id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	uname	nginx_version
2418	ngx_http_upstream_process_non_buffered_request do_write Dead-loop	bullerdu@…		"In non-buffering transport mode(**proxy_request_buffering off**), I encountered a write dead loop problem with the following code path:

{{{
src/http/ngx_http_upstream.c:3666 ngx_http_upstream_process_non_buffered_request
  ->
     src/http/ngx_http_upstream.c:3691
     rc = ngx_http_output_filter(r, u->out_bufs);  /* u->out_bufs=0x0 */
         ->
         src/http/ngx_http_write_filter_module.c:48
         ngx_http_write_filter
            ->
            src/http/ngx_http_write_filter_module.c:295
            chain = c->send_chain(c, r->out, limit);
               src/os/uinx/ngx_linux_sendfile_chain.c:50
               ngx_linux_sendfile_chain
               /* code here return
                  if (!wev->ready) {
                      return in;
                  } 
               */

             src/http/ngx_http_write_filter_module.c:342
             /* code here return
                if (chain) {
                    c->buffered |= NGX_HTTP_WRITE_BUFFERED;
                    return NGX_AGAIN;
                }
              */

src/http/ngx_http_upstream.c:3735
n = upstream->recv(upstream, b->last, size);
/* code here, return n = -1 */

->

src/http/ngx_http_upstream.c:3753
/* code here, continue */

-> 

src/http/ngx_http_upstream.c:3691
rc = ngx_http_output_filter(r, u->out_bufs);

}}}

**My analysis is as follows:**

First:
when the downstream is written, ngx_linux_sendfile_chain calls ngx_writev and returns NGX_AGAIN, setting wev->ready =0
src/os/uinx/ngx_linux_sendfile_chain.c:204
if (n == NGX_AGAIN) {
wev->ready = 0;
return in;
}

Second:
src/http/ngx_http_upstream.c:3691
ngx_http_output_filter returns NGX_AGAIN, u->out_bufs = NULL, u->busy_bufs! = NULL

size > 0 && upstream->read->ready = 1, upstream->recv returns -1, So I keep writing downstream(ngx_http_output_filter), but downstream->write->ready = 0 so ngx_http_output_filter keeps returning -2

**The resulting problems：**
ngx_http_output_filter loop call, some filter_modules have cpu consumption problem, CPU 100%

**The Solution：**

Check the return value of ngx_http_output_filter. If it is NGX_AGAIN, break the write loop

src/http/ngx_http_upstream.c:3691

rc = ngx_http_output_filter(r, u->out_bufs);
if (rc == NGX_ERROR) {
   ngx_http_upstream_finalize_request(r, u, NGX_ERROR);
   return;
}
if (rc == NGX_AGAIN) {
   ...

   break;
}


"	defect	closed	minor		documentation	1.23.x	fixed	proxy_request_buffering off	bullerdu@…	Linux cdn-dev011164234021.na61 4.19.91-008.ali4000.alios7.x86_64 #1 SMP Fri Sep 4 17:33:26 CST 2020 x86_64 x86_64 x86_64 GNU/Linux	"nginx version: nginx/1.23.2
built by gcc 6.5.1 20220324 (GCC)
configure arguments: --prefix=/home/yefei.dyf/nginx"