[gnutls-help] Truly non-blocking example of gnutls usage

[Nikos Mavrogiannopoulos]

On Fri, 2016-04-08 at 17:16 +0100, Alex Bligh wrote:
> Nikos,
> 
> Thanks for your reply.
> 
> On 8 Apr 2016, at 14:35, Nikos Mavrogiannopoulos <nmav at gnutls.org> 
> wrote:
> 
> > On Fri, Apr 8, 2016 at 10:36 AM, Alex Bligh <alex at alex.org.uk> 
> > wrote:
> > > Is there a simple, easy to reuse, example of gnu-tls acting like 
> > > a
> > > proxy which is truly non-blocking? By truly non-blocking I mean 
> > > using
> > > non-blocking writes as well as non-blocking reads. The danger I
> > > am concerned about is receiving a large amount of plain-text,
> > > gnutls converting that to cypher-text, attempting to write it
> > > but blocking because the remote side is not ready to receive it.
> > > The remote side is not ready to receive it because it has its
> > > own output blocked as gnutls is not polling for reads as
> > > it's blocked above, meaning deadlock.
> > 
> > Blocking is a matter of the underlying socket functions.
> 
> Perhaps I should have used the word 'asynchronous'
> 
> > If you set
> > the sockets to non blocking mode gnutls operates in a non-blocking 
> > way
> > almost identically to berkeley sockets. Have you checked the 
> > manual?
> > https://www.gnutls.org/manual/html_node/Asynchronous-operation.html
> 
> I had done, but it was not fantastically helpful.
> 
> The manual says "GnuTLS does not keep a write buffer, thus when 
> writing
> no additional actions are required." which I took to mean it was 
> writing
> synchronously (but reading async).

That sentence is indeed confusing. I've removed it.

> 
> > The simplest example is mini-eagain.c from the test suite which
> > verifies the asynchronous operation of gnutls_record_send and recv.
> Thanks - that was helpful. But it doesn't do a select loop
> as far as I can tell.

No. A select loop will be complex and I don't know if one could have a
reasonable example. If you have one consider contributing it.

To see a real world example check ocserv's main loop:
https://gitlab.com/ocserv/ocserv/blob/master/src/worker-vpn.c#L1892

It uses both TLS and DTLS sockets in async mode (with poll, there is no
reason to use select() as it has terrible semantics).

regards,
Nikos