Make Transformers use cache files when hf.co is down (#16362)

* Make Transformers use cache files when hf.co is down

* Fix tests

* Was there a random circleCI failure?

* Isolate patches

* Style

* Comment out the failure since it doesn't fail anymore

* Better comment

tests/test_modeling_common.py

@@ -23,6 +23,7 @@ import random
 import sys
 import tempfile
 import unittest
+import unittest.mock as mock
 import warnings
 from pathlib import Path
 from typing import Dict, List, Tuple
@@ -2272,6 +2273,22 @@ class ModelUtilsTest(TestCasePlus):
         for p1, p2 in zip(model.parameters(), new_model.parameters()):
             self.assertTrue(torch.equal(p1, p2))
 
+    def test_cached_files_are_used_when_internet_is_down(self):
+        # A mock response for an HTTP head request to emulate server down
+        response_mock = mock.Mock()
+        response_mock.status_code = 500
+        response_mock.headers = []
+        response_mock.raise_for_status.side_effect = HTTPError
+
+        # Download this model to make sure it's in the cache.
+        _ = BertModel.from_pretrained("hf-internal-testing/tiny-random-bert")
+
+        # Under the mock environment we get a 500 error when trying to reach the model.
+        with mock.patch("transformers.utils.hub.requests.head", return_value=response_mock) as mock_head:
+            _ = BertModel.from_pretrained("hf-internal-testing/tiny-random-bert")
+            # This check we did call the fake head request
+            mock_head.assert_called()
+
 
 @require_torch
 @is_staging_test