[CFR] reflect resolv.conf update to running application

[Matthew Reimer]

Robert Watson wrote:

> 
> On Sat, 20 Aug 2005, Hajimu UMEMOTO wrote:
> 
>> Our resolver reads resolv.conf once, and never re-read it.  Recent
>> OpenBSD changed to re-read resolv.conf when it is updated.  I believe it
>> is useful specially for mobile environment.  So, I made a patch for our
>> resolver.  Please review it.
> 
> Two concerns:
> 
> (1) Has anyone characterized the significant of the cost of doing a stat()
>      for every DNS lookup for a significant workload?  Does it matter?
>      Most performance-critical network paths don't do name lookups in
>      order to prevent indefinite stalls in lookup, but doing, say, 1,000
>      additional stats a second is not a small issue.
> 
> (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.

How about just stat'ing resolv.conf if more than X seconds has passed since 
the last stat, rather than for every lookup?

Matt