pgsql: Allow parallel workers to cope with a newly-created session user

Allow parallel workers to cope with a newly-created session user ID.

Parallel workers failed after a sequence like
BEGIN;
CREATE USER foo;
SET SESSION AUTHORIZATION foo;
because check_session_authorization could not see the uncommitted
pg_authid row for "foo". This is because we ran RestoreGUCState()
in a separate transaction using an ordinary just-created snapshot.
The same disease afflicts any other GUC that requires catalog lookups
and isn't forgiving about the lookups failing.

To fix, postpone RestoreGUCState() into the worker's main transaction
after we've set up a snapshot duplicating the leader's. This affects
check_transaction_isolation and check_transaction_deferrable, which
think they should only run during transaction start. Make them
act like check_transaction_read_only, which already knows it should
silently accept the value when InitializingParallelWorker.

This un-reverts commit f5f30c22e. The original plan was to back-patch
that, but the fact that 0ae5b763e proved to be a pre-requisite shows
that the subtle API change for GUC hooks might actually break some of
them. The problem we're trying to fix seems not worth taking such a
risk for in stable branches.

Per bug #18545 from Andrey Rachitskiy.

Discussion: https://postgr.es/m/18545-feba138862f19aaa@postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6e086fa2e771ad4c0b2d3cca4e2de209a92c719b

Modified Files
--------------
src/backend/access/transam/parallel.c | 12 ++++++++----
src/backend/commands/variable.c | 10 ++++++++--
src/test/regress/expected/select_parallel.out | 18 ++++++++++++++++++
src/test/regress/sql/select_parallel.sql | 9 +++++++++
4 files changed, 43 insertions(+), 6 deletions(-)