HLS proxying multiple sources

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Hello,

I need a way to provide the 'proxy_pass' argument from external script 
for hls streaming. One way will be to use exec_pull with rtmp module, 
but this means to convert hls to rtmp, and run ffmpeg - pretty 
cumbersome. If there is a way to do something like ** proxy_pass 
`my_script original_request` ** (my_script is in backticks), it will be 
ideal.

Situation: I need to choose dynamically the server where I pull the hls 
video stream from. Some of the upstream servers (usually 2-4 in total) 
may be down or slow at the moment where the request comes to the local 
nginx, so I need to dynamically choose the 'preferred' one. On top, I 
have different login credentials for those, and a limit of logins per 
server.

Almost sure nginx will have some way of doing it - it does anything 
imaginable - and few things beyond.

Any suggestion appreciated,
Thanks, George.