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.wartak@enterprisedb.com>
Co-authored-by: Jeevan Chalke <jeevan.chalke@enterprisedb.com>
Backpatch-through: 14
Discussion: http://postgr.es/m/CAKZiRmwfVripa3FGo06=5D1EddpsLu9JY2iJOTgbsxUQ339ogQ@mail.gmail.com

src/backend/access/transam/parallel.c

@@ -503,8 +503,12 @@ InitializeParallelDSM(ParallelContext *pcxt)
 void
 ReinitializeParallelDSM(ParallelContext *pcxt)
 {
+	MemoryContext oldcontext;
 	FixedParallelState *fps;
 
+	/* We might be running in a very short-lived memory context. */
+	oldcontext = MemoryContextSwitchTo(TopTransactionContext);
+
 	/* Wait for any old workers to exit. */
 	if (pcxt->nworkers_launched > 0)
 	{
@@ -542,6 +546,9 @@ ReinitializeParallelDSM(ParallelContext *pcxt)
 			pcxt->worker[i].error_mqh = shm_mq_attach(mq, pcxt->seg, NULL);
 		}
 	}
+
+	/* Restore previous memory context. */
+	MemoryContextSwitchTo(oldcontext);
 }
 
 /*