Re: [Libevent-users] random crashed by [err] event_queue_remove:not on queue 8

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Sun, Feb 17, 2013 at 10:09 PM, Tony Qin <anqin.qin@xxxxxxxxx> wrote:
> Hi all,
>
> I uses the libevent-2.0.19-stable in my rpc server to send data from
> client-end to server-end. However, my server is always "exist" by
> libevent (by event_errx()).
>
> Checked log and fond the follow message:
>
> event_queue_remove: 0x7ea588(fd 17) not on queue 8
>
> So, after add abort() in event_exit() and dump the core, the core
> stack is as following:

Hm.  This means that while we were processing active events (in
event_process_active_single_queue), when we went to remove one from
the queue of active events (EVLIST_ACTIVE, queue 8), it didn't have
the right flag set.

This really shoudldn't be happening unless there's an internal error
in the code somewhere: in your code, or in Libevent's code.  For one
thing, 2.0.19-stable is a little old, but none of the fixes in
2.0.20-stable or 2.0.21-stable look like they'd fix a bug that could
cause this.

One thing that can cause bogus behavior like this is if an event is
passed to event_set() or event_assign(), or if you edit the event's
internals, without first passing the event to event_del().  Some ways
you could try to diagnose that to find out if it's happening:

 * Make sure you built libevent with debugging support, and then use
event_enable_debug_mode() to get extra run-time debugging checks.
 * Try using valgrind or a similar memory access checking tool to see
if there could be any premature free() issues.
 * Try running with debug-level logs from libevent to track the
life-history of the offending event object, and see whether anything
strange happens to it before it crashes your program.

Best of luck,
-- 
Nick
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