[tf.data] Prevent mutation of numpy arrays returned through as_numpy_iterator.

PiperOrigin-RevId: 355551968
Change-Id: I3ce0c4f014ab47088d44d9f75013b3b63b3bce76

tensorflow/python/data/kernel_tests/as_numpy_iterator_test.py

@@ -38,6 +38,14 @@ class AsNumpyIteratorTest(test_base.DatasetTestBase, parameterized.TestCase):
     ds = dataset_ops.Dataset.range(3)
     self.assertEqual([0, 1, 2], list(ds.as_numpy_iterator()))
 
+  @combinations.generate(test_base.eager_only_combinations())
+  def testImmutable(self):
+    ds = dataset_ops.Dataset.from_tensors([1, 2, 3])
+    arr = next(ds.as_numpy_iterator())
+    with self.assertRaisesRegex(ValueError,
+                                'assignment destination is read-only'):
+      arr[0] = 0
+
   @combinations.generate(test_base.eager_only_combinations())
   def testNestedStructure(self):
     point = collections.namedtuple('Point', ['x', 'y'])

tensorflow/python/data/ops/dataset_ops.py

@@ -4004,7 +4004,9 @@ class _NumpyIterator(object):
     def to_numpy(x):
       numpy = x._numpy()  # pylint: disable=protected-access
       if isinstance(numpy, np.ndarray):
-        return np.asarray(memoryview(numpy))
+        # `numpy` shares the same underlying buffer as the `x` Tensor.
+        # Tensors are expected to be immutable, so we disable writes.
+        numpy.setflags(write=False)
       return numpy
 
     return nest.map_structure(to_numpy, next(self._iterator))