References to old and new blocks were mixed in xlog_heap_update handler (#5312)

## Problem

See https://neondb.slack.com/archives/C05L7D1JAUS/p1694614585955029

https://www.notion.so/neondatabase/Duplicate-key-issue-651627ce843c45188fbdcb2d30fd2178

## Summary of changes

Swap old/new block references

## Checklist before requesting a review

- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

test_runner/regress/test_vm_bits.py

@@ -19,18 +19,40 @@ def test_vm_bit_clear(neon_simple_env: NeonEnv):
     # Install extension containing function needed for test
     cur.execute("CREATE EXTENSION neon_test_utils")
 
-    # Create a test table and freeze it to set the VM bit.
+    # Create a test table for a few different scenarios and freeze it to set the VM bits.
     cur.execute("CREATE TABLE vmtest_delete (id integer PRIMARY KEY)")
     cur.execute("INSERT INTO vmtest_delete VALUES (1)")
     cur.execute("VACUUM FREEZE vmtest_delete")
 
-    cur.execute("CREATE TABLE vmtest_update (id integer PRIMARY KEY)")
-    cur.execute("INSERT INTO vmtest_update SELECT g FROM generate_series(1, 1000) g")
-    cur.execute("VACUUM FREEZE vmtest_update")
+    cur.execute("CREATE TABLE vmtest_hot_update (id integer PRIMARY KEY, filler text)")
+    cur.execute("INSERT INTO vmtest_hot_update VALUES (1, 'x')")
+    cur.execute("VACUUM FREEZE vmtest_hot_update")
+
+    cur.execute("CREATE TABLE vmtest_cold_update (id integer PRIMARY KEY)")
+    cur.execute("INSERT INTO vmtest_cold_update SELECT g FROM generate_series(1, 1000) g")
+    cur.execute("VACUUM FREEZE vmtest_cold_update")
+
+    cur.execute(
+        "CREATE TABLE vmtest_cold_update2 (id integer PRIMARY KEY, filler text) WITH (fillfactor=100)"
+    )
+    cur.execute("INSERT INTO vmtest_cold_update2 SELECT g, '' FROM generate_series(1, 1000) g")
+    cur.execute("VACUUM FREEZE vmtest_cold_update2")
 
     # DELETE and UPDATE the rows.
     cur.execute("DELETE FROM vmtest_delete WHERE id = 1")
-    cur.execute("UPDATE vmtest_update SET id = 5000 WHERE id = 1")
+    cur.execute("UPDATE vmtest_hot_update SET filler='x' WHERE id = 1")
+    cur.execute("UPDATE vmtest_cold_update SET id = 5000 WHERE id = 1")
+
+    # Clear the VM bit on the last page with an INSERT. Then clear the VM bit on
+    # the page where row 1 is (block 0), by doing an UPDATE. The UPDATE is a
+    # cold update, and the new tuple goes to the last page, which already had
+    # its VM bit cleared. The point is that the UPDATE *only* clears the VM bit
+    # on the page containing the old tuple. We had a bug where we got the old
+    # and new pages mixed up, and that only shows up when one of the bits is
+    # cleared, but not the other one.
+    cur.execute("INSERT INTO vmtest_cold_update2 VALUES (9999, 'x')")
+    # Clears the VM bit on the old page
+    cur.execute("UPDATE vmtest_cold_update2 SET id = 5000, filler=repeat('x', 200) WHERE id = 1")
 
     # Branch at this point, to test that later
     fork_at_current_lsn(env, endpoint, "test_vm_bit_clear_new", "test_vm_bit_clear")
@@ -50,9 +72,13 @@ def test_vm_bit_clear(neon_simple_env: NeonEnv):
     """
     )
 
-    cur.execute("SELECT * FROM vmtest_delete WHERE id = 1")
+    cur.execute("SELECT id FROM vmtest_delete WHERE id = 1")
     assert cur.fetchall() == []
-    cur.execute("SELECT * FROM vmtest_update WHERE id = 1")
+    cur.execute("SELECT id FROM vmtest_hot_update WHERE id = 1")
+    assert cur.fetchall() == [(1,)]
+    cur.execute("SELECT id FROM vmtest_cold_update WHERE id = 1")
+    assert cur.fetchall() == []
+    cur.execute("SELECT id FROM vmtest_cold_update2 WHERE id = 1")
     assert cur.fetchall() == []
 
     cur.close()
@@ -77,7 +103,11 @@ def test_vm_bit_clear(neon_simple_env: NeonEnv):
     """
     )
 
-    cur_new.execute("SELECT * FROM vmtest_delete WHERE id = 1")
+    cur_new.execute("SELECT id FROM vmtest_delete WHERE id = 1")
     assert cur_new.fetchall() == []
-    cur_new.execute("SELECT * FROM vmtest_update WHERE id = 1")
+    cur_new.execute("SELECT id FROM vmtest_hot_update WHERE id = 1")
+    assert cur_new.fetchall() == [(1,)]
+    cur_new.execute("SELECT id FROM vmtest_cold_update WHERE id = 1")
+    assert cur_new.fetchall() == []
+    cur_new.execute("SELECT id FROM vmtest_cold_update2 WHERE id = 1")
     assert cur_new.fetchall() == []