Add failing test

src/python/grpcio_tests/tests/unit/BUILD.bazel

@@ -13,6 +13,7 @@ GRPCIO_TESTS_UNIT = [
     "_channel_connectivity_test.py",
     "_channel_ready_future_test.py",
     "_compression_test.py",
+    "_contextvars_propagation_test.py",
     "_credentials_test.py",
     "_dns_resolver_test.py",
     "_empty_message_test.py",

src/python/grpcio_tests/tests/unit/_contextvars_propagation_test.py

@@ -0,0 +1,112 @@
+# Copyright 2020 The gRPC authors.
+#
+# 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.
+"""Test of propagation of contextvars between threads."""
+
+import contextlib
+import logging
+import sys
+import unittest
+
+import grpc
+
+from tests.unit import test_common
+
+_UNARY_UNARY = "/test/UnaryUnary"
+_REQUEST = b"0000"
+
+
+def _unary_unary_handler(request, context):
+    return request
+
+
+# TODO: Test for <3.7 and 3.7+.
+
+
+def contextvars_supported():
+    return sys.version_info[0] == 3 and sys.version_info[1] >= 7
+
+
+class _GenericHandler(grpc.GenericRpcHandler):
+
+    def service(self, handler_call_details):
+        if handler_call_details.method == _UNARY_UNARY:
+            return grpc.unary_unary_rpc_method_handler(_unary_unary_handler)
+        else:
+            raise NotImplementedError()
+
+
+@contextlib.contextmanager
+def _server():
+    try:
+        server = test_common.test_server()
+        target = '[::]:0'
+        port = server.add_insecure_port(target)
+        server.add_generic_rpc_handlers((_GenericHandler(),))
+        server.start()
+        yield port
+    finally:
+        server.stop(None)
+
+
+if contextvars_supported():
+    import contextvars
+
+    _EXPECTED_VALUE = 24601
+    test_var = contextvars.ContextVar("test_var", default=None)
+    test_var.set(_EXPECTED_VALUE)
+
+    class TestCallCredentials(grpc.AuthMetadataPlugin):
+
+        def __init__(self):
+            self._recorded_value = None
+
+        def __call__(self, context, callback):
+            self._recorded_value = test_var.get()
+            callback((), None)
+
+        def assert_called(self, test):
+            test.assertEqual(_EXPECTED_VALUE, self._recorded_value)
+
+else:
+
+    class TestCallCredentials(grpc.AuthMetadataPlugin):
+
+        def __call__(self, context, callback):
+            callback((), None)
+
+        def assert_called(self, test):
+            pass
+
+
+class ContextVarsPropagationTest(unittest.TestCase):
+
+    def test_propagation_to_auth_plugin(self):
+        with _server() as port:
+            target = "localhost:{}".format(port)
+            local_credentials = grpc.local_channel_credentials()
+            test_call_credentials = TestCallCredentials()
+            call_credentials = grpc.metadata_call_credentials(
+                test_call_credentials, "test call credentials")
+            composite_credentials = grpc.composite_channel_credentials(
+                local_credentials, call_credentials)
+            with grpc.secure_channel(target, composite_credentials) as channel:
+                stub = channel.unary_unary(_UNARY_UNARY)
+                response = stub(_REQUEST)
+                self.assertEqual(_REQUEST, response)
+                test_call_credentials.assert_called(self)
+
+
+if __name__ == '__main__':
+    logging.basicConfig()
+    unittest.main(verbosity=2)