gh-129559: Add bytearray.resize()

Doc/library/stdtypes.rst

@@ -2845,10 +2845,10 @@ objects.
 
       Resize the :class:`bytearray` using :ref:`Memory Management <memory>` APIs
       to contain *size* bytes. Data from the existing :class:`bytearray` will be
-      kept up to *size*. Any new bytes (the :class:`bytearray` grew) will be
-      unchanged from the memory allocator except one byte beyond the requested
-      *size* which will be set to NULL to ensure the :class:`bytearray` is
-      always NULL terminated.
+      kept up to *size*. If :class:`bytearray` needs to grow, all new bytes will
+      be zeroed / set to the NULL byte (``b"\0"``). One byte beyond *size* will
+      always be set to NULL to ensure the :class:`bytearray` is always NULL
+      terminated regardless of contents.
 
       .. versionadded:: next
 

Lib/test/test_bytes.py

@@ -1365,15 +1365,19 @@ def test_resize(self):
         self.assertEqual(ba, bytearray(b'abc'))
         self.assertIsNone(ba.resize(10))
         self.assertEqual(len(ba), 10)
-        # Check bytes are intact after resize including the NULL from .resize(3)
-        self.assertEqual(ba[:4], bytearray(b'abc\0'))
+        # Bytes beyond set values must be cleared.
+        self.assertEqual(ba, bytearray(b'abc\0\0\0\0\0\0\0'))
+        expected_bytearray = bytearray(b'abcdeabcde')
+        ba[3:10] = b'deabcde'
+        self.assertEqual(ba, expected_bytearray)
         self.assertIsNone(ba.resize(2**20))
         self.assertEqual(len(ba), 2**20)
-        self.assertEqual(ba[:4], bytearray(b'abc\0'))
-        # Check for NULL from .resize(10)
-        self.assertEqual(ba[10], 0)
+        self.assertEqual(ba[:10], expected_bytearray)
+        self.assertEqual(ba[10:], bytearray(b'\0' * (2 ** 20 - 10)))
         self.assertIsNone(ba.resize(0))
         self.assertEqual(ba, bytearray())
+        self.assertIsNone(ba.resize(10))
+        self.assertEqual(ba, bytearray(b'\0' * 10))
 
         ba = ByteArraySubclass(b'abcdef')
         self.assertIsNone(ba.resize(3))

Objects/bytearrayobject.c

@@ -1405,10 +1405,15 @@ static PyObject *
 bytearray_resize_impl(PyByteArrayObject *self, Py_ssize_t size)
 /*[clinic end generated code: output=f73524922990b2d9 input=75fd4d17c4aa47d3]*/
 {
+    Py_ssize_t start_size = PyByteArray_GET_SIZE(self);
     int result = PyByteArray_Resize((PyObject *)self, size);
     if (result == -1) {
         return NULL;
     }
+    // Set new bytes to provide consistent / safer behavior in Python version.
+    if (size > start_size) {
+        memset(PyByteArray_AS_STRING(self) + start_size, 0, size-start_size);
+    }
     Py_RETURN_NONE;
 }