[CFR] reflect resolv.conf update to running application

[Brooks Davis]

On Sun, Aug 21, 2005 at 07:17:06PM +0200, Poul-Henning Kamp wrote:
> In message <20050821115454.55441a64 at Magellan.Leidinger.net>, Alexander Leidinger writes:
> >On Sun, 21 Aug 2005 00:37:56 +0100 (BST)
> >Robert Watson <rwatson at freebsd.org> wrote:
> >
> >> (2) By reading the configuration file more frequently and more quickly
> >>      after a change, we increase the chances of a race condition in which
> >>      the resolve reads a partially written resolv.conf file during an
> >>      update.  Does this happen in practice?  I've always been very leery of
> >>      re-reading configuration files automatically based on a time-stamp, as
> >>      updates to files are not atomic at all.
> >
> >Can kqueue be used instead of polling?
> 
> Programs writing resolv.conf should just this the right way:
> 
> 	1. Write new contents to temorary file.
> 	2. Rename temporary file to resolv.conf.

The one issue with this is that we sortof support a read-only /etc with
resolv.conf as a symlink to somewhere else.  Short of following the
symlink by hand in dhclient-script, you have to do the current cat
trick.  It should cause the file to be replaced in one write() though so
I don't think the race exists (unless someone comes up with a dhclient
config that generates a resolv.conf larger then 512-bytes).

-- Brooks

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