how to proxy a proxy (subrequest with corporate proxy)

[CeeGeeDev]

Greetings,

Our custom nginx module implements a number of subrequests (REST calls to
other servers to obtain data for our business logic). Everything is working
correctly, except one customer requires a corporate HTTP web proxy for the
URL (running on a different server) that our subrequests will be hitting.
It's not clear to us how to configure a "web proxy" for a subrequest, since
the subrequest itself is already basically a "proxy" call.

Our subrequests would look like this:

location /subrequest {
    internal;
    resolver 127.0.0.1;
    proxy_pass http://rest_server/...;
}

We're aware of this kind of thing for the main request... but unclear if/how
it applies to subrequests?

http {
  upstream corporate_proxy  {
      server proxy.my.company.net:8080;
  }
  server {
    ...
    location /custom/main/request/url {
          proxy_buffering off;
          proxy_pass_header on;
          proxy_set_header Host "www.origin-server.com";
          proxy_pass http://corporate_proxy;
    }
  }
}

How would we force the subrequest to use the corporate proxy?

Thanks

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,273300,273300#msg-273300