BUG#37627508: mysql/connector python fetchmany() has an off by one bug

SubCoder1 · SubCoder1 · commit c70277a5c643 · 2025-05-26T14:25:09.000+05:30
when argument given as 1

This patch fixes the issue where c-extension based default cursor's
fetchmany method gets stuck in an infinite loop when one result row
at a time is being fetched.

Change-Id: I2290dc1330c50d39fdb75617e7fcbf8a8cf7c6de
diff --git a/CHANGES.txt b/CHANGES.txt
@@ -13,6 +13,7 @@ v9.4.0
 
 - BUG#37820231: Text based django ORM filters doesn't work with Connector/Python
 - BUG#37806057: Rename extra option (when installing wheel package) to install webauthn functionality dependencies
+- BUG#37627508: mysql/connector python fetchmany() has an off by one bug when argument given as 1
 - BUG#37047789: Python connector does not support Django enum
 - BUG#36452514: Missing version info resources
 - BUG#34950958: MySQL Python Connector doesn't work with ssh in the same process
diff --git a/mysql-connector-python/lib/mysql/connector/cursor_cext.py b/mysql-connector-python/lib/mysql/connector/cursor_cext.py
@@ -704,17 +704,16 @@ def fetchmany(self, size: int = 1) -> List[RowType]:
         if size and self._connection.unread_result:
             rows.extend(self._connection.get_rows(size, raw=self._raw)[0])
 
-        if size:
-            if self._connection.unread_result:
-                self._nextrow = self._connection.get_row()
-                if (
-                    self._nextrow
-                    and not self._nextrow[0]
-                    and not self._connection.more_results
-                ):
-                    self._connection.free_result()
-            else:
-                self._nextrow = (None, None)
+        if self._connection.unread_result:
+            self._nextrow = self._connection.get_row()
+            if (
+                self._nextrow
+                and not self._nextrow[0]
+                and not self._connection.more_results
+            ):
+                self._connection.free_result()
+        else:
+            self._nextrow = (None, None)
 
         if not rows:
             self._handle_eof()
diff --git a/mysql-connector-python/tests/cext/test_cext_cursor.py b/mysql-connector-python/tests/cext/test_cext_cursor.py
@@ -37,10 +37,11 @@
 import logging
 import sys
 import unittest
-import warnings
 
 import tests
 
+import mysql.connector
+
 from mysql.connector import errorcode, errors
 
 try:
@@ -1161,3 +1162,54 @@ def test_executemany(self):
         self.assertEqual(len(rows), 4)
         self.assertEqual(rows[0], dict(zip(self.column_names, self.exp)))
         self.assertEqual(rows[1], dict(zip(self.column_names, self.exp)))
+
+
+class BugOra37627508(tests.MySQLConnectorTests):
+    """BUG#37627508: mysql/connector python fetchmany() has an off by one bug when argument given as 1
+
+    While single result is being fetched using the c-extension based cursor module's fetchmany() method,
+    the application crashes into a infinite loop of same results being returned by the cursor.
+
+    This patch fixes the issue by flushing out the cached result (next row) after it is being used up.
+    """
+
+    table_name = "Bug37627508"
+
+    exp_fetchmany_result_1 = [[(1,)], [(2,)], [(3,)], [(4,)], [(5,)], []]
+    exp_fetchmany_result_2 = [[(1,), (2,)], [(3,), (4,)], [(5,)], []]
+
+    @classmethod
+    def setUpClass(cls):
+        with mysql.connector.connect(**tests.get_mysql_config()) as cnx:
+            cnx.cmd_query(f"CREATE TABLE IF NOT EXISTS {cls.table_name} (id INTEGER)")
+            cnx.cmd_query(f"INSERT INTO {cls.table_name} (id) VALUES (1), (2), (3), (4), (5)")
+            cnx.commit()
+
+    @classmethod
+    def tearDownClass(cls):
+        with mysql.connector.connect(**tests.get_mysql_config()) as cnx:
+            cnx.cmd_query(f"DROP TABLE IF EXISTS {cls.table_name}")
+
+    @tests.foreach_cnx()
+    def test_fetchmany_cext_resultsize_1(self):
+        with self.cnx.cursor() as cur:
+            cur.execute(f"SELECT * FROM {self.table_name}")
+            res = []
+            try:
+                for _ in range(6):
+                    res.append(cur.fetchmany(1))
+                self.assertEqual(res, self.exp_fetchmany_result_1)
+            except Exception as e:
+                self.fail(e)
+
+    @tests.foreach_cnx()
+    def test_fetchmany_cext_resultsize_2(self):
+        with self.cnx.cursor() as cur:
+            cur.execute(f"SELECT * FROM {self.table_name}")
+            res = []
+            try:
+                for _ in range(4):
+                    res.append(cur.fetchmany(2))
+                self.assertEqual(res, self.exp_fetchmany_result_2)
+            except Exception as e:
+                self.fail(e)
diff --git a/mysql-connector-python/tests/test_bugs.py b/mysql-connector-python/tests/test_bugs.py
@@ -41,7 +41,6 @@
 to be created first.
 """
 
-import asyncio
 import collections
 import gc
 import os
@@ -52,7 +51,7 @@
 import traceback
 import unittest
 
-from datetime import date, datetime, time, timedelta, timezone
+from datetime import date, datetime, timedelta, timezone
 from decimal import Decimal
 from threading import Thread
 from time import sleep
