Re: [Libevent-users] Signals and priority queues

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Tue, Jan 3, 2012 at 6:45 PM, Ralph Castain <rhc@xxxxxxxxxxxx> wrote:
> Hi folks
>
> I'm trying to use libevent 2.0.13 with priorities, and am having a problem when signal events are defined. Basically, this is what I do:
>
> 1. create an event base
>
> 2. call event_base_priority_init(base, 8)
>
> 3. event_assign(&event, base, SIGTERM, EV_SIGNAL|EV_PERSIST, cbfunc, &event)
>
> 4. event_priority_set(&event, 1)
>
> 5. event_add(&event, NULL)
>
> This is accepted just fine. However, when I subsequently call event_base_free(base), libevent crashes:
>
> 729                     struct event *next = TAILQ_NEXT(ev, ev_next);
> (gdb) where
> #0  0x0000000100157be2 in event_base_free (base=0x1003046f0) at event.c:729
>
>
> Any ideas on what might be happening?

Not really.  I just tried to do  what you said you were doing above,
and I can't reproduce the same problem with 2.0.16-stable.  I tried it
again with Libevent 2.0.13-stable, and got the same result: worked
fine.  I'm attaching my test code so you can see if you get different
results.

> My program works fine (all other events cause no problem) so long as I remove these signal events. Do signal events not work with priority queues, at least in 2.0.13?

(First, a terminology aside: "priority queue" isn't what implements
priorities in libevent.  A priority queue is just a kind of abstract
data structure that we use to implement timeouts for events.)

But yeah, signals are supposed to work with event priorities, and in
my (attached) test, it seems they do.  Something else might be going
on here.

To try to debug this more, here are some ideas:
  * Try to see if your code works when you turn on Libevent's debugging mode.
  * See if valgrind (or a similar tool) finds any memory corruption issues
  * Try to tweak my (attached) test so that it reproduces the crash
you're seeing.

hoping this helps,
-- 
Nick

#include <event2/event.h>
#include <event2/event_struct.h>

#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

static void
cbfunc(evutil_socket_t fd, short what, void *arg)
{
	event_base_loopbreak(arg);
}

static void
die(const char *msg)
{
	fprintf(stderr, "%s\n", msg);
	exit(1);
}

int
main(int argc, char **argv)
{

	struct event_base *base;
	struct event ev;

	printf("Libevent %s\n", event_get_version());

	if (!(base = event_base_new()))
		die("event_base_new");
	if (event_base_priority_init(base, 8) < 0)
		die("event_base_priority_init");
	if (event_assign(&ev, base, SIGTERM, EV_SIGNAL|EV_PERSIST, cbfunc, base)<0)
		die("event_assign");
	if (event_priority_set(&ev, 1) < 0)
		die("event_priority_set");
	if (event_add(&ev, NULL) < 0)
		die("event_add");

	event_base_loop(base, 0);

	event_base_free(base);
	return 0;
}