Correctly set t_self for heap tuples in expand_tuple

Commit 16828d5c incorrectly set an invalid pointer for t_self for heap
tuples. This patch correctly copies it from the source tuple, and
includes a regression test that relies on it being set correctly.

Backpatch to release 11.

Fixes bug #15448 reported by Tillmann Schulz

Diagnosis and test case by Amit Langote

src/backend/access/common/heaptuple.c

@@ -856,7 +856,7 @@ expand_tuple(HeapTuple *targetHeapTuple,
 			= (HeapTupleHeader) ((char *) *targetHeapTuple + HEAPTUPLESIZE);
 		(*targetHeapTuple)->t_len = len;
 		(*targetHeapTuple)->t_tableOid = sourceTuple->t_tableOid;
-		ItemPointerSetInvalid(&((*targetHeapTuple)->t_self));
+		(*targetHeapTuple)->t_self = sourceTuple->t_self;
 
 		targetTHeader->t_infomask = sourceTHeader->t_infomask;
 		targetTHeader->t_hoff = hoff;

src/test/regress/expected/fast_default.out

@@ -727,7 +727,17 @@ SELECT * FROM t;
 (1 row)
 
 DROP TABLE t;
+-- make sure expanded tuple has correct self pointer
+-- it will be required by the RI tigger doing the cascading delete
+CREATE TABLE leader (a int PRIMARY KEY, b int);
+CREATE TABLE follower (a int REFERENCES leader ON DELETE CASCADE, b int);
+INSERT INTO leader VALUES (1, 1), (2, 2);
+ALTER TABLE leader ADD c int;
+ALTER TABLE leader DROP c;
+DELETE FROM leader;
 -- cleanup
+DROP TABLE follower;
+DROP TABLE leader;
 DROP FUNCTION test_trigger();
 DROP TABLE t1;
 DROP FUNCTION set(name);

src/test/regress/sql/fast_default.sql

@@ -471,7 +471,19 @@ UPDATE t SET y = 2;
 SELECT * FROM t;
 DROP TABLE t;
 
+-- make sure expanded tuple has correct self pointer
+-- it will be required by the RI tigger doing the cascading delete
+
+CREATE TABLE leader (a int PRIMARY KEY, b int);
+CREATE TABLE follower (a int REFERENCES leader ON DELETE CASCADE, b int);
+INSERT INTO leader VALUES (1, 1), (2, 2);
+ALTER TABLE leader ADD c int;
+ALTER TABLE leader DROP c;
+DELETE FROM leader;
+
 -- cleanup
+DROP TABLE follower;
+DROP TABLE leader;
 DROP FUNCTION test_trigger();
 DROP TABLE t1;
 DROP FUNCTION set(name);