Offer to run JS code at the rewrite phase

[TPXP]

Is your feature request related to a problem? Please describe

I'm trying to implement a reverse proxy with NJS, however each backend I can send the request to needs a different header value. So far, I can define a backend and get nginx to forward requests to it with proxy_pass like this (I need to use a proxy directive since the backend server can return large files):

js_set $upstream myCode.getUpstream;
js_set $freshHeader myCode.getFreshHeader;

proxy_pass $upstream;
proxy_set_header X-Custom-Fresh-Header $freshHeader;

I'd like to handle upstream server errors and get Javascript code to run when that happens, to determine a new upstream to try (and compute the new header). To that extent, I'm performing an internal redirect on error. (Cannot use upstream since headers need to change, also I need to have control on the order in which servers are tried).

location / {
  js_set $upstream myCode.getUpstream;

  proxy_pass $upstream;

  # If something goes wrong, start over
  proxy_intercept_errors on;
  recursive_error_pages on;
  error_page 502 503 504 = /;
}

I hoped to be able to use a variable like upstream_addr to determine the amount of servers tried and adapt the value returned by js_set. However, since js_set and js_var return a cached variable, the get handler will never be called again and nginx will reuse its value for every round of processing. So $upstream does not change for every attempt.

Describe the solution you'd like

I'd like to be able to explicitely state I want some JS code to run at a certain phase (here, I think the rewrite would be great) to be able to run code in a more predictable way.

Describe alternatives you've considered

OpenResty/nginx-lua seems to handle this with the rewrite_by_lua directive. We can possibly take some inspiration.

Additional context

Nothing special, thanks for bringing JS to nginx. It makes it quite fun to script nginx behaviour!

[TPXP]

After browsing deeper through the request, I found out that the recommended way to achieve this is to rely on an auth_request with js_content. This doesn't work well in my case because the subrequest used by auth_request hides a few variables from the context (for example $upstream_code, location capture groups, or even the original request URI!).

Also, being able to call JS code in the middle of an if block would be super cool!

[TPXP]

With #771 I'm able to workaround this by calling my variable in a set $target $js_variable; directive (evaluated at rewrite phase) so this is less of a problem now. Also, I see there are some similar issues in #475 and #474 so I'll close this issue.

In case someone has a similar problem, here's how I achieve it on my side

js_set $upstream myCode.getUpstream nocache;
# myCode.getUpstream can update otherVar through `r.variables`
js_var $otherVar;

location / {
  set $cached_upstream $upstream; # avoid duplicate calls since the variable is not cached
  if ($upstream = "") {
    return 503 "Not available";
  }
  proxy_set_header X-custom $otherVar;
  proxy_pass $upstream;
}