Fix crashes on plans with multiple Gather (Merge) nodes.

es_query_dsa turns out to be broken by design, because it supposes
that there is only one DSA for the whole query, whereas there is
actually one per Gather (Merge) node.  For now, work around that
problem by setting and clearing the pointer around the sections of
code that might need it.  It's probably a better idea to get rid of
es_query_dsa altogether in favor of having each node keep track
individually of which DSA is relevant, but that seems like more than
we would want to back-patch.

Thomas Munro, reviewed and tested by Andreas Seltenreich, Amit
Kapila, and by me.

Discussion: http://postgr.es/m/CAEepm=1U6as=brnVvMNixEV2tpi8NuyQoTmO8Qef0-VV+=7MDA@mail.gmail.com

Author: robertmhaas

src/backend/executor/execParallel.c

@@ -543,12 +543,6 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 										pcxt->seg);
 	}
 
-	/*
-	 * Make the area available to executor nodes running in the leader.  See
-	 * also ParallelQueryMain which makes it available to workers.
-	 */
-	estate->es_query_dsa = pei->area;
-
 	/*
 	 * Give parallel-aware nodes a chance to initialize their shared data.
 	 * This also initializes the elements of instrumentation->ps_instrument,
@@ -557,7 +551,11 @@ ExecInitParallelPlan(PlanState *planstate, EState *estate, int nworkers)
 	d.pcxt = pcxt;
 	d.instrumentation = instrumentation;
 	d.nnodes = 0;
+
+	/* Install our DSA area while initializing the plan. */
+	estate->es_query_dsa = pei->area;
 	ExecParallelInitializeDSM(planstate, &d);
+	estate->es_query_dsa = NULL;
 
 	/*
 	 * Make sure that the world hasn't shifted under our feet.  This could
@@ -609,6 +607,8 @@ void
 ExecParallelReinitialize(PlanState *planstate,
 						 ParallelExecutorInfo *pei)
 {
+	EState	   *estate = planstate->state;
+
 	/* Old workers must already be shut down */
 	Assert(pei->finished);
 
@@ -618,7 +618,9 @@ ExecParallelReinitialize(PlanState *planstate,
 	pei->finished = false;
 
 	/* Traverse plan tree and let each child node reset associated state. */
+	estate->es_query_dsa = pei->area;
 	ExecParallelReInitializeDSM(planstate, pei->pcxt);
+	estate->es_query_dsa = NULL;
 }
 
 /*

src/backend/executor/nodeGather.c

@@ -278,7 +278,13 @@ gather_getnext(GatherState *gatherstate)
 
 		if (gatherstate->need_to_scan_locally)
 		{
+			EState *estate = gatherstate->ps.state;
+
+			/* Install our DSA area while executing the plan. */
+			estate->es_query_dsa =
+				gatherstate->pei ? gatherstate->pei->area : NULL;
 			outerTupleSlot = ExecProcNode(outerPlan);
+			estate->es_query_dsa = NULL;
 
 			if (!TupIsNull(outerTupleSlot))
 				return outerTupleSlot;

src/backend/executor/nodeGatherMerge.c

@@ -627,8 +627,12 @@ gather_merge_readnext(GatherMergeState *gm_state, int reader, bool nowait)
 		{
 			PlanState  *outerPlan = outerPlanState(gm_state);
 			TupleTableSlot *outerTupleSlot;
+			EState *estate = gm_state->ps.state;
 
+			/* Install our DSA area while executing the plan. */
+			estate->es_query_dsa = gm_state->pei ? gm_state->pei->area : NULL;
 			outerTupleSlot = ExecProcNode(outerPlan);
+			estate->es_query_dsa = NULL;
 
 			if (!TupIsNull(outerTupleSlot))
 			{