc as the "slate" stream, it can of course be replaced with any other reliable source.

This example has been tested with live RTMP streams, and uses interpipe elements to isolate the source and mixing pipelines.

This requires patches from https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/620

Build with:

cargo build

Display help:

cargo run -- --help

Behaviour on error in the source pipeline

Test with:

cargo run -- --live-rtmp-uri rtmp://192.168.1.107:1935/live/myStreamd --error-after 300 --discard-after 2

Expected behaviour:

• the slate should be displayed while the RTMP pipeline is buffering

• then 300 buffers (10 seconds with a 30 fps framerate) should be displayed from the live RTMP stream.

• the last buffer should stay displayed for 2 seconds

• the fallback slate should be displayed again while the source pipeline rebuffers

• As the bus handler restarts the pipeline upon error, the RTMP stream should be displayed again after buffering

You can change --discard-after to 0 to fall back to the slate without freezing, not specifying it will freeze the last received buffer for ever, which means the slate will only be displayed at the start.

Behaviour on EOS from the source pipeline

Test with:

cargo run -- --live-rtmp-uri rtmp://192.168.1.107:1935/live/myStreamd --eos-after 300 --discard-after 2

The behaviour with this implementation is for EOS to be handled the same way as error. If this is not desirable, the EOS message handler should be modified to not restart the pipeline, and forward-eos should be set to TRUE on interpipesink.