Rearrange CustomScan API.

Make it work more like FDW plans do: instead of assuming that there are
expressions in a CustomScan plan node that the core code doesn't know
about, insist that all subexpressions that need planner attention be in
a "custom_exprs" list in the Plan representation.  (Of course, the
custom plugin can break the list apart again at executor initialization.)
This lets us revert the parts of the patch that exposed setrefs.c and
subselect.c processing to the outside world.

Also revert the GetSpecialCustomVar stuff in ruleutils.c; that concept
may work in future, but it's far from fully baked right now.

src/backend/optimizer/plan/createplan.c

@@ -77,7 +77,7 @@ static WorkTableScan *create_worktablescan_plan(PlannerInfo *root, Path *best_pa
 						  List *tlist, List *scan_clauses);
 static ForeignScan *create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
 						List *tlist, List *scan_clauses);
-static Plan *create_customscan_plan(PlannerInfo *root,
+static CustomScan *create_customscan_plan(PlannerInfo *root,
 					   CustomPath *best_path,
 					   List *tlist, List *scan_clauses);
 static NestLoop *create_nestloop_plan(PlannerInfo *root, NestPath *best_path,
@@ -86,6 +86,7 @@ static MergeJoin *create_mergejoin_plan(PlannerInfo *root, MergePath *best_path,
 					  Plan *outer_plan, Plan *inner_plan);
 static HashJoin *create_hashjoin_plan(PlannerInfo *root, HashPath *best_path,
 					 Plan *outer_plan, Plan *inner_plan);
+static Node *replace_nestloop_params(PlannerInfo *root, Node *expr);
 static Node *replace_nestloop_params_mutator(Node *node, PlannerInfo *root);
 static void process_subquery_nestloop_params(PlannerInfo *root,
 								 List *subplan_params);
@@ -413,10 +414,10 @@ create_scan_plan(PlannerInfo *root, Path *best_path)
 			break;
 
 		case T_CustomScan:
-			plan = create_customscan_plan(root,
-										  (CustomPath *) best_path,
-										  tlist,
-										  scan_clauses);
+			plan = (Plan *) create_customscan_plan(root,
+												   (CustomPath *) best_path,
+												   tlist,
+												   scan_clauses);
 			break;
 
 		default:
@@ -2022,11 +2023,11 @@ create_foreignscan_plan(PlannerInfo *root, ForeignPath *best_path,
  *
  * Transform a CustomPath into a Plan.
  */
-static Plan *
+static CustomScan *
 create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 					   List *tlist, List *scan_clauses)
 {
-	Plan	   *plan;
+	CustomScan *cplan;
 	RelOptInfo *rel = best_path->path.parent;
 
 	/*
@@ -2045,23 +2046,35 @@ create_customscan_plan(PlannerInfo *root, CustomPath *best_path,
 	 * Invoke custom plan provider to create the Plan node represented by the
 	 * CustomPath.
 	 */
-	plan = best_path->methods->PlanCustomPath(root, rel, best_path, tlist,
-											  scan_clauses);
-
-	/*
-	 * NOTE: unlike create_foreignscan_plan(), it is the responsibility of the
-	 * custom plan provider to replace outer-relation variables with nestloop
-	 * params, because we cannot know what expression trees may be held in
-	 * private fields.
-	 */
+	cplan = (CustomScan *) best_path->methods->PlanCustomPath(root,
+															  rel,
+															  best_path,
+															  tlist,
+															  scan_clauses);
+	Assert(IsA(cplan, CustomScan));
 
 	/*
 	 * Copy cost data from Path to Plan; no need to make custom-plan providers
 	 * do this
 	 */
-	copy_path_costsize(plan, &best_path->path);
+	copy_path_costsize(&cplan->scan.plan, &best_path->path);
 
-	return plan;
+	/*
+	 * Replace any outer-relation variables with nestloop params in the qual
+	 * and custom_exprs expressions.  We do this last so that the custom-plan
+	 * provider doesn't have to be involved.  (Note that parts of custom_exprs
+	 * could have come from join clauses, so doing this beforehand on the
+	 * scan_clauses wouldn't work.)
+	 */
+	if (best_path->path.param_info)
+	{
+		cplan->scan.plan.qual = (List *)
+			replace_nestloop_params(root, (Node *) cplan->scan.plan.qual);
+		cplan->custom_exprs = (List *)
+			replace_nestloop_params(root, (Node *) cplan->custom_exprs);
+	}
+
+	return cplan;
 }
 
 
@@ -2598,7 +2611,7 @@ create_hashjoin_plan(PlannerInfo *root,
  * root->curOuterRels are replaced by Params, and entries are added to
  * root->curOuterParams if not already present.
  */
-Node *
+static Node *
 replace_nestloop_params(PlannerInfo *root, Node *expr)
 {
 	/* No setup needed for tree walk, so away we go */

src/backend/optimizer/plan/setrefs.c

@@ -94,6 +94,7 @@ static Plan *set_subqueryscan_references(PlannerInfo *root,
 							SubqueryScan *plan,
 							int rtoffset);
 static bool trivial_subqueryscan(SubqueryScan *plan);
+static Node *fix_scan_expr(PlannerInfo *root, Node *node, int rtoffset);
 static Node *fix_scan_expr_mutator(Node *node, fix_scan_expr_context *context);
 static bool fix_scan_expr_walker(Node *node, fix_scan_expr_context *context);
 static void set_join_references(PlannerInfo *root, Join *join, int rtoffset);
@@ -580,23 +581,15 @@ set_plan_refs(PlannerInfo *root, Plan *plan, int rtoffset)
 
 		case T_CustomScan:
 			{
-				CustomScan *cscan = (CustomScan *) plan;
+				CustomScan *splan = (CustomScan *) plan;
 
-				cscan->scan.scanrelid += rtoffset;
-				cscan->scan.plan.targetlist =
-					fix_scan_list(root, cscan->scan.plan.targetlist, rtoffset);
-				cscan->scan.plan.qual =
-					fix_scan_list(root, cscan->scan.plan.qual, rtoffset);
-
-				/*
-				 * The core implementation applies the routine to fixup varno
-				 * on the target-list and scan qualifier. If custom-scan has
-				 * additional expression nodes on its private fields, it has
-				 * to apply same fixup on them. Otherwise, the custom-plan
-				 * provider can skip this callback.
-				 */
-				if (cscan->methods->SetCustomScanRef)
-					cscan->methods->SetCustomScanRef(root, cscan, rtoffset);
+				splan->scan.scanrelid += rtoffset;
+				splan->scan.plan.targetlist =
+					fix_scan_list(root, splan->scan.plan.targetlist, rtoffset);
+				splan->scan.plan.qual =
+					fix_scan_list(root, splan->scan.plan.qual, rtoffset);
+				splan->custom_exprs =
+					fix_scan_list(root, splan->custom_exprs, rtoffset);
 			}
 			break;
 
@@ -1182,7 +1175,7 @@ fix_param_node(PlannerInfo *root, Param *p)
  * looking up operator opcode info for OpExpr and related nodes,
  * and adding OIDs from regclass Const nodes into root->glob->relationOids.
  */
-Node *
+static Node *
 fix_scan_expr(PlannerInfo *root, Node *node, int rtoffset)
 {
 	fix_scan_expr_context context;

src/backend/optimizer/plan/subselect.c

@@ -2284,24 +2284,9 @@ finalize_plan(PlannerInfo *root, Plan *plan, Bitmapset *valid_params,
 			break;
 
 		case T_CustomScan:
-			{
-				CustomScan *custom_scan = (CustomScan *) plan;
-
-				context.paramids = bms_add_members(context.paramids,
-												   scan_params);
-				/*
-				 * custom-scan provider is responsible to apply
-				 * finalize_primnode() on the expression node of
-				 * its private fields, but no need to apply it
-				 * on the tlist and qual of Plan node because it
-				 * is already done above.
-				 */
-				if (custom_scan->methods->FinalizeCustomScan)
-					custom_scan->methods->FinalizeCustomScan(root,
-															 custom_scan,
-															 finalize_primnode,
-															 (void *)&context);
-			}
+			finalize_primnode((Node *) ((CustomScan *) plan)->custom_exprs,
+							  &context);
+			context.paramids = bms_add_members(context.paramids, scan_params);
 			break;
 
 		case T_ModifyTable: