Re: Restore enforce_generic_type_consistency's breaks a farms

I wrote:
> ... But this type of problem has come
> up before.  I wonder if we shouldn't do what was previously discussed:
> make has_table_privilege and related functions silently return FALSE,
> instead of throwing error, when given a "nonexistent" OID.

On checking the archives, it seems most of the previous complaints have
been about "cache lookup failed" errors arising from pg_table_is_visible
and its ilk; those are used in the queries generated by psql's \d
commands.

So my proposal is to change the OID-accepting variants of
has_table_privilege and friends, as well as pg_table_is_visible and
friends, to silently return FALSE instead of failing when given a
bad OID.  I had originally thought we might change the underlying
functions (pg_class_aclcheck, RelationIsVisible, etc) but it looks
like it'd be better to expend a SearchSysCacheExists call in the
SQL-accessible functions.  (This doesn't leave a race condition
since there'd not be any check for syscache invalidation messages
between the SearchSysCacheExists and the ensuing lookup.)
        regards, tom lane