Bugs in CREATE/DROP INDEX CONCURRENTLY

1. These operations think they can use ordinary heap_update operations
to change pg_index entries when they don't have exclusive lock on the
parent table. The lack of ex-lock means that another backend could be
currently loading up its list of index OIDs for the table --- and since
it scans pg_index with SnapshotNow to do that, the heap_update could
result in the other backend failing to see this index *at all*. That's
okay if it causes the other backend to not use the index for scanning...
but not okay if it causes the other backend to fail to make index
entries it is supposed to make.

I think this could possibly be fixed by using nontransactional
update-in-place when we're trying to change indisvalid and/or
indisready, but I've not really thought through the details.

2. DROP INDEX CONCURRENTLY doesn't bother to do
TransferPredicateLocksToHeapRelation until long after it's invalidated
the index. Surely that's no good? Is it even possible to do that
correctly, when we don't have a lock that will prevent new predicate
locks from being taken out meanwhile?

regards, tom lane