[RFC] listen on errors but not read about libevent

[nmathewson]

I don't think that's possible with the way that most underlying socket APIs work: the functions that would notify you of errors are the same ones that try to do reading and writing.

If you know of some portable way to detect errors without doing reads/writes, then we can try to figure something out.

[davidsteinsland]

So basically I have no choice other than reading directly from the buffers? How will that work if the socket in question is SSL?

[JohnOhl]

Libevent can handle all the heavy lifting /w managing SSL based sockets; see http://www.wangafu.net/~nickm/libevent-book/Ref6a_advanced_bufferevents.html for a few OpenSSL examples. After setting up the bufferevent you'd simply write to the output buffer just as you would for a normal bufferevent socket.

[errzey]

have my own layer around sockets

Is it a layer around libevent?

[errzey]

Oh crap, I forgot I already researched this topic a long time ago.

Potential new flags for turning off read but still get errors.

epoll:

 EPOLLRDHUP works without EPOLLIN

kqueue:

EV_SET(&event, infd, EVFILT_READ, EV_ADD|EV_EOF, NOTE_LOWAT, INTMAX_MAX, NULL);

That looks like a hack, but it waits for INTMAX_MAX bytes before triggering the event, but still triggers EV_EOF

Would be interesting to look into a (kqueue and epoll) specific flag like "EV_READ_ERROR".

bufferevent_disable(EV_READ);
bufferevent_enable(EV_READ_ERROR);

[azat]

Potential new flags for turning off read but still get errors.

Some backends will not support this, and there will be all why-this-does-not-work questions, though we can have workaround for others, but does it worth it?

[errzey]

Some backends will not support this

Like when I said

a (kqueue and epoll) specific flag

Also whats this I hear all about `the various flags specific to an event backend?

There will be all why-this-does-not-work questions,

We always get those; Even for things that do work - just document it and refer them to the libevent-book link.

but does it worth it?

No workarounds for backends that do not support it. Period.

The ability to have read disabled and still get socket errors would help every proxy developer in the world (53242423423.32432E) to be exact.

Joking aside, yes, this is definitely worth it (I've wanted this for a long time, never knew how to do it, then I realized I did, while browsing my documents folder).

[errzey]

Replacing and closing this ticket with #345