gh-94028: Clear and reset sqlite3 statements properly in cursor iternext (GH-94042)

(cherry picked from commit 94eeac0)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@protonmail.com>

Author: miss-islington

Lib/test/test_sqlite3/test_transactions.py

@@ -139,6 +139,45 @@ def test_rollback_cursor_consistency(self):
         con.rollback()
         self.assertEqual(cur.fetchall(), [(1,), (2,), (3,)])
 
+    def test_multiple_cursors_and_iternext(self):
+        # gh-94028: statements are cleared and reset in cursor iternext.
+
+        # Provoke the gh-94028 by using a cursor cache.
+        CURSORS = {}
+        def sql(cx, sql, *args):
+            cu = cx.cursor()
+            cu.execute(sql, args)
+            CURSORS[id(sql)] = cu
+            return cu
+
+        self.con1.execute("create table t(t)")
+        sql(self.con1, "insert into t values (?), (?), (?)", "u1", "u2", "u3")
+        self.con1.commit()
+
+        # On second connection, verify rows are visible, then delete them.
+        count = sql(self.con2, "select count(*) from t").fetchone()[0]
+        self.assertEqual(count, 3)
+        changes = sql(self.con2, "delete from t").rowcount
+        self.assertEqual(changes, 3)
+        self.con2.commit()
+
+        # Back in original connection, create 2 new users.
+        sql(self.con1, "insert into t values (?)", "u4")
+        sql(self.con1, "insert into t values (?)", "u5")
+
+        # The second connection cannot see uncommitted changes.
+        count = sql(self.con2, "select count(*) from t").fetchone()[0]
+        self.assertEqual(count, 0)
+
+        # First connection can see its own changes.
+        count = sql(self.con1, "select count(*) from t").fetchone()[0]
+        self.assertEqual(count, 2)
+
+        # The second connection can now see the changes.
+        self.con1.commit()
+        count = sql(self.con2, "select count(*) from t").fetchone()[0]
+        self.assertEqual(count, 2)
+
 
 class RollbackTests(unittest.TestCase):
     """bpo-44092: sqlite3 now leaves it to SQLite to resolve rollback issues"""

Misc/NEWS.d/next/Library/2022-06-20-23-14-43.gh-issue-94028.UofEcX.rst

@@ -0,0 +1,3 @@
+Fix a regression in the :mod:`sqlite3` where statement objects were not
+properly cleared and reset after use in cursor iters. The regression was
+introduced by PR 27884 in Python 3.11a1. Patch by Erlend E. Aasland.

Modules/_sqlite/cursor.c

@@ -1129,10 +1129,13 @@ pysqlite_cursor_iternext(pysqlite_Cursor *self)
             self->rowcount = (long)sqlite3_changes(self->connection->db);
         }
         (void)stmt_reset(self->statement);
+        Py_CLEAR(self->statement);
     }
     else if (rc != SQLITE_ROW) {
         (void)_pysqlite_seterror(self->connection->state,
                                  self->connection->db);
+        (void)stmt_reset(self->statement);
+        Py_CLEAR(self->statement);
         Py_DECREF(row);
         return NULL;
     }