Re: Ncat/Nsock notification of connection reset

[Henri Doreau <henri.doreau () gmail com>]

2017-03-18 5:41 GMT+01:00 Daniel Miller <bonsaiviking () gmail com>:
> I really think there ought to be some way to catch the fact that the server
> has killed the connection. The first step would be making sure Nsock notices
> this. With the default select engine [2], I don't see any indication of the
> event with strace. But with the epoll engine, I think I see something that
> looks like this:
>
> epoll_wait(3, {{EPOLLERR|EPOLLHUP, {u32=13173952, u64=13173952}}}, 128, -1)
> = 1
>
> This is not present in other situations such as the first line sent after
> the server is killed. So maybe we could do something with that?
>
> The second step would be making Ncat handle the situation properly. Instead
> of trying to read from STDIN, we could just exit, since the other side is
> fully shut down.
>
> Thoughts?
> Dan

Hi Dan,

this bug has been there forever, but I don't believe anyone relies on
that behavior in a way or another[1], so let's fix it.

I can see three things:

* First, nsock internal handling of EV_EXCEPT is half broken (and it's
my fault) . It should be an output flag (ie. automatically watched and
returned) and nsock_core should check it systematically.

* Second, about ncat using select by default. It can indeed work with
others but select is the only nsock engine that works for stdin on
windows - thanks to our fselect wrapper in nbase - and also if stdin
is a file on unix like (ncat localhost < somefile) which are the two
tricky cases IIRC. I wouldn't change that.

* Ncat read handler can exit on EOF on the socket but only in
recv-only mode or non-TCP. I don't think we really want that (needs
more thoughts though). Commenting that check out and a dirty-fix of
nsock internals seems to resolve the issue. Ncat client exits as soon
as the RST from the dying server is received.


HTH

[1] (the obligatory xkcd) https://xkcd.com/1172

-- 
Henri
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