fix: MVCC cursor ignored left join null flag

Author: jussisaurio

core/vdbe/execute.rs

@@ -3082,6 +3082,10 @@ pub fn op_row_id(
                     .get_mut(*cursor_id)
                     .expect("cursor_id should be valid")
                 {
+                    if btree_cursor.get_null_flag() {
+                        state.registers[*dest] = Register::Value(Value::Null);
+                        break;
+                    }
                     if let Some(ref rowid) = return_if_io!(btree_cursor.rowid()) {
                         state.registers[*dest].set_int(*rowid);
                     } else {

testing/runner/turso-tests/mvcc_left_join_null_row.sqltest

@@ -0,0 +1,16 @@
+# Regression test for MVCC LEFT JOIN NullRow bug.
+# MvccLazyCursor::rowid() was not checking null_flag, so after NullRow
+# it returned the stale rowid of the last seeked row instead of NULL.
+
+@database :memory:
+
+test mvcc-left-join-null-row {
+    CREATE TABLE t1(x INTEGER PRIMARY KEY);
+    CREATE TABLE t2(x INTEGER PRIMARY KEY, v TEXT);
+    INSERT INTO t1 VALUES (1);
+    INSERT INTO t2 VALUES (1, 'hello');
+    SELECT t1.x, t2.x FROM t1 LEFT JOIN t2 ON t1.x = t2.x AND t2.v = 'nonexistent';
+}
+expect {
+    1|
+}