ngx_http_script_run() question

Chris Newton cnewton at
Mon Jul 18 15:02:53 UTC 2022

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.

I can make a change to ngx_http_proxy_eval() :

*--- a/nginx/files/nginx/src/http/modules/ngx_http_proxy_module.c*

*+++ b/nginx/files/nginx/src/http/modules/ngx_http_proxy_module.c*

@@ -1098,9 +1098,10 @@ ngx_http_proxy_eval(ngx_http_request_t *r,
ngx_http_proxy_ctx_t *ctx,

     ngx_url_t             url;

     ngx_http_upstream_t  *u;

     if (ngx_http_script_run(r, &proxy, plcf->proxy_lengths->elts, 0,


-        == NULL)

+        == NULL || proxy.len == 0)


         return NGX_ERROR;


although I think it might be good to make a more general fix - ie., have
ngx_http_script_run() return NULL if there was no output from the script:

*--- a/nginx/files/nginx/src/http/ngx_http_script.c*

*+++ b/nginx/files/nginx/src/http/ngx_http_script.c*

@@ -641,6 +641,9 @@ ngx_http_script_run(ngx_http_request_t *r, ngx_str_t

         len += lcode(&e);


+    if (len == 0) {

+        return NULL;

+    }

Would that be a reasonable change to make ?


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