[Libevent-users] Re: libevent 2.1.11-stable breaks the links browser

[Mikulas Patocka <mikulas@xxxxxxxxxxxxx>]

On Tue, 3 Sep 2019, Azat Khuzhin wrote:

> > OK, so I will just add a code that blacklists version 2.1.11 and assume 
> > that all the following versions will be ok.

I did some more testing and it turns out that libevent 1.4.15-stable 
shipped with OpenBSD 6.4 using the kqueue backend also doesn't handle 
event_reinit from callback. event_reinit succeeds, but it reports a bad 
file descriptor error when we return to the event loop.

So I modified the links browser to call event_reinit out of the callbacks 
- that works on OpenBSD.

> FYI: upstream commit is ddacaef59ab6808a0801007d0a681f2415af4871
> 
> > BTW - I'd like to ask - is it OK to call event_base_free from the libevent 
> > callback after fork? (assuming that we never return back to the callback)
> > 
> > I want to avoid leaking file descriptors to external programs started by 
> > links
> 
> Hm, what about O_CLOEXEC?

You are right - libevent uses O_CLOEXEC, so the file descriptor doesn't 
leak into subprocesses.

> > - so in the forked child I call event_reinit(base) and 
> > event_base_free(base) just to clean the open file descriptors. Is it OK or 
> > is it unsupported?
> 
> Hm, AFAIR you have analog of daemon(3), so in this case parent is gone
> and child process doing all the job, then why do you call
> event_base_free() just after event_reinit(), what did I miss?
> 
> But anyway you cannot call event_base_free() from the loop, but if you
> call _exit() just after event_base_free(), than who cares?
> 
> -- 
>   Azat.

There are two uses for fork() in the links browser - one is to "daemonize" 
itself when the user closes the master session and there are other 
sessions attached to the AF_UNIX socket.

Another use of fork() is for dns lookups and spawning external viewers.

For the first use, event_reinit is needed (but the daemonizing could be 
deferred until we are out of the event callbacks).

For the second use, it never returns back to the event loop. I used 
event_reinit(), event_base_free() in the child just after fork() - but it 
is not really required, so I deleted it.

Mikulas
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