[PATCH] Check stale write events in ngx_epoll_module

[Maxim Dounin]

Hello!

On Mon, Nov 07, 2011 at 07:48:29PM +0800, agentzh wrote:

> Hello!
> 
> We've run into an issue with stale write event in ngx_epoll_module.
> Here's a scenario for the issue:
> 
> 1. epoll_wait returns a read event and a write event for a connection,
> 2. ngx_epoll_process_events checks if the read event is stale and sees
> it is not, then invoke the read event handler.
> 3. the read event handler closes the current connection and makes the
> write event not yet processed stale,
> 4. ngx_epoll_process_events blindly continues to run the write event
> handler even if the write event is already stale.
> 
> I've already attached a patch for the nginx 1.0.9 core to fix this issue.
> 
> Thanks!
> -agentzh
> 
> --- nginx-1.0.9/src/event/modules/ngx_epoll_module.c	2011-09-30
> 22:12:53.000000000 +0800
> +++ nginx-1.0.9-patched/src/event/modules/ngx_epoll_module.c	2011-11-07
> 18:07:04.764111952 +0800
> @@ -681,6 +681,18 @@
> 
>          wev = c->write;
> 
> +        if (c->fd == -1 || wev->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;
> +        }
> +
>          if ((revents & EPOLLOUT) && wev->active) {
> 
>              if (flags & NGX_POST_THREAD_EVENTS) {

Patch looks fine for me.  Could you please provide some details 
about user-visible effects observed if the problem occurs?

Maxim Dounin