Re: open-{gnutls,network}-stream backwards compatibility

[Eli Zaretskii <eliz@gnu.org>]

> From: Robert Pluim <rpluim@gmail.com>
> Cc: emacs-devel@gnu.org
> Date: Wed, 02 Jan 2019 19:56:25 +0100
> 
> >   change open-network-stream such that not specifying
> >   :client-certificate is the same as specifying t
> >
> > Doesn't this mean an incompatible change in behavior?
> 
> For what I think is a tiny (perhaps non-existent) minority of Emacs
> users.

We have no reliable way of estimating the number of affected users.
Besides, even if there's only a single user affected, it requires us
to consider this an incompatible change, which should be avoided if
possible.

> Currently, specifying ':client-certificate t' to open-network-stream
> causes client certificates to be looked up via auth-source up when
> using external gnutls-cli.
> 
> My fix intends to extend that behaviour to do the same when using
> built-in GnuTLS. So far I think thatʼs uncontroversial.

Yes.

> My follow-on to that fix would be to assume that not specifying
> :client-certificate at all when calling open-network-stream is the
> same as specifying ':client certificate t'. Only people with existing
> client certificate entries in their auth-source provider (such as
> .authinfo) would see a change in behaviour.

It's still a change.  At the very least, we should provide a way to
have the old behavior back.

> If that last change is unacceptable, Iʼd want to change all the
> callers of open-network-stream inside Emacs to specify
> ':client-certificate t'. People using built-in Emacs packages would
> then be able to use client certificates by changing configuration data
> only. People using packages developed outside Emacs would need to
> update those external packages to versions which specify
> ':client-certificate t', which is what Iʼd like to avoid.
> 
> Short version: we assume a username/password entry in auth-source for
> a specific server means 'use this username/password', so we should do
> the same for a client-certificate specification.

I don't necessarily disagree, provided that we give users who
want/need that a way to get back the old behavior.  Maybe I'm
misunderstanding, but it sounds like both of your alternatives are
backward-incompatible, so they both need such a "fire escape".

Thanks.