"status 000" sometimes occurs when using eval with proxy_pass

[pmarks-net]

We were using nginx-eval-module with NGINX 1.16.0 and a configuration of this form:

location / {
  proxy_pass https://some_upstream_server/;

  eval_override_content_type application/octet-stream;
  eval $foo {
    proxy_pass http://foo;
    proxy_pass_request_headers off;
  }
  proxy_set_header Authorization $foo;
}

Clients send us POST requests, the eval module fetches a value, and we attach a header before forwarding the request.

Some fraction of the time, for some fraction of client implementations, NGINX would simply close the incoming TCP connection, while logging these values:

$status "000"
$upstream_status "-"

Lines like this also appeared in the --debug log:

*517 http client request body rest 217  
*517 http client request body recv -2  

I'm not trying to debug this anymore, because "This module is experimental and production use is discouraged", and the problem went away when I replaced eval with rewrite_by_lua_block + ngx.location.capture().

I'm just posting this in case someone encounters similar errors in the future.