input required on proxy_next_upstream

[kaustubh]

Thanks for reply. But I checked upstreams and second instance is working
fine but does not receive retry request.

I did small setup where one upstream instance responds early with 503 and
other instance processes requests,
and I observe that the request never comes to working upstream server on
early 503 from first upstream.

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,272440,272495#msg-272495