offline mode for firewalled envs (#10407)

* offline mode start

* add specific values

* fix fallback

* add test

* better values check and range

* test that actually works

* document the offline mode

* Apply suggestions from code review

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* more strict check

* cleaner test

* pt-only test

* style

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

tests/test_offline.py

@@ -0,0 +1,53 @@
+# Copyright 2020 The HuggingFace Team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import subprocess
+import sys
+
+from transformers.testing_utils import TestCasePlus, require_torch
+
+
+class OfflineTests(TestCasePlus):
+    @require_torch
+    def test_offline_mode(self):
+
+        # this test is a bit tricky since TRANSFORMERS_OFFLINE can only be changed before
+        # `transformers` is loaded, and it's too late for inside pytest - so we are changing it
+        # while running an external program
+
+        # python one-liner segments
+        load = "from transformers import BertConfig, BertModel, BertTokenizer;"
+        run = "mname = 'lysandre/tiny-bert-random'; BertConfig.from_pretrained(mname) and BertModel.from_pretrained(mname) and BertTokenizer.from_pretrained(mname);"
+        mock = 'import socket; exec("def offline_socket(*args, **kwargs): raise socket.error(\\"Offline mode is enabled.\\")"); socket.socket = offline_socket;'
+
+        # baseline - just load from_pretrained with normal network
+        cmd = [sys.executable, "-c", f"{load} {run}"]
+
+        # should succeed
+        env = self.get_env()
+        result = subprocess.run(cmd, env=env, check=False, capture_output=True)
+        self.assertEqual(result.returncode, 0, result.stderr)
+
+        # next emulate no network
+        cmd = [sys.executable, "-c", f"{load} {mock} {run}"]
+
+        # should normally fail as it will fail to lookup the model files w/o the network
+        env["TRANSFORMERS_OFFLINE"] = "0"
+        result = subprocess.run(cmd, env=env, check=False, capture_output=True)
+        self.assertEqual(result.returncode, 1, result.stderr)
+
+        # should succeed as TRANSFORMERS_OFFLINE=1 tells it to use local files
+        env["TRANSFORMERS_OFFLINE"] = "1"
+        result = subprocess.run(cmd, env=env, check=False, capture_output=True)
+        self.assertEqual(result.returncode, 0, result.stderr)