pgsql: Fix mistaken failure to allow parallelism in corner case.

Fix mistaken failure to allow parallelism in corner case.

If we try to run a parallel plan in serial mode because, for example,
it's going to be scanned via a cursor, but for some reason we're
already in parallel mode (for example because an outer query is
running in parallel), we'd incorrectly try to launch workers.
Fix by adding a flag to the EState, so that we can be certain that
ExecutePlan() and ExecGather()/ExecGatherMerge() will have the same
idea about whether we are executing serially or in parallel.

Report and fix by Amit Kapila with help from Kuntal Ghosh. A few
tweaks by me.

Discussion: http://postgr.es/m/CAA4eK1+_BuZrmVCeua5Eqnm4Co9DAXdM5HPAOE2J19ePbR912Q@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/639c1a6bb9ee08fe4757a6fab1ddbd01291515e1

Modified Files
--------------
src/backend/executor/execMain.c | 1 +
src/backend/executor/execUtils.c | 2 ++
src/backend/executor/nodeGather.c | 2 +-
src/backend/executor/nodeGatherMerge.c | 2 +-
src/include/nodes/execnodes.h | 2 ++
5 files changed, 7 insertions(+), 2 deletions(-)