Fix ON CONFLICT bugs that manifest when used in rules.

Specifically the tlist and rti of the pseudo "excluded" relation weren't
properly treated by expression_tree_walker, which lead to errors when
excluded was referenced inside a rule because the varnos where not
properly adjusted.  Similar omissions in OffsetVarNodes and
expression_tree_mutator had less impact, but should obviously be fixed
nonetheless.

A couple tests of for ON CONFLICT UPDATE into INSERT rule bearing
relations have been added.

In passing I updated a couple comments.

src/backend/executor/nodeModifyTable.c

@@ -1675,6 +1675,7 @@ ExecInitModifyTable(ModifyTable *node, EState *estate, int eflags)
 		ExecSetSlotDescriptor(mtstate->mt_existing,
 							  resultRelInfo->ri_RelationDesc->rd_att);
 
+		/* carried forward solely for the benefit of explain */
 		mtstate->mt_excludedtlist = node->exclRelTlist;
 
 		/* create target slot for UPDATE SET projection */

src/backend/nodes/nodeFuncs.c

@@ -1922,6 +1922,8 @@ expression_tree_walker(Node *node,
 					return true;
 				if (walker(onconflict->onConflictWhere, context))
 					return true;
+				if (walker(onconflict->exclRelTlist, context))
+					return true;
 			}
 			break;
 		case T_JoinExpr:
@@ -2642,6 +2644,7 @@ expression_tree_mutator(Node *node,
 				MUTATE(newnode->arbiterWhere, oc->arbiterWhere, Node *);
 				MUTATE(newnode->onConflictSet, oc->onConflictSet, List *);
 				MUTATE(newnode->onConflictWhere, oc->onConflictWhere, Node *);
+				MUTATE(newnode->exclRelTlist, oc->exclRelTlist, List *);
 
 				return (Node *) newnode;
 			}

src/backend/optimizer/plan/setrefs.c

@@ -740,9 +740,9 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 
 				/*
 				 * We treat ModifyTable with ON CONFLICT as a form of 'pseudo
-				 * join', where the inner side is the EXLUDED tuple. Therefore
-				 * use fix_join_expr to setup the relevant variables to
-				 * INNER_VAR. We explicitly don't create any OUTER_VARs as
+				 * join', where the inner side is the EXCLUDED tuple.
+				 * Therefore use fix_join_expr to setup the relevant variables
+				 * to INNER_VAR. We explicitly don't create any OUTER_VARs as
 				 * those are already used by RETURNING and it seems better to
 				 * be non-conflicting.
 				 */
@@ -763,6 +763,9 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 									  NULL, itlist,
 									  linitial_int(splan->resultRelations),
 									  rtoffset);
+
+					splan->exclRelTlist =
+						fix_scan_list(root, splan->exclRelTlist, rtoffset);
 				}
 
 				splan->nominalRelation += rtoffset;

src/backend/rewrite/rewriteManip.c

@@ -426,9 +426,9 @@ OffsetVarNodes(Node *node, int offset, int sublevels_up)
 		/*
 		 * If we are starting at a Query, and sublevels_up is zero, then we
 		 * must also fix rangetable indexes in the Query itself --- namely
-		 * resultRelation and rowMarks entries.  sublevels_up cannot be zero
-		 * when recursing into a subquery, so there's no need to have the same
-		 * logic inside OffsetVarNodes_walker.
+		 * resultRelation, exclRelIndex and rowMarks entries.  sublevels_up
+		 * cannot be zero when recursing into a subquery, so there's no need
+		 * to have the same logic inside OffsetVarNodes_walker.
 		 */
 		if (sublevels_up == 0)
 		{
@@ -436,6 +436,10 @@ OffsetVarNodes(Node *node, int offset, int sublevels_up)
 
 			if (qry->resultRelation)
 				qry->resultRelation += offset;
+
+			if (qry->onConflict && qry->onConflict->exclRelIndex)
+				qry->onConflict->exclRelIndex += offset;
+
 			foreach(l, qry->rowMarks)
 			{
 				RowMarkClause *rc = (RowMarkClause *) lfirst(l);
@@ -617,6 +621,11 @@ ChangeVarNodes(Node *node, int rt_index, int new_index, int sublevels_up)
 
 			if (qry->resultRelation == rt_index)
 				qry->resultRelation = new_index;
+
+			/* this is unlikely to ever be used, but ... */
+			if (qry->onConflict && qry->onConflict->exclRelIndex == rt_index)
+				qry->onConflict->exclRelIndex = new_index;
+
 			foreach(l, qry->rowMarks)
 			{
 				RowMarkClause *rc = (RowMarkClause *) lfirst(l);