Make process a string getter in input.external.{rawaudio, rawvideo}

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Discussed in #2860

^{Originally posted by krangm January 23, 2023}
Hello.

Please help to find out how to reuse source in rotate.merge after it's exit. As it states in docs, rotate.merge should loops a sequence of sources. But after source exit 0 on the end of track, stream goes safe_blank instead of looping sources.

Some info:

$ uname -a
Linux localhost 6.1.0-1-amd64 #1 SMP PREEMPT_DYNAMIC Debian 6.1.4-1 (2023-01-07) x86_64 GNU/Linux

$ liquidsoap --version
Liquidsoap 2.1.3

LS code:

#!/usr/bin/liquidsoap
log.file.path.set("/home/user/radio/radio.log")
log.stdout.set(true)

def input.airsonic(~id="input.airsonic",
		~restart=false,~restart_on_error=false,
		~buffer=0.2,~max=20.,s) =
	input.external.rawaudio(id=id,restart=restart,
		restart_on_error=restart_on_error,
		buffer=buffer,max=max,
		"/home/user/radio/streamer #{string.quote(s)}")
end

trackids = (process.read.lines("cat /home/user/radio/total_songs.list"))

def input.airsonic.random(~id="input.airsonic.random",
		~restart=false,~restart_on_error=false) =
	input.airsonic(id=id,restart=restart,
		restart_on_error=restart_on_error,
		list.pick(list.shuffle(trackids)))
end

radio = rotate.merge([input.airsonic.random(),input.airsonic.random()])
radio = mksafe(radio)

output.icecast(%fdkaac(channels=2, samplerate=44100,
		bandwidth="auto", bitrate=128, afterburner=true,
		aot="mpeg4_he_aac_v2", transmux="adts", sbr_mode=true),
	host = "x",
	port = x,
	password = "x",
	mount = "x",
radio)

With current code, LS plays two random tracks and goes safe_blank after each of sources exit. If i try to set
rotate.merge([input.airsonic.random(restart=true),input.airsonic.random(restart=true)])
only first source plays and it constantly repeats 1 track, so it does not perform list.pick() every time as expected.

Is there a way to reuse source without restart argument? Or to re-read variable on source repeat? Or maybe there is another approach?