afexists()

[Hiroki Sato]

Doug Barton <dougb at FreeBSD.org> wrote
  in <4DE588B4.7090908 at FreeBSD.org>:

do> >> Attached is a patch which caches a positive result for support for a
do> >> given address family. I don't think caching negative results is a good
do> >> idea since that could change as the boot progresses.
do> >
do> > Not yet for inet or inet6 (or ipx I think) but atm might be loadable.
do> > Looking ahead that's certainly true though maybe also considering
do> > virtualization maybe.
do>
do> I think it's generally safer not to cache the negative answer, and
do> from what you're saying it sounds like it may add some future-proofing
do> as well. And yes, I did also have VMs in mind, since I'm doing a lot
do> of work in that area atm.

 Caching the results looks good to me, but I think it is rather safer
 to keep the results unchanged while a script is running regardless of
 the value.  This is because the rc.d scripts do not assume afexists()
 returns different values in a run (at least at this moment) , and
 writing a script to support such a dynamically-changed condition
 would be quite difficult.

do> >> I plan to commit this on Friday if there are no objections.
do> >
do> > I am not sure it helps but I see no regression, so if you want, feel
do> > free to go ahead.
do>
do> If you can assume that each call to the sysctl takes 100 ms (which is
do> a WAG for sake of argument), then saving 25 of them will result in us
do> booting 2.5 seconds faster. I'd ultimately like to cut the
do> rc.d-related portion of the boot in half, if not more, so every little
do> bit helps.

 I think it would be great if it is possible to create a wrapper
 function for testing and caching.  I expects testing kern.features.*
 is likely added again to somewhere in rc.d scripts, and adding a long
 lines of "[ -n ... ] && return 0; if sysctl...; fi" each time
 degrades readability.

-- Hiroki
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