nginx 100% cpu and it may be a nginx bug

[wangbin579]

The function ngx_http_upstream_get_round_robin_peer in nginx will loop
for a long period of times when pc->tries equals to 0.

Well then, under what conditions does pc->tries equal to zero?

When executing ngx_http_upstream_get_round_robin_peer for the first
time, in some cases it returns NGX_OK while pc->tries equals to 1. 

When in ngx_http_upstream_process_header, nginx process may enter the
following:
1524         if (n == NGX_ERROR || n == 0) {
1525             ngx_http_upstream_next(r, u,
NGX_HTTP_UPSTREAM_FT_ERROR);
1526             return;
1527         }

then enter ngx_http_upstream_next function

In the ngx_http_upstream_next,nginx process will call
ngx_http_upstream_free_round_robin_peer

2796     if (ft_type != NGX_HTTP_UPSTREAM_FT_NOLIVE) {
2797         u->peer.free(&u->peer, u->peer.data, state);
2798     }


we now enter ngx_http_upstream_free_round_robin_peer.

Pay attention to the following codes in the function:
if (pc->tries) {
   pc->tries--;
}

After executing the above, pc->tries changes from 1 to 0.

then the process returns to ngx_http_upstream_next

2892     ngx_http_upstream_connect(r, u);
2893 }

we enter ngx_http_upstream_connect here:

this function will call 

1081     rc = ngx_event_connect_peer(&u->peer);


...


It finally enter ngx_http_upstream_get_round_robin_peer again. And then
the process will loop for a long time which is unpredictable.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,218736,218736#msg-218736