how to handle stale event in nginx - nginx

Question

These days, I am studying the nginx source code.
But there is a question about stale event.
If there is some coming event : #1, #2,#3 .. #40,
When we deal with #1, #40 will shut down and it's variables instance is 0
and #2.#3 is a new connection,
Accept function to allocate a new descriptor which is just free (#40),
when we deal with #2,
we need to invoke function named ngx_event_accept, and then invoke ngx_get_connection,
but unfortunately after that ,we failed which mean we need to free connection,but we have invoked ngx_get_connection once which means variables instance have changed once.
just like the following code (success and failed )
void ngx_event_accept(ngx_event_t *ev)
{
...
/* success */
c = ngx_get_connection(s, ev->log);
if (c == NULL) {
if (ngx_close_socket(s) == -1) {
ngx_log_error(NGX_LOG_ALERT, ev->log, ngx_socket_errno,
ngx_close_socket_n " failed");
}
return;
}
c->type = SOCK_STREAM;
...
/* failed */
c->pool = ngx_create_pool(ls->pool_size, ev->log);
if (c->pool == NULL) {
ngx_close_accepted_connection(c);
return;
}
c->sockaddr = ngx_palloc(c->pool, socklen);
if (c->sockaddr == NULL) {
ngx_close_accepted_connection(c);
return;
}
ngx_memcpy(c->sockaddr, sa, socklen);
/* or failed here */
log = ngx_palloc(c->pool, sizeof(ngx_log_t));
if (log == NULL) {
ngx_close_accepted_connection(c);
return;
}
....
}
when we deal with #3,we success, the fd set to 40, and the value of instance is changed again.
Now it's the same with before, so the following Judgment statement in the function named ngx_epoll_process_events will not work,
if (c->fd == -1 || rev->instance != instance) {
/*
* the stale event from a file descriptor
* that was just closed in this iteration
*/
ngx_log_debug1(NGX_LOG_DEBUG_EVENT, cycle->log, 0,
"epoll: stale event %p", c);
continue;
}
I know it can post event which put in in queue not handle,but if i don't use ngx_use_accept_mutex,which means the flag in function ngx_process_events_and_timers doesn't include NGX_POST_EVENTS,
in this case,it will not post event but handle event immediately and it's wrong because #40 is stale event.