Re: [Libevent-users] Clarify new behavior?

[Nick Mathewson <nickm@xxxxxxxxxxxxx>]

On Sat, Oct 23, 2010 at 5:14 AM, Ralph Castain <rhc@xxxxxxxxxxxx> wrote:
> Hi folks
>
> I successfully updated our libevent integration in Open MPI, but have encountered a problem with one use-case that used to work and now doesn't. Before proceeding to devise a fix, I just wanted to confirm that I accurately understand the issue.
>
> The problem arises from this scenario:
>
> 1. we receive a command via a message that we receive in a file descriptor event. We "push" the command message into a timer event (duration zero time) to help break a threading issue, and then return from the file descriptor event.
>
> 2. the event library is called with LOOP_ONCE, causing the timer event to fire.
>
> 3. from within the timer event, the command causes us to execute a procedure that results in us having to wait for another event to occur. We "block" in that position, running a loop that includes a call to progress the event library (i.e., a call to event_loop(LOOP_ONCE)).
>
> This last step used to work just fine, yet now yields the following warning:
>
> warn] event_base_loop: reentrant invocation.  Only one event_base_loop can run on each event_base at once.: Resource temporarily unavailable
>
> and no progression of the event library occurs.
>
> I can see a few ways around this problem, but all involve considerable code rewrite. Before I do so, I just wanted to confirm my understanding that this behavior changed (i.e., recursion is no longer allowed), and see if any alternative approaches to revising such codes are recommended.

Hi, Ralph!  I'll try to answer in more detail later on -- I can't
write too much now, but I wanted to drop you a line before you started
rewriting your code.

I think that the warning you hit may be overzealous.  It was meant to
detect errors that would previously pass silently and produce
hard-to-diagnose crashes, such as running event_base_dispatch on the
same event base from multiple threads at once.  But I think your use
case may either a) work right, or b) be trivial to make work right,
and if so, we should fix Libevent not to warn about it.

I'll look harder at the code here and try to see what's going on here,
but calling the loop from a callback really was working fine with
Libevent 1.4, then I think 2.0 should support it.

Thanks for the heads-up, and thanks to everybody else who's hunting
for regressions and other bugs in 2.0.8-rc.

yrs,
-- 
Nick
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