That is fine. Such "upstream server down" events should be rare and the extra
notices in the logs will not impact things.

Thank you. Can you provide a hint as to what .c file(s) I would need to modify
and what types of system calls I should look at? Maybe all I need to do is find
the spot where nginx deems it should skip a downed server, and immediately after
that add this code: if (fork() == 0) execl("/bin/sh","/path/to/upstream_down.sh","ID of down upsteam goes here",
NULL);

Or should I be using the system() function?

Any other considerations I need to worry about?

Also if I make this change might it be adopted and incorporated into the
product? I would not want to have to keep applying my own patch every time I
compile a new version of nginx and am happy to contribute my work to this
excellent product and project. Thank you.