Improve division of labor between execParallel.c and nodeGather[Merge].c.

Move the responsibility for creating/destroying TupleQueueReaders into
execParallel.c, to avoid duplicative coding in nodeGather.c and
nodeGatherMerge.c.  Also, instead of having DestroyTupleQueueReader do
shm_mq_detach, do it in the caller (which is now only ExecParallelFinish).
This means execParallel.c does both the attaching and detaching of the
tuple-queue-reader shm_mqs, which seems less weird than the previous
arrangement.

These changes also eliminate a vestigial memory leak (of the pei->tqueue
array).  It's now demonstrable that rescans of Gather or GatherMerge don't
leak memory.

Discussion: https://postgr.es/m/8670.1504192177@sss.pgh.pa.us

src/backend/executor/nodeGather.c

@@ -130,7 +130,6 @@ ExecGather(PlanState *pstate)
 {
 	GatherState *node = castNode(GatherState, pstate);
 	TupleTableSlot *fslot = node->funnel_slot;
-	int			i;
 	TupleTableSlot *slot;
 	ExprContext *econtext;
 
@@ -173,33 +172,30 @@ ExecGather(PlanState *pstate)
 			LaunchParallelWorkers(pcxt);
 			/* We save # workers launched for the benefit of EXPLAIN */
 			node->nworkers_launched = pcxt->nworkers_launched;
-			node->nreaders = 0;
-			node->nextreader = 0;
 
 			/* Set up tuple queue readers to read the results. */
 			if (pcxt->nworkers_launched > 0)
 			{
-				node->reader = palloc(pcxt->nworkers_launched *
-									  sizeof(TupleQueueReader *));
-
-				for (i = 0; i < pcxt->nworkers_launched; ++i)
-				{
-					shm_mq_set_handle(node->pei->tqueue[i],
-									  pcxt->worker[i].bgwhandle);
-					node->reader[node->nreaders++] =
-						CreateTupleQueueReader(node->pei->tqueue[i],
-											   fslot->tts_tupleDescriptor);
-				}
+				ExecParallelCreateReaders(node->pei,
+										  fslot->tts_tupleDescriptor);
+				/* Make a working array showing the active readers */
+				node->nreaders = pcxt->nworkers_launched;
+				node->reader = (TupleQueueReader **)
+					palloc(node->nreaders * sizeof(TupleQueueReader *));
+				memcpy(node->reader, node->pei->reader,
+					   node->nreaders * sizeof(TupleQueueReader *));
 			}
 			else
 			{
 				/* No workers?	Then never mind. */
-				ExecShutdownGatherWorkers(node);
+				node->nreaders = 0;
+				node->reader = NULL;
 			}
+			node->nextreader = 0;
 		}
 
 		/* Run plan locally if no workers or not single-copy. */
-		node->need_to_scan_locally = (node->reader == NULL)
+		node->need_to_scan_locally = (node->nreaders == 0)
 			|| !gather->single_copy;
 		node->initialized = true;
 	}
@@ -258,11 +254,11 @@ gather_getnext(GatherState *gatherstate)
 	MemoryContext tupleContext = gatherstate->ps.ps_ExprContext->ecxt_per_tuple_memory;
 	HeapTuple	tup;
 
-	while (gatherstate->reader != NULL || gatherstate->need_to_scan_locally)
+	while (gatherstate->nreaders > 0 || gatherstate->need_to_scan_locally)
 	{
 		CHECK_FOR_INTERRUPTS();
 
-		if (gatherstate->reader != NULL)
+		if (gatherstate->nreaders > 0)
 		{
 			MemoryContext oldContext;
 
@@ -319,19 +315,15 @@ gather_readnext(GatherState *gatherstate)
 		tup = TupleQueueReaderNext(reader, true, &readerdone);
 
 		/*
-		 * If this reader is done, remove it, and collapse the array.  If all
-		 * readers are done, clean up remaining worker state.
+		 * If this reader is done, remove it from our working array of active
+		 * readers.  If all readers are done, we're outta here.
 		 */
 		if (readerdone)
 		{
 			Assert(!tup);
-			DestroyTupleQueueReader(reader);
 			--gatherstate->nreaders;
 			if (gatherstate->nreaders == 0)
-			{
-				ExecShutdownGatherWorkers(gatherstate);
 				return NULL;
-			}
 			memmove(&gatherstate->reader[gatherstate->nextreader],
 					&gatherstate->reader[gatherstate->nextreader + 1],
 					sizeof(TupleQueueReader *)
@@ -378,37 +370,25 @@ gather_readnext(GatherState *gatherstate)
 /* ----------------------------------------------------------------
  *		ExecShutdownGatherWorkers
  *
- *		Destroy the parallel workers.  Collect all the stats after
- *		workers are stopped, else some work done by workers won't be
- *		accounted.
+ *		Stop all the parallel workers.
  * ----------------------------------------------------------------
  */
 static void
 ExecShutdownGatherWorkers(GatherState *node)
 {
-	/* Shut down tuple queue readers before shutting down workers. */
-	if (node->reader != NULL)
-	{
-		int			i;
-
-		for (i = 0; i < node->nreaders; ++i)
-			DestroyTupleQueueReader(node->reader[i]);
-
-		pfree(node->reader);
-		node->reader = NULL;
-	}
-
-	/* Now shut down the workers. */
 	if (node->pei != NULL)
 		ExecParallelFinish(node->pei);
+
+	/* Flush local copy of reader array */
+	if (node->reader)
+		pfree(node->reader);
+	node->reader = NULL;
 }
 
 /* ----------------------------------------------------------------
  *		ExecShutdownGather
  *
  *		Destroy the setup for parallel workers including parallel context.
- *		Collect all the stats after workers are stopped, else some work
- *		done by workers won't be accounted.
  * ----------------------------------------------------------------
  */
 void