RFC: Automatically Reloading /etc/resolv.conf

[Eric van Gyzen]

I would like to change the libc resolver to automatically reload
/etc/resolv.conf when the latter changes.  I would like to hear opinions
about the implementation.  Broadly, I see two approaches.

== "stat" ==

When loading the file, record the mod time.  Before each query, stat()
the file to see if it has changed.

Advantage:  It uses no extra persistently allocated objects.

Disadvantage:  It incurs a stat() on every query.  I don't see this as a
major disadvantage, since the resolver already does a lot of work on
every query.  (For example, it creates and destroys a kqueue and a socket.)

OpenBSD uses this approach.  It also uses clock_gettime(CLOCK_MONOTONIC)
to rate-limit the stat() calls to one per several seconds.

== "kqueue" ==

When loading the file, open a kqueue and register for the appropriate
events.  Before each query, check for kevents.

Advantage:  The per-query overhead is fairly small.

Disadvantage:  This would persistently allocate an open file and a
kqueue for every thread that ever uses the resolver, for the life of the
thread.  This seems fairly expensive.

NetBSD uses this approach.  It mitigates most of the space-cost by using
a shared pool of res_state objects, instead of one per thread [that uses
the resolver].  On each query, a thread allocates/borrows a res_state
from the pool, uses it, and returns it.  So, the number of objects is
only the high water mark of the number of threads _concurrently_ issuing
resolver queries.

There are probably several variations on each theme, of course.  I would
appreciate your thoughts on these approaches and others I missed, as
well as variations and details.

FYI, I'm leaning toward the "stat" approach.

Cheers,

Eric