ngx_http_script_run() question

[Maxim Dounin]

Hello!

On Mon, Jul 18, 2022 at 04:02:53PM +0100, Chris Newton via nginx-devel wrote:

> I'm adding a variable that wants to provide an absolute URL to proxy_pass,
> whose value is based in part on a database lookup - but if the search key
> is not found in that database, my handler function is returning an
> NGX_ERROR which seemed reasonable (there is no good default I can return),
> so just causing a 500 error at that point is not unreasonable.
> 
> However, that also causes an error to be logged by ngx_http_proxy_eval()
> for every request, which i'd like to avoid.

NGX_ERROR means a fatal error and it is expected to be logged - 
by the code where the error appears.  Right now scripting 
infrastructure silently converts errors from variable handlers to 
empty values - but this is something to change, since this 
isn't a safe approach and can cause various issues.  Consider 
returning a value instead - something like an empty value or a 
value with the not_found flag.

The logging you see is, however, unrelated.  It's 
ngx_http_proxy_eval() which complains about an invalid URL after 
variable evaluation.  That's because proxy_pass is expected to be 
reasonably configured to always result in a valid URL.  If it 
doesn't, this is considered to be a configuration error and 
reported, both to the user with 500 (Internal Server Error) and to 
the error log.

If in your case a valid URL for proxying is not always available, 
consider re-configuring nginx to avoid the error condition.  In 
particular, providing a dummy URL which immediately returns an 
error or an explicit check before proxying might be considered.

-- 
Maxim Dounin
http://mdounin.ru/