[PATCH] autocache patch for mock

[Jason L Tibbitts III]

OK, since I do a bunch of builds I just had to try this out.  It took
me a while to get building again after moving from mock 0.4 to 0.5;
I'm still not sure where the buildsys-build package is supposed to
come from, so I just built the spec I found in mock CVS and stuck it
in my local repo.

Basic summary: this saves me 30 seconds per build; the time goes from
2:40 to 2:10 when building a basic package (perl-Expect) which has
only one dependency outside of base.  These timings are stable and run
cache-warm.

I'm building on a dual core quad socket Opteron 880 (2.4GHz) machine
with 16GB of RAM; disk is a plain throwaway 250GB Western Digital
connected via SATA to an onboard SiI 3114 controller.  The machine is
otherwise idle.  It has sufficient memory to cache pretty much
everything involved in the build process.

Things are mostly IO bound on this machine, except for unpacking the
cached buildroot which is completely CPU bound.  I think that
switching to gzip might help.

Of course, building the cache takes some time; bzip is just incredibly
slow.  Time for a build after deleting the cache file is 5:43.  Again,
gzip would probably help; bzip2 is just so slow.

I noticed one issue: tar is traversing proc.  I thought I saw a patch
where it was called with -l (--one-file-system) which would prevent this:

tar: root/proc/acpi/event: Cannot open: Device or resource busy
tar: root/proc/irq/11: file changed as we read it
tar: root/proc/irq: file changed as we read it
tar: root/proc/sys/dev/scsi: file changed as we read it
tar: root/proc/sys/dev: file changed as we read it
tar: root/proc/sys/net/ipv6/neigh/default: file changed as we read it
tar: root/proc/sys/net/ipv6/neigh: file changed as we read it

 - J<