[PATCH] Fixed 413 custom error page for HTTP/2 and HTTP/3 (ticket #2609)

[Roman Arutyunyan]

# HG changeset patch
# User Roman Arutyunyan <arut at nginx.com>
# Date 1709563405 -14400
#      Mon Mar 04 18:43:25 2024 +0400
# Node ID 3b0be477ab7246caba4c5152286b8be520ee0418
# Parent  44da04c2d4db94ad4eefa84b299e07c5fa4a00b9
Fixed 413 custom error page for HTTP/2 and HTTP/3 (ticket #2609).

Previously an attempt to return a custom 413 error page for these protocols
resulted in the standard 413 page (if recursive_error_pages was off) or
otherwise internal redirection cycle followed by the 500 error.

Discarding request body for HTTP/1 starts by setting r->discard_body which
indicates the body is currently being discarded.  If and when the entire body
is read and discarded, the flag is cleared and r->headers_in.content_length_n
is set to zero.  Both r->discard_body and r->headers_in.content_length_n
prevent nginx from re-generating 413 error after internal redirect in
ngx_http_core_find_config_phase().

However the above does not work for HTTP/2 and HTTP/3.  Discarding request
body for these protocols does not affect the above mentioned fields, which is
why there's no protection against re-generating the 413 error.  The fix is to
assign zero to r->headers_in.content_length_n much like in HTTP/1 case after
the body is entirely read and discarded, except for these protocols no active
discard is needed.

diff --git a/src/http/ngx_http_request_body.c b/src/http/ngx_http_request_body.c
--- a/src/http/ngx_http_request_body.c
+++ b/src/http/ngx_http_request_body.c
@@ -640,12 +640,14 @@ ngx_http_discard_request_body(ngx_http_r
 #if (NGX_HTTP_V2)
     if (r->stream) {
         r->stream->skip_data = 1;
+        r->headers_in.content_length_n = 0;
         return NGX_OK;
     }
 #endif
 
 #if (NGX_HTTP_V3)
     if (r->http_version == NGX_HTTP_VERSION_30) {
+        r->headers_in.content_length_n = 0;
         return NGX_OK;
     }
 #endif