[PATCH] Runtime control of CLOBBER_CACHE_ALWAYS

Hi all

While working on extensions I've often wanted to enable cache clobbering
for a targeted piece of code, without paying the price of running it for
all backends during postgres startup and any initial setup tasks that are
required.

So here's a patch that, when CLOBBER_CACHE_ALWAYS or
CLOBBER_CACHE_RECURSIVE are defined, adds a GUC named clobber_cache_depth .
It defaults to 1 for CLOBBER_CACHE_ALWAYS or 3 for CLOBBER_CACHE_RECURSIVE
to match the existing compiled-in behaviour. But with this change it's now
possible to run Pg with clobber_cache_depth=0 then set it to 1 only for
targeted tests.

clobber_cache_depth is treated as an unknown GUC if Pg was not built with
CLOBBER_CACHE_ALWAYS or CLOBBER_CACHE_RECURSIVE defined.

-----

On a side note, to make things like this easier to use, I personally patch
all pg_regress tests to include the following at the start of each sql
input file:

\set ECHO none
-- Put per-test settings or overrides here
\set ECHO queries

then patch the expected files accordingly. That way it's easy for me to
make per-test adjustments while still running the whole suite. It's not
always practical to run just one targeted test with TESTS=foo.

-- 
 Craig Ringer                   http://www.2ndQuadrant.com/
 2ndQuadrant - PostgreSQL Solutions for the Enterprise