Allowing the etag directive within if in location

[Maxim Dounin]

Hello!

On Sun, Nov 27, 2022 at 01:01:54AM +0800, Danila Vershinin wrote:

> Hi,
> 
> For our purposes, we need to be able to turn off etag under specific
> conditions, e.g.:
> 
> location / {
>     if ($condition) {
>         etag off;
>     }
> }
> 
> However, this syntax is invalid because this directive doesn't include
> the NGX_HTTP_LIF_CONF flag.
> 
> We added that flag. Then recompiled NGINX. So now:
> 
> * the syntax is accepted by NGINX
> * at runtime it works as desired
> 
> Are there any potential problems with this approach? Performance or
> otherwise?

The generic issue with the "if" directive within a location 
context is that it creates a separate configuration, and therefore 
works in a counter-intuitive way in many cases (including being 
counter-intuitive for developers, which results in various bugs), 
see IfIsEvil wiki article for the details.  Due to this, no new 
directives are allowed to work in the if-in-location context.

Other than that, it probably should work fine assuming it works 
for you now and you are ok to maintain the patch yourself.

A more portable solution might be to use

    map $condition $etag {
        default    $sent_http_etag;
        1          "";
    }

    add_header ETag $etag;

to conditionally remove the ETag header.

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