evdns_cancel_request may not fully cancel

[Erik Fears]

Hello,

I've run into an issue in my codebase where it appears evdns is calling
a callback on a request that I've canceled.

This is an issue for me, because once I call evdns_cancel_request, I've
also freed my own context associated with the request (i.e. the
void* passed back by libevent to the callback).

If the result was always DNS_ERR_CANCEL on a cancelled request,
this wouldn't be a problem, because I could abort. It seems that in some
cases the result could be DNS_ERR_NONE if the callback was
already pending


I believe this code may be to blame?

void
evdns_cancel_request(struct evdns_base *base, struct evdns_request *handle)
{
...

EVDNS_LOCK(base);
if (handle->pending_cb) {
EVDNS_UNLOCK(base);
return;
}

...
}

Is this expected?

Thanks!
Erik

[Nick Mathewson]

Post by Erik Fears
Hello,
I've run into an issue in my codebase where it appears evdns is calling
a callback on a request that I've canceled.
This is an issue for me, because once I call evdns_cancel_request, I've
also freed my own context associated with the request (i.e. the
void* passed back by libevent to the callback).
If the result was always DNS_ERR_CANCEL on a cancelled request,
this wouldn't be a problem, because I could abort. It seems that in some
cases the result could be DNS_ERR_NONE if the callback was
already pending
I believe this code may be to blame?
void
evdns_cancel_request(struct evdns_base *base, struct evdns_request *handle)
{
...
EVDNS_LOCK(base);
if (handle->pending_cb) {
EVDNS_UNLOCK(base);
return;
}
...
}

Hm. By the time that pending_cb is set, the deferred_cb for the evdns
request is already scheduled and ready to run. Maybe
evdns_cancel_request needs to cancel the deferred_cb object if
possible by invoking evend_deferred_cb_cancel on it.. (Even so, there
is going to be an issue for multithreaded programs, where the callback
might be running at the same time you're trying to cancel it.)

Possibly evdns_cancel_request needs to return a value to indicate
whether cancelling the request succeeded.