bug#40343: 28.0.50; dns-set-servers fails on IPv6 available Windows

[Kazuhiro Ito <kzhr@d1.dion.ne.jp>]

>     Kazuhiro> For example, when I tested on my standalone Debian (sid) box,
>     Kazuhiro> evaluating the below code just after starting Emacs returns annoying
>     Kazuhiro> result.
> 
>     Kazuhiro> (progn
>     Kazuhiro>   (require 'dns)
>     Kazuhiro>   (dns-set-servers)
>     Kazuhiro>   dns-servers)
> 
>     -> ("c")
> 
> Yes, but the API exposed by dns.el is 'dns-query' (and maybe
> 'dns-query-cached'). 'dns-set-servers' should probably be called
> 'dns--set-servers' to indicate that itʼs an internal function.
> 
>     Kazuhiro> We need to validate dns-servers's value to judge whether
>     Kazuhiro> dns-set-servers succeeded contorary docstring of dns-set-servers,
>     Kazuhiro> which says dns-servers is non-nil when dns-set-servers succeeded.  I
>     Kazuhiro> think docstring or actual behavior of dns-set-servers should be fixed.
> 
> See previous paragraph: itʼs an internal function.

I don't understand what you mean by "itʼs an internal function".  I
don't think it is okay that internal functions don't behave as
described in docstring.

>     Kazuhiro> I noticed another problem in dns.el.  dns-query function returns
>     Kazuhiro> message's string ("No DNS server configuration found") when
>     Kazuhiro> dns-servers is nil.
> 
> Yes, it should probably return nil, and this is actually a valid
> reason for why 'dns-set-servers' should set dns-servers to nil on
> failure. Can you determine why itʼs failing on your Debian box?

I tested on Debian box as below.

1. /etc/init.d/networking stop
2. remove nameserver entry from /etc/resolv.conf

On such condition, nslookup program outputs as below

> ;; connection timed out; no servers could be reached

Here is code snippet communicating with nslookup in dns-set-servers.

> (with-temp-buffer
>   (call-process "nslookup" nil t nil "localhost")
>   (goto-char (point-min))
>   (re-search-forward
>    "^Address:[ \t]*\\([0-9]+\\.[0-9]+\\.[0-9]+\\.[0-9]+\\)" nil t)
>   (setq dns-servers (list (match-string 1))))

On the above condition, re-search-forward doesn't match anything.  But
next match-string is called unconditionally.  As far as I tested,
match-string's result when last search didn't match is undefined.  it
may return nil, string or raise an error.  Additionally, even if
match-string returns nil, dns-servers is never set to nil.  It is set
to (nil).

-- 
Kazuhiro Ito