pgsql: Fix problems with the "role" GUC and parallel query.

Fix problems with the "role" GUC and parallel query.

Without this fix, dropping a role can sometimes result in parallel
query failures in sessions that have used "SET ROLE" to assume the
dropped role, even if that setting isn't active any more.

Report by Pavan Deolasee. Patch by Amit Kapila, reviewed by me.

Discussion: http://postgr.es/m/CABOikdOomRcZsLsLK+Z+qENM1zxyaWnAvFh3MJZzZnnKiF+REg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/846fcc85167c417873865099d70068ed85f758a8

Modified Files
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src/backend/access/transam/parallel.c | 11 +++++++++++
src/backend/utils/misc/guc.c | 25 +++++++++----------------
src/include/utils/guc.h | 1 +
src/test/regress/expected/select_parallel.out | 16 ++++++++++++++++
src/test/regress/sql/select_parallel.sql | 11 +++++++++++
5 files changed, 48 insertions(+), 16 deletions(-)