pgsql: Switch memory contexts in ReinitializeParallelDSM.

Switch memory contexts in ReinitializeParallelDSM.

We already do this in CreateParallelContext, InitializeParallelDSM, and
LaunchParallelWorkers. I suspect the reason why the matching logic was
omitted from ReinitializeParallelDSM is that I failed to realize that
any memory allocation was happening here -- but shm_mq_attach does
allocate, which could result in a shm_mq_handle being allocated in a
shorter-lived context than the ParallelContext which points to it.

That could result in a crash if the shorter-lived context is freed
before the parallel context is destroyed. As far as I am currently
aware, there is no way to reach a crash using only code that is
present in core PostgreSQL, but extensions could potentially trip
over this. Fixing this in the back-branches appears low-risk, so
back-patch to all supported versions.

Author: Jakub Wartak <jakub(dot)wartak(at)enterprisedb(dot)com>
Co-authored-by: Jeevan Chalke <jeevan(dot)chalke(at)enterprisedb(dot)com>
Backpatch-through: 14
Discussion: http://postgr.es/m/CAKZiRmwfVripa3FGo06=5D1EddpsLu9JY2iJOTgbsxUQ339ogQ@mail.gmail.com

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bd2726d3e15fdf8245f63bdbaddf4e3700f86577

Modified Files
--------------
src/backend/access/transam/parallel.c | 7 +++++++
1 file changed, 7 insertions(+)