Re: [Libevent-users] Event callback from evbuffer_add_printf

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Wed, Apr 3, 2013 at 4:26 AM, Sebastian Sjöberg
<sebastian.sjoberg@xxxxxxxx> wrote:
> Hello,
>
> Is it possible that evbuffer_add_printf triggers an event callback on a bufferevent before returning?

Yes.  When deferred callbacks aren't enabled on an event, then it's
possible that when you queue data, that data can be flushed to the
backend immediately, particularly with filtering bufferevents.  If
that data is written right after you queue it, then a write callback
can happen immediately.

This is not something I regard as good behavior on Libevent's part; see below.

> We've had a system based on libevent running for years and suddenly tuesday morning it started crashing. Gathering information from core files and logs we've concluded that the event callback on the SSL bufferevents is called when we call evbuffer_add_printf.
>
> After changing to deferred callbacks on the bufferevents the issue is gone. The biggest question we're asking ourselves is, why did the callbacks suddenly fire now after years of not firing?

That's a hard one.  If it's truly sudden change in behavior with no
change in libevent or your codebase or your OpenSSL version, I'd look
for changes on the other side, or changes in network behavior, or
something.




Incidentally, I would like to use Libevent 2.1's new smarter callback
logic to make it so Libevent 2.1 (or 2.2, or whenever) has a better
notion of when to invoke callbacks like that.  Instead of having a
choice between never delaying the callback (the default), or always
delaying the callback (as happens with the deferred flag), I'd like to
have a design where the callback is deferred if it would give
surprising results to user code, but can get called immediately if
it's invoked from within libevent callback.  That way we avoid
performance penalties *and* surprising results.

yrs,
-- 
Nick
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