Restore sane locking behavior during parallel query.

Commit 9a3cebea changed things so that parallel workers didn't obtain
any lock of their own on tables they access.  That was clearly a bad
idea, but I'd mistakenly supposed that it was the intended end result
of the series of patches for simplifying the executor's lock management.
Undo that change in relation_open(), and adjust ExecOpenScanRelation()
so that it gets the correct lock if inside a parallel worker.

In passing, clean up some more obsolete comments about when locks
are acquired.

Discussion: https://postgr.es/m/468c85d9-540e-66a2-1dde-fec2b741e688@lab.ntt.co.jp

src/backend/access/heap/heapam.c

@@ -1140,13 +1140,9 @@ relation_open(Oid relationId, LOCKMODE lockmode)
 	/*
 	 * If we didn't get the lock ourselves, assert that caller holds one,
 	 * except in bootstrap mode where no locks are used.
-	 *
-	 * Also, parallel workers currently assume that their parent holds locks
-	 * for tables used in the parallel query (a mighty shaky assumption).
 	 */
 	Assert(lockmode != NoLock ||
 		   IsBootstrapProcessingMode() ||
-		   IsParallelWorker() ||
 		   CheckRelationLockedByMe(r, AccessShareLock, true));
 
 	/* Make note that we've accessed a temporary relation */

src/backend/executor/execMain.c

@@ -1622,8 +1622,8 @@ ExecEndPlan(PlanState *planstate, EState *estate)
 	}
 
 	/*
-	 * close whatever rangetable Relations have been opened.  We did not
-	 * acquire locks in ExecGetRangeTableRelation, so don't release 'em here.
+	 * close whatever rangetable Relations have been opened.  We do not
+	 * release any locks we might hold on those rels.
 	 */
 	num_relations = estate->es_range_table_size;
 	for (i = 0; i < num_relations; i++)

src/backend/executor/execUtils.c

@@ -732,16 +732,30 @@ ExecGetRangeTableRelation(EState *estate, Index rti)
 
 		Assert(rte->rtekind == RTE_RELATION);
 
-		rel = estate->es_relations[rti - 1] = heap_open(rte->relid, NoLock);
+		if (!IsParallelWorker())
+		{
+			/*
+			 * In a normal query, we should already have the appropriate lock,
+			 * but verify that through an Assert.  Since there's already an
+			 * Assert inside heap_open that insists on holding some lock, it
+			 * seems sufficient to check this only when rellockmode is higher
+			 * than the minimum.
+			 */
+			rel = heap_open(rte->relid, NoLock);
+			Assert(rte->rellockmode == AccessShareLock ||
+				   CheckRelationLockedByMe(rel, rte->rellockmode, false));
+		}
+		else
+		{
+			/*
+			 * If we are a parallel worker, we need to obtain our own local
+			 * lock on the relation.  This ensures sane behavior in case the
+			 * parent process exits before we do.
+			 */
+			rel = heap_open(rte->relid, rte->rellockmode);
+		}
 
-		/*
-		 * Verify that appropriate lock was obtained before execution.
-		 *
-		 * In the case of parallel query, only the leader would've obtained
-		 * the lock (that needs to be fixed, though).
-		 */
-		Assert(IsParallelWorker() ||
-			   CheckRelationLockedByMe(rel, rte->rellockmode, false));
+		estate->es_relations[rti - 1] = rel;
 	}
 
 	return rel;

src/backend/executor/nodeBitmapHeapscan.c

@@ -899,16 +899,12 @@ ExecInitBitmapHeapScan(BitmapHeapScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &scanstate->ss.ps);
 
 	/*
-	 * open the base relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
 
 	/*
 	 * initialize child nodes
-	 *
-	 * We do this after ExecOpenScanRelation because the child nodes will open
-	 * indexscans on our relation's indexes, and we want to be sure we have
-	 * acquired a lock on the relation first.
 	 */
 	outerPlanState(scanstate) = ExecInitNode(outerPlan(node), estate, eflags);
 

src/backend/executor/nodeCustom.c

@@ -55,7 +55,7 @@ ExecInitCustomScan(CustomScan *cscan, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &css->ss.ps);
 
 	/*
-	 * open the base relation, if any, and acquire an appropriate lock on it
+	 * open the scan relation, if any
 	 */
 	if (scanrelid > 0)
 	{

src/backend/executor/nodeForeignscan.c

@@ -156,8 +156,8 @@ ExecInitForeignScan(ForeignScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &scanstate->ss.ps);
 
 	/*
-	 * open the base relation, if any, and acquire an appropriate lock on it;
-	 * also acquire function pointers from the FDW's handler
+	 * open the scan relation, if any; also acquire function pointers from the
+	 * FDW's handler
 	 */
 	if (scanrelid > 0)
 	{

src/backend/executor/nodeIndexonlyscan.c

@@ -511,7 +511,7 @@ ExecInitIndexOnlyScan(IndexOnlyScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &indexstate->ss.ps);
 
 	/*
-	 * open the base relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
 

src/backend/executor/nodeIndexscan.c

@@ -933,7 +933,7 @@ ExecInitIndexScan(IndexScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &indexstate->ss.ps);
 
 	/*
-	 * open the base relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);
 

src/backend/executor/nodeSamplescan.c

@@ -134,10 +134,7 @@ ExecInitSampleScan(SampleScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &scanstate->ss.ps);
 
 	/*
-	 * Initialize scan relation.
-	 *
-	 * Get the relation object id from the relid'th entry in the range table,
-	 * open that relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	scanstate->ss.ss_currentRelation =
 		ExecOpenScanRelation(estate,

src/backend/executor/nodeSeqscan.c

@@ -163,10 +163,7 @@ ExecInitSeqScan(SeqScan *node, EState *estate, int eflags)
 	ExecAssignExprContext(estate, &scanstate->ss.ps);
 
 	/*
-	 * Initialize scan relation.
-	 *
-	 * Get the relation object id from the relid'th entry in the range table,
-	 * open that relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	scanstate->ss.ss_currentRelation =
 		ExecOpenScanRelation(estate,

src/backend/executor/nodeTidscan.c

@@ -531,7 +531,7 @@ ExecInitTidScan(TidScan *node, EState *estate, int eflags)
 	tidstate->tss_TidPtr = -1;
 
 	/*
-	 * open the base relation and acquire appropriate lock on it.
+	 * open the scan relation
 	 */
 	currentRelation = ExecOpenScanRelation(estate, node->scan.scanrelid, eflags);