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

src/backend/executor/execMain.c

@@ -1702,6 +1702,7 @@ ExecutePlan(EState *estate,
 	if (!execute_once)
 		use_parallel_mode = false;
 
+	estate->es_use_parallel_mode = use_parallel_mode;
 	if (use_parallel_mode)
 		EnterParallelMode();
 

src/backend/executor/execUtils.c

@@ -156,6 +156,8 @@ CreateExecutorState(void)
 	estate->es_epqScanDone = NULL;
 	estate->es_sourceText = NULL;
 
+	estate->es_use_parallel_mode = false;
+
 	/*
 	 * Return the executor state structure
 	 */

src/backend/executor/nodeGather.c

@@ -150,7 +150,7 @@ ExecGather(PlanState *pstate)
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gather->num_workers > 0 && IsInParallelMode())
+		if (gather->num_workers > 0 && estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 

src/backend/executor/nodeGatherMerge.c

@@ -194,7 +194,7 @@ ExecGatherMerge(PlanState *pstate)
 		 * Sometimes we might have to run without parallelism; but if parallel
 		 * mode is active then we can try to fire up some workers.
 		 */
-		if (gm->num_workers > 0 && IsInParallelMode())
+		if (gm->num_workers > 0 && estate->es_use_parallel_mode)
 		{
 			ParallelContext *pcxt;
 

src/include/nodes/execnodes.h

@@ -507,6 +507,8 @@ typedef struct EState
 	bool	   *es_epqTupleSet; /* true if EPQ tuple is provided */
 	bool	   *es_epqScanDone; /* true if EPQ tuple has been fetched */
 
+	bool		es_use_parallel_mode; /* can we use parallel workers? */
+
 	/* The per-query shared memory area to use for parallel execution. */
 	struct dsa_area *es_query_dsa;
 } EState;