Fix metadata tests

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

@@ -255,8 +255,8 @@ class MetadataCodeDetailsTest(unittest.TestCase):
 
         response_iterator_call = self._unary_stream(
             _SERIALIZED_REQUEST, metadata=_CLIENT_METADATA)
-        received_initial_metadata = response_iterator_call.initial_metadata()
         list(response_iterator_call)
+        received_initial_metadata = response_iterator_call.initial_metadata()
 
         self.assertTrue(
             test_common.metadata_transmitted(
@@ -347,13 +347,17 @@ class MetadataCodeDetailsTest(unittest.TestCase):
             self._servicer.set_details(_DETAILS)
             self._servicer.set_abort_call()
 
+
             response_iterator_call = self._unary_stream(
                 _SERIALIZED_REQUEST, metadata=_CLIENT_METADATA)
-            received_initial_metadata = \
-                response_iterator_call.initial_metadata()
+            # NOTE: In the single-threaded case, we cannot grab the initial_metadata
+            # without running the RPC first (or concurrently, in another
+            # thread).
             with self.assertRaises(grpc.RpcError):
                 self.assertEqual(len(list(response_iterator_call)), 0)
 
+            received_initial_metadata = \
+                response_iterator_call.initial_metadata()
             self.assertTrue(
                 test_common.metadata_transmitted(
                     _CLIENT_METADATA,
@@ -454,9 +458,9 @@ class MetadataCodeDetailsTest(unittest.TestCase):
 
         response_iterator_call = self._unary_stream(
             _SERIALIZED_REQUEST, metadata=_CLIENT_METADATA)
-        received_initial_metadata = response_iterator_call.initial_metadata()
         with self.assertRaises(grpc.RpcError):
             list(response_iterator_call)
+        received_initial_metadata = response_iterator_call.initial_metadata()
 
         self.assertTrue(
             test_common.metadata_transmitted(
@@ -547,9 +551,9 @@ class MetadataCodeDetailsTest(unittest.TestCase):
 
         response_iterator_call = self._unary_stream(
             _SERIALIZED_REQUEST, metadata=_CLIENT_METADATA)
-        received_initial_metadata = response_iterator_call.initial_metadata()
         with self.assertRaises(grpc.RpcError):
             list(response_iterator_call)
+        received_initial_metadata = response_iterator_call.initial_metadata()
 
         self.assertTrue(
             test_common.metadata_transmitted(

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

@@ -202,6 +202,9 @@ class MetadataTest(unittest.TestCase):
     def testUnaryStream(self):
         multi_callable = self._channel.unary_stream(_UNARY_STREAM)
         call = multi_callable(_REQUEST, metadata=_INVOCATION_METADATA)
+        # NOTE(gnossen): In the single-threaded case, we must consume at least
+        # one message before the initial metadata will show up.
+        next(call)
         self.assertTrue(
             test_common.metadata_transmitted(_EXPECTED_INITIAL_METADATA,
                                              call.initial_metadata()))