check_exclusion_constraint didn't actually work correctly for index

expressions: FormIndexDatum requires the estate's scantuple to already point
at the tuple the values are supposedly being extracted from.  Adjust test
case so that this type of confusion will be exposed.
Per report from hubert depesz lubaczewski.

src/backend/executor/execUtils.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/execUtils.c,v 1.168 2010/01/02 16:57:41 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/execUtils.c,v 1.169 2010/01/02 17:53:56 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1167,7 +1167,9 @@ check_exclusion_constraint(Relation heap, Relation index, IndexInfo *indexInfo,
 	int				i;
 	bool			conflict;
 	bool			found_self;
+	ExprContext	   *econtext;
 	TupleTableSlot *existing_slot;
+	TupleTableSlot *save_scantuple;
 
 	/*
 	 * If any of the input values are NULL, the constraint check is assumed
@@ -1194,9 +1196,19 @@ check_exclusion_constraint(Relation heap, Relation index, IndexInfo *indexInfo,
 					values[i]);
 	}
 
-	/* Need a TupleTableSlot to put existing tuples in */
+	/*
+	 * Need a TupleTableSlot to put existing tuples in.
+	 *
+	 * To use FormIndexDatum, we have to make the econtext's scantuple point
+	 * to this slot.  Be sure to save and restore caller's value for
+	 * scantuple.
+	 */
 	existing_slot = MakeSingleTupleTableSlot(RelationGetDescr(heap));
 
+	econtext = GetPerTupleExprContext(estate);
+	save_scantuple = econtext->ecxt_scantuple;
+	econtext->ecxt_scantuple = existing_slot;
+
 	/*
 	 * May have to restart scan from this point if a potential
 	 * conflict is found.
@@ -1311,6 +1323,8 @@ retry:
 						RelationGetRelationName(index)),
 				 errhint("This may be because of a non-immutable index expression.")));
 
+	econtext->ecxt_scantuple = save_scantuple;
+
 	ExecDropSingleTupleTableSlot(existing_slot);
 
 	return !conflict;

src/test/regress/input/constraints.source

@@ -376,26 +376,26 @@ CREATE TABLE circles (
   c1 CIRCLE,
   c2 TEXT,
   EXCLUDE USING gist
-    (c1 WITH &&, (c2::circle) WITH ~=)
+    (c1 WITH &&, (c2::circle) WITH &&)
     WHERE (circle_center(c1) <> '(0,0)')
 );
 
 -- these should succeed because they don't match the index predicate
 INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 5>');
-INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 5>');
+INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 4>');
 
 -- succeed
 INSERT INTO circles VALUES('<(10,10), 10>', '<(0,0), 5>');
 -- fail, overlaps
-INSERT INTO circles VALUES('<(20,20), 10>', '<(0,0), 5>');
+INSERT INTO circles VALUES('<(20,20), 10>', '<(0,0), 4>');
 -- succeed because c1 doesn't overlap
 INSERT INTO circles VALUES('<(20,20), 1>', '<(0,0), 5>');
--- succeed because c2 is not the same
-INSERT INTO circles VALUES('<(20,20), 10>', '<(1,1), 5>');
+-- succeed because c2 doesn't overlap
+INSERT INTO circles VALUES('<(20,20), 10>', '<(10,10), 5>');
 
 -- should fail on existing data without the WHERE clause
 ALTER TABLE circles ADD EXCLUDE USING gist
-  (c1 WITH &&, (c2::circle) WITH ~=);
+  (c1 WITH &&, (c2::circle) WITH &&);
 
 DROP TABLE circles;
 

src/test/regress/output/constraints.source

@@ -520,29 +520,29 @@ CREATE TABLE circles (
   c1 CIRCLE,
   c2 TEXT,
   EXCLUDE USING gist
-    (c1 WITH &&, (c2::circle) WITH ~=)
+    (c1 WITH &&, (c2::circle) WITH &&)
     WHERE (circle_center(c1) <> '(0,0)')
 );
 NOTICE:  CREATE TABLE / EXCLUDE will create implicit index "circles_c1_c2_exclusion" for table "circles"
 -- these should succeed because they don't match the index predicate
 INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 5>');
-INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 5>');
+INSERT INTO circles VALUES('<(0,0), 5>', '<(0,0), 4>');
 -- succeed
 INSERT INTO circles VALUES('<(10,10), 10>', '<(0,0), 5>');
 -- fail, overlaps
-INSERT INTO circles VALUES('<(20,20), 10>', '<(0,0), 5>');
+INSERT INTO circles VALUES('<(20,20), 10>', '<(0,0), 4>');
 ERROR:  conflicting key value violates exclusion constraint "circles_c1_c2_exclusion"
-DETAIL:  Key (c1, (c2::circle))=(<(20,20),10>, <(0,0),5>) conflicts with existing key (c1, (c2::circle))=(<(10,10),10>, <(0,0),5>).
+DETAIL:  Key (c1, (c2::circle))=(<(20,20),10>, <(0,0),4>) conflicts with existing key (c1, (c2::circle))=(<(10,10),10>, <(0,0),5>).
 -- succeed because c1 doesn't overlap
 INSERT INTO circles VALUES('<(20,20), 1>', '<(0,0), 5>');
--- succeed because c2 is not the same
-INSERT INTO circles VALUES('<(20,20), 10>', '<(1,1), 5>');
+-- succeed because c2 doesn't overlap
+INSERT INTO circles VALUES('<(20,20), 10>', '<(10,10), 5>');
 -- should fail on existing data without the WHERE clause
 ALTER TABLE circles ADD EXCLUDE USING gist
-  (c1 WITH &&, (c2::circle) WITH ~=);
+  (c1 WITH &&, (c2::circle) WITH &&);
 NOTICE:  ALTER TABLE / ADD EXCLUDE will create implicit index "circles_c1_c2_exclusion1" for table "circles"
 ERROR:  could not create exclusion constraint "circles_c1_c2_exclusion1"
-DETAIL:  Key (c1, (c2::circle))=(<(0,0),5>, <(0,0),5>) conflicts with key (c1, (c2::circle))=(<(0,0),5>, <(0,0),5>).
+DETAIL:  Key (c1, (c2::circle))=(<(0,0),5>, <(0,0),5>) conflicts with key (c1, (c2::circle))=(<(0,0),5>, <(0,0),4>).
 DROP TABLE circles;
 -- Check deferred exclusion constraint
 CREATE TABLE deferred_excl (