Incorrect special case for "return 204" removed. The special case in question leads to replies without body in configuration like location / { error_page 404 /zero; return 404; } location /zero { return 204; } while replies with empty body are expected per protocol specs. Correct one will look like if (status == NGX_HTTP_NO_CONTENT) { rc = ngx_http_send_header(r); if (rc == NGX_ERROR || r->header_only) { return rc; } return ngx_http_send_special(r, NGX_HTTP_LAST); } though it looks like it's better to drop this special case at all.

--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -1790,11 +1790,6 @@ ngx_http_send_response(ngx_http_request_
 
     r->headers_out.status = status;
 
-    if (status == NGX_HTTP_NO_CONTENT) {
-        r->header_only = 1;
-        return ngx_http_send_header(r);
-    }
-
     if (ngx_http_complex_value(r, cv, &val) != NGX_OK) {
         return NGX_HTTP_INTERNAL_SERVER_ERROR;
     }