pgsql: Avoid potential deadlock in InitCatCachePhase2().

Avoid potential deadlock in InitCatCachePhase2().

Opening a catcache's index could require reading from that cache's own
catalog, which of course would acquire AccessShareLock on the catalog.
So the original coding here risks locking index before heap, which could
deadlock against another backend trying to get exclusive locks in the
normal order.  Because InitCatCachePhase2 is only called when a backend
has to start up without a relcache init file, the deadlock was seldom seen
in the field.  (And by the same token, there's no need to worry about any
performance disadvantage; so not much point in trying to distinguish
exactly which catalogs have the risk.)

Bug report, diagnosis, and patch by Nikhil Sontakke.  Additional commentary
by me.  Back-patch to all supported branches.

Branch
------
REL8_2_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/e2d64f81a6a6502799e0cf7b7b8aa4b764e657f9

Modified Files
--------------
src/backend/utils/cache/catcache.c |    9 +++++++++
src/backend/utils/cache/relcache.c |    6 ++++++
2 files changed, 15 insertions(+), 0 deletions(-)