[Libevent-users] Re: libevent-2.0.8-rc/evdns.c:2672: Assertion (req)->handle && (req)->handle->current_req == (req) failed in evdns_cancel_request

[Denis Bilenko <denis.bilenko@xxxxxxxxx>]

So what happens here, is that eventually reply_handle is called, which calls

reply_schedule_callback(req, ttl, 0, reply); in evdns.c:872
and then
request_finished(req, &REQ_HEAD(req->base, req->trans_id), 1); in evdns.c:876
which calls mm_free(req->handle)

When I call cancel I pass this freed handle to evdns_cancel_request
and that causes crash.

So the problem here is that the evdns_request structure is freed
before the user's callback is called and there's no way for the client
code to know when it's OK to cancel the request.

I might be missing something, but why go through
reply_schedule_callback at all? Why not call the user's callback
immediately and avoid extra step which makes cancel() unsafe?


On Wed, Nov 3, 2010 at 12:47 PM, Denis Bilenko <denis.bilenko@xxxxxxxxx> wrote:
> I've been working on making gevent use libevent's getaddrinfo and
> occasionally get the above message in one of the test cases. What
> could trigger it? How can I avoid it?
>
> I don't call evdns_getaddrinfo_cancel if the callback was already
> executed, so I wonder how could I arrive at invalid request?
>
> libevent version: 2.0.8-rc
>
> A bit more context:
>
> #0  0x002cc422 in __kernel_vsyscall ()
> No symbol table info available.
> #1  0x0013a651 in raise () from /lib/tls/i686/cmov/libc.so.6
> No symbol table info available.
> #2  0x0013da82 in abort () from /lib/tls/i686/cmov/libc.so.6
> No symbol table info available.
> #3  0x00329385 in event_exit (errcode=-559030611) at
> /home/denis/src/libevent-all/libevent-2.0.8-rc/log.c:79
> No locals.
> #4  0x003293b7 in event_errx (eval=-559030611, fmt=0x3402f4 "%s:%d:
> Assertion %s failed in %s") at
> /home/denis/src/libevent-all/libevent-2.0.8-rc/log.c:136
> No locals.
> #5  0x00334468 in evdns_cancel_request (base=0x8cac928,
> handle=0x8cb9530) at
> /home/denis/src/libevent-all/libevent-2.0.8-rc/evdns.c:2672
>        req = 0x267410
>        __func__ = "evdns_cancel_request"
> #6  0x003344a6 in evdns_getaddrinfo_cancel (data=0x8d0e588) at
> /home/denis/src/libevent-all/libevent-2.0.8-rc/evdns.c:4465
> No locals.
> #7  0x002e7773 in __pyx_pf_6gevent_4core_19getaddrinfo_request__cancel
> (__pyx_v_self=<gevent.core.getaddrinfo_request at remote 0x8c74aec>,
> unused=0x0)
>    at gevent/core.c:5227
>
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