test: Add tests for trailing null preservation and multi-byte UTF-8 in ShareableList

Add test_shared_memory_ShareableList_trailing_nulls covering gh-106939
(bytes/str values with trailing null bytes preserved correctly) and
test_shared_memory_ShareableList_multibyte_utf8 covering gh-145261
(multi-byte UTF-8 strings stored and retrieved without corruption).
Both tests include cross-process verification via name-based attachment.

Author: stefanzetzsche

Lib/test/_test_multiprocessing.py

@@ -4899,6 +4899,58 @@ def test_shared_memory_ShareableList_pickling_dead_object(self):
                 with self.assertRaises(FileNotFoundError):
                     pickle.loads(serialized_sl)
 
+    def test_shared_memory_ShareableList_trailing_nulls(self):
+        # gh-106939: ShareableList should preserve trailing null bytes
+        # in bytes and str values.
+        sl = shared_memory.ShareableList([
+            b'\x03\x02\x01\x00\x00\x00',
+            '?\x00',
+            b'\x00\x00\x00',
+            b'',
+            b'no nulls',
+        ])
+        self.addCleanup(sl.shm.unlink)
+        self.addCleanup(sl.shm.close)
+
+        self.assertEqual(sl[0], b'\x03\x02\x01\x00\x00\x00')
+        self.assertEqual(sl[1], '?\x00')
+        self.assertEqual(sl[2], b'\x00\x00\x00')
+        self.assertEqual(sl[3], b'')
+        self.assertEqual(sl[4], b'no nulls')
+
+        sl2 = shared_memory.ShareableList(name=sl.shm.name)
+        self.addCleanup(sl2.shm.close)
+        self.assertEqual(sl2[0], b'\x03\x02\x01\x00\x00\x00')
+        self.assertEqual(sl2[1], '?\x00')
+        self.assertEqual(sl2[2], b'\x00\x00\x00')
+        self.assertEqual(sl2[3], b'')
+        self.assertEqual(sl2[4], b'no nulls')
+
+    def test_shared_memory_ShareableList_multibyte_utf8(self):
+        # gh-145261: ShareableList should correctly handle multi-byte
+        # UTF-8 strings without corruption or spillage.
+        sl = shared_memory.ShareableList([
+            'ascii',        # 1-byte per char (5 bytes)
+            'café',         # 2-byte char: é (5 bytes)
+            '中文测试',     # 3-byte per char (12 bytes)
+            '𐀀𐀁',         # 4-byte per char (8 bytes)
+        ])
+        self.addCleanup(sl.shm.unlink)
+        self.addCleanup(sl.shm.close)
+
+        self.assertEqual(sl[0], 'ascii')
+        self.assertEqual(sl[1], 'café')
+        self.assertEqual(sl[2], '中文测试')
+        self.assertEqual(sl[3], '𐀀𐀁')
+
+        # Verify cross-process access via name-based attachment.
+        sl2 = shared_memory.ShareableList(name=sl.shm.name)
+        self.addCleanup(sl2.shm.close)
+        self.assertEqual(sl2[0], 'ascii')
+        self.assertEqual(sl2[1], 'café')
+        self.assertEqual(sl2[2], '中文测试')
+        self.assertEqual(sl2[3], '𐀀𐀁')
+
     def test_shared_memory_cleaned_after_process_termination(self):
         cmd = '''if 1:
             import os, time, sys