From a354629c1ffe3e57b766943aaee42f243844fe1c Mon Sep 17 00:00:00 2001
From: arithmetic1728 <58957152+arithmetic1728@users.noreply.github.com>
Date: Wed, 8 Apr 2020 14:54:23 -0700
Subject: [PATCH] Add mTLS support to generator (#359)

Add preliminary support for mTLS to the generated surface.
mTLS provides mutual authentication between a client and a service using certificates.
Unless an alternative or custom endpoint is provided, the client surface assumes that the mTLS endpoint for a service is the same as the non-mtls variant with the 'mtls' moniker prepended to the domain.

Client certificates can be passed explicitly or yielded via a callback.
---
 .circleci/config.yml                          |  14 +-
 .../%sub/services/%service/client.py.j2       |  75 ++++-
 .../services/%service/transports/grpc.py.j2   |  50 +++-
 gapic/templates/setup.py.j2                   |   1 +
 .../%name_%version/%sub/test_%service.py.j2   | 267 ++++++++++++++----
 noxfile.py                                    |   4 +
 6 files changed, 332 insertions(+), 79 deletions(-)

diff --git a/.circleci/config.yml b/.circleci/config.yml
index b6891e3670..1bb5158969 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -253,7 +253,7 @@ jobs:
             ln -s /usr/src/protoc/bin/protoc /usr/local/bin/protoc
       - run:
           name: Run showcase tests.
-          command: nox -s showcase_alternative_templates          
+          command: nox -s showcase_alternative_templates
   showcase-unit-3.6:
     docker:
       - image: python:3.6-slim
@@ -263,7 +263,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
@@ -287,7 +287,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
@@ -311,7 +311,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
@@ -335,7 +335,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
@@ -359,7 +359,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
@@ -383,7 +383,7 @@ jobs:
           name: Install system dependencies.
           command: |
             apt-get update
-            apt-get install -y curl pandoc unzip
+            apt-get install -y curl pandoc unzip git
       - run:
           name: Install protoc 3.7.1.
           command: |
diff --git a/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2 b/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
index 1f821b8af9..f0b7d05381 100644
--- a/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
+++ b/gapic/templates/%namespace/%name_%version/%sub/services/%service/client.py.j2
@@ -2,7 +2,8 @@
 
 {% block content %}
 from collections import OrderedDict
-from typing import Dict, {% if service.any_server_streaming %}Iterable, {% endif %}{% if service.any_client_streaming %}Iterator, {% endif %}Sequence, Tuple, Type, Union
+import re
+from typing import Callable, Dict, {% if service.any_server_streaming %}Iterable, {% endif %}{% if service.any_client_streaming %}Iterator, {% endif %}Sequence, Tuple, Type, Union
 import pkg_resources
 
 import google.api_core.client_options as ClientOptions # type: ignore
@@ -57,7 +58,40 @@ class {{ service.client_name }}Meta(type):
 class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
     """{{ service.meta.doc|rst(width=72, indent=4) }}"""
 
-    DEFAULT_OPTIONS = ClientOptions.ClientOptions({% if service.host %}api_endpoint='{{ service.host }}'{% endif %})
+    @staticmethod
+    def _get_default_mtls_endpoint(api_endpoint):
+        """Convert api endpoint to mTLS endpoint.
+        Convert "*.sandbox.googleapis.com" and "*.googleapis.com" to
+        "*.mtls.sandbox.googleapis.com" and "*.mtls.googleapis.com" respectively.
+        Args:
+            api_endpoint (Optional[str]): the api endpoint to convert.
+        Returns:
+            str: converted mTLS api endpoint.
+        """
+        if not api_endpoint:
+            return api_endpoint
+
+        mtls_endpoint_re = re.compile(
+            r"(?P<name>[^.]+)(?P<mtls>\.mtls)?(?P<sandbox>\.sandbox)?(?P<googledomain>\.googleapis\.com)?"
+        )
+
+        m = mtls_endpoint_re.match(api_endpoint)
+        name, mtls, sandbox, googledomain = m.groups()
+        if mtls or not googledomain:
+            return api_endpoint
+
+        if sandbox:
+            return api_endpoint.replace(
+                "sandbox.googleapis.com", "mtls.sandbox.googleapis.com"
+            )
+
+        return api_endpoint.replace(".googleapis.com", ".mtls.googleapis.com")
+
+    DEFAULT_ENDPOINT = {% if service.host %}'{{ service.host }}'{% else %}None{% endif %}
+    DEFAULT_MTLS_ENDPOINT = _get_default_mtls_endpoint.__func__(  # type: ignore
+        DEFAULT_ENDPOINT
+    )
+    DEFAULT_OPTIONS = ClientOptions.ClientOptions(api_endpoint=DEFAULT_ENDPOINT)
 
     @classmethod
     def from_service_account_file(cls, filename: str, *args, **kwargs):
@@ -106,23 +140,56 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
                 transport to use. If set to None, a transport is chosen
                 automatically.
             client_options (ClientOptions): Custom options for the client.
+                (1) The ``api_endpoint`` property can be used to override the
+                default endpoint provided by the client.
+                (2) If ``transport`` argument is None, ``client_options`` can be
+                used to create a mutual TLS transport. If ``api_endpoint`` is
+                provided and different from the default endpoint, or the
+                ``client_cert_source`` property is provided,  mutual TLS
+                transport will be created if client SSL credentials are found.
+                Client SSL credentials are obtained from ``client_cert_source``
+                or application default SSL credentials.
+
+        Raises:
+            google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+                creation failed for any reason.
         """
         if isinstance(client_options, dict):
             client_options = ClientOptions.from_dict(client_options)
 
+        # Set default api endpoint if not set.
+        if client_options.api_endpoint is None:
+            client_options.api_endpoint = self.DEFAULT_ENDPOINT
+
         # Save or instantiate the transport.
         # Ordinarily, we provide the transport, but allowing a custom transport
         # instance provides an extensibility point for unusual situations.
         if isinstance(transport, {{ service.name }}Transport):
+            # transport is a {{ service.name }}Transport instance.
             if credentials:
                 raise ValueError('When providing a transport instance, '
                                  'provide its credentials directly.')
             self._transport = transport
-        else:
+        elif transport is not None or (
+            client_options.api_endpoint == self.DEFAULT_ENDPOINT
+            and client_options.client_cert_source is None
+        ):
+            # Don't trigger mTLS.
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
+                credentials=credentials, host=client_options.api_endpoint
+            )
+        else:
+            # Trigger mTLS. If the user overrides endpoint, use it as the mTLS
+            # endpoint, otherwise use the default mTLS endpoint.
+            option_endpoint = client_options.api_endpoint
+            api_mtls_endpoint = self.DEFAULT_MTLS_ENDPOINT if option_endpoint == self.DEFAULT_ENDPOINT else option_endpoint
+
+            self._transport = {{ service.name }}GrpcTransport(
                 credentials=credentials,
-                host=client_options.api_endpoint{% if service.host %} or '{{ service.host }}'{% endif %},
+                host=client_options.api_endpoint,
+                api_mtls_endpoint=api_mtls_endpoint,
+                client_cert_source=client_options.client_cert_source,
             )
 
     {% for method in service.methods.values() -%}
diff --git a/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2 b/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2
index bd3b074e74..eb47dbdc52 100644
--- a/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2
+++ b/gapic/templates/%namespace/%name_%version/%sub/services/%service/transports/grpc.py.j2
@@ -1,13 +1,15 @@
 {% extends '_base.py.j2' %}
 
 {% block content %}
-from typing import Callable, Dict
+from typing import Callable, Dict, Tuple
 
 from google.api_core import grpc_helpers   # type: ignore
 {%- if service.has_lro %}
 from google.api_core import operations_v1  # type: ignore
 {%- endif %}
 from google.auth import credentials        # type: ignore
+from google.auth.transport.grpc import SslCredentials  # type: ignore
+
 
 import grpc  # type: ignore
 
@@ -35,7 +37,9 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
     def __init__(self, *,
             host: str{% if service.host %} = '{{ service.host }}'{% endif %},
             credentials: credentials.Credentials = None,
-            channel: grpc.Channel = None) -> None:
+            channel: grpc.Channel = None,
+            api_mtls_endpoint: str = None,
+            client_cert_source: Callable[[], Tuple[bytes, bytes]] = None) -> None:
         """Instantiate the transport.
 
         Args:
@@ -49,19 +53,51 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
                 This argument is ignored if ``channel`` is provided.
             channel (Optional[grpc.Channel]): A ``Channel`` instance through
                 which to make calls.
+            api_mtls_endpoint (Optional[str]): The mutual TLS endpoint. If
+                provided, it overrides the ``host`` argument and tries to create
+                a mutual TLS channel with client SSL credentials from
+                ``client_cert_source`` or applicatin default SSL credentials.
+            client_cert_source (Optional[Callable[[], Tuple[bytes, bytes]]]): A
+                callback to provide client SSL certificate bytes and private key
+                bytes, both in PEM format. It is ignored if ``api_mtls_endpoint``
+                is None.
+
+        Raises:
+          google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+              creation failed for any reason.
         """
-        # Sanity check: Ensure that channel and credentials are not both
-        # provided.
         if channel:
+            # Sanity check: Ensure that channel and credentials are not both
+            # provided.
             credentials = False
 
+            # If a channel was explicitly provided, set it.
+            self._grpc_channel = channel
+        elif api_mtls_endpoint:
+            host = api_mtls_endpoint if ":" in api_mtls_endpoint else api_mtls_endpoint + ":443"
+
+            # Create SSL credentials with client_cert_source or application
+            # default SSL credentials.
+            if client_cert_source:
+                cert, key = client_cert_source()
+                ssl_credentials = grpc.ssl_channel_credentials(
+                    certificate_chain=cert, private_key=key
+                )
+            else:
+                ssl_credentials = SslCredentials().ssl_credentials
+
+            # create a new channel. The provided one is ignored.
+            self._grpc_channel = grpc_helpers.create_channel(
+                host,
+                credentials=credentials,
+                ssl_credentials=ssl_credentials,
+                scopes=self.AUTH_SCOPES,
+            )
+
         # Run the base constructor.
         super().__init__(host=host, credentials=credentials)
         self._stubs = {}  # type: Dict[str, Callable]
 
-        # If a channel was explicitly provided, set it.
-        if channel:
-            self._grpc_channel = channel
 
     @classmethod
     def create_channel(cls,
diff --git a/gapic/templates/setup.py.j2 b/gapic/templates/setup.py.j2
index 9d408a23dd..91bff5f82b 100644
--- a/gapic/templates/setup.py.j2
+++ b/gapic/templates/setup.py.j2
@@ -16,6 +16,7 @@ setuptools.setup(
     platforms='Posix; MacOS X; Windows',
     include_package_data=True,
     install_requires=(
+        'google-auth >= 1.13.1',
         'google-api-core >= 1.8.0, < 2.0.0dev',
         'googleapis-common-protos >= 1.5.8',
         'grpcio >= 1.10.0',
diff --git a/gapic/templates/tests/unit/%name_%version/%sub/test_%service.py.j2 b/gapic/templates/tests/unit/%name_%version/%sub/test_%service.py.j2
index 3916e795d9..e55cc99909 100644
--- a/gapic/templates/tests/unit/%name_%version/%sub/test_%service.py.j2
+++ b/gapic/templates/tests/unit/%name_%version/%sub/test_%service.py.j2
@@ -15,6 +15,7 @@ from google.oauth2 import service_account
 from {{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }} import {{ service.client_name }}
 from {{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }} import transports
 from google.api_core import client_options
+from google.api_core import grpc_helpers
 {% if service.has_lro -%}
 from google.api_core import future
 from google.api_core import operations_v1
@@ -30,6 +31,25 @@ from google.longrunning import operations_pb2
 {% endfilter %}
 
 
+def client_cert_source_callback():
+    return b"cert bytes", b"key bytes"
+
+
+def test__get_default_mtls_endpoint():
+    api_endpoint = "example.googleapis.com"
+    api_mtls_endpoint = "example.mtls.googleapis.com"
+    sandbox_endpoint = "example.sandbox.googleapis.com"
+    sandbox_mtls_endpoint = "example.mtls.sandbox.googleapis.com"
+    non_googleapi = "api.example.com"
+
+    assert {{ service.client_name }}._get_default_mtls_endpoint(None) == None
+    assert {{ service.client_name }}._get_default_mtls_endpoint(api_endpoint) == api_mtls_endpoint
+    assert {{ service.client_name }}._get_default_mtls_endpoint(api_mtls_endpoint) == api_mtls_endpoint
+    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_endpoint) == sandbox_mtls_endpoint
+    assert {{ service.client_name }}._get_default_mtls_endpoint(sandbox_mtls_endpoint) == sandbox_mtls_endpoint
+    assert {{ service.client_name }}._get_default_mtls_endpoint(non_googleapi) == non_googleapi
+
+
 def test_{{ service.client_name|snake_case }}_from_service_account_file():
     creds = credentials.AnonymousCredentials()
     with mock.patch.object(service_account.Credentials, 'from_service_account_file') as factory:
@@ -45,25 +65,65 @@ def test_{{ service.client_name|snake_case }}_from_service_account_file():
 
 def test_{{ service.client_name|snake_case }}_client_options():
     # Check the default options have their expected values.
-    {% if service.host %}assert {{ service.client_name }}.DEFAULT_OPTIONS.api_endpoint == '{{ service.host }}'{% endif %}
+    assert {{ service.client_name }}.DEFAULT_OPTIONS.api_endpoint == {% if service.host %}'{{ service.host }}'{% else %}None{% endif %}
+    assert {{ service.client_name }}.DEFAULT_OPTIONS.api_endpoint == {{ service.client_name }}.DEFAULT_ENDPOINT
 
-    # Check that options can be customized.
-    options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
+    # Check that if channel is provided we won't create a new one.
+    with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.{{ service.client_name }}.get_transport_class') as gtc:
+        transport = transports.{{ service.name }}GrpcTransport(
+            credentials=credentials.AnonymousCredentials()
+        )
+        client = {{ service.client_name }}(transport=transport)
+        gtc.assert_not_called()
+
+    # Check mTLS is not triggered with empty client options.
+    options = client_options.ClientOptions()
     with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.{{ service.client_name }}.get_transport_class') as gtc:
         transport = gtc.return_value = mock.MagicMock()
-        client = {{ service.client_name }}(
-            client_options=options
+        client = {{ service.client_name }}(client_options=options)
+        transport.assert_called_once_with(
+            credentials=None,
+            host=client.DEFAULT_ENDPOINT,
         )
-        transport.assert_called_once_with(credentials=None, host="squid.clam.whelk")
 
+    # Check mTLS is triggered with api endpoint override.
+    options = client_options.ClientOptions(api_endpoint="squid.clam.whelk")
+    with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.transports.{{ service.name }}GrpcTransport.__init__') as grpc_transport:
+        grpc_transport.return_value = None
+        client = {{ service.client_name }}(client_options=options)
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint="squid.clam.whelk",
+            client_cert_source=None,
+            credentials=None,
+            host="squid.clam.whelk",
+        )
+
+    # Check mTLS is triggered if client_cert_source is provided.
+    options = client_options.ClientOptions(
+        client_cert_source=client_cert_source_callback
+    )
+    with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.transports.{{ service.name }}GrpcTransport.__init__') as grpc_transport:
+        grpc_transport.return_value = None
+        client = {{ service.client_name }}(client_options=options)
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
+            client_cert_source=client_cert_source_callback,
+            credentials=None,
+            host=client.DEFAULT_ENDPOINT,
+        )
 
 def test_{{ service.client_name|snake_case }}_client_options_from_dict():
-    with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.{{ service.client_name }}.get_transport_class') as gtc:
-        transport = gtc.return_value = mock.MagicMock()
+    with mock.patch('{{ (api.naming.module_namespace + (api.naming.versioned_module_name,) + service.meta.address.subpackage)|join(".") }}.services.{{ service.name|snake_case }}.transports.{{ service.name }}GrpcTransport.__init__') as grpc_transport:
+        grpc_transport.return_value = None
         client = {{ service.client_name }}(
             client_options={'api_endpoint': 'squid.clam.whelk'}
         )
-        transport.assert_called_once_with(credentials=None, host="squid.clam.whelk")
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint="squid.clam.whelk",
+            client_cert_source=None,
+            credentials=None,
+            host="squid.clam.whelk",
+        )
 
 
 {% for method in service.methods.values() -%}
@@ -154,7 +214,7 @@ def test_{{ method.name|snake_case }}_field_headers():
             '__call__') as call:
         call.return_value = {{ method.output.ident }}()
         client.{{ method.name|snake_case }}(request)
-        
+
         # Establish that the underlying gRPC stub method was called.
         assert len(call.mock_calls) == 1
         _, args, _ = call.mock_calls[0]
@@ -200,57 +260,57 @@ def test_{{ method.name|snake_case }}_from_dict():
 
 {% endif %}
 
-{% if method.flattened_fields %} 
-def test_{{ method.name|snake_case }}_flattened(): 
-    client = {{ service.client_name }}( 
-        credentials=credentials.AnonymousCredentials(), 
-    ) 
-
-    # Mock the actual call within the gRPC stub, and fake the request. 
-    with mock.patch.object( 
-            type(client._transport.{{ method.name|snake_case }}), 
-            '__call__') as call: 
-        # Designate an appropriate return value for the call. 
-        {% if method.void -%} 
-        call.return_value = None 
-        {% elif method.lro -%} 
-        call.return_value = operations_pb2.Operation(name='operations/op') 
-        {% elif method.server_streaming -%} 
-        call.return_value = iter([{{ method.output.ident }}()]) 
-        {% else -%} 
-        call.return_value = {{ method.output.ident }}() 
-        {% endif %} 
-        # Call the method with a truthy value for each flattened field, 
-        # using the keyword arguments to the method. 
-        response = client.{{ method.name|snake_case }}( 
-            {%- for field in method.flattened_fields.values() %} 
-            {{ field.name }}={{ field.mock_value }}, 
-            {%- endfor %} 
-        ) 
-
-        # Establish that the underlying call was made with the expected 
-        # request object values. 
-        assert len(call.mock_calls) == 1 
-        _, args, _ = call.mock_calls[0] 
+{% if method.flattened_fields %}
+def test_{{ method.name|snake_case }}_flattened():
+    client = {{ service.client_name }}(
+        credentials=credentials.AnonymousCredentials(),
+    )
+
+    # Mock the actual call within the gRPC stub, and fake the request.
+    with mock.patch.object(
+            type(client._transport.{{ method.name|snake_case }}),
+            '__call__') as call:
+        # Designate an appropriate return value for the call.
+        {% if method.void -%}
+        call.return_value = None
+        {% elif method.lro -%}
+        call.return_value = operations_pb2.Operation(name='operations/op')
+        {% elif method.server_streaming -%}
+        call.return_value = iter([{{ method.output.ident }}()])
+        {% else -%}
+        call.return_value = {{ method.output.ident }}()
+        {% endif %}
+        # Call the method with a truthy value for each flattened field,
+        # using the keyword arguments to the method.
+        response = client.{{ method.name|snake_case }}(
+            {%- for field in method.flattened_fields.values() %}
+            {{ field.name }}={{ field.mock_value }},
+            {%- endfor %}
+        )
+
+        # Establish that the underlying call was made with the expected
+        # request object values.
+        assert len(call.mock_calls) == 1
+        _, args, _ = call.mock_calls[0]
         {% for key, field in method.flattened_fields.items() -%}
-        assert args[0].{{ key }} == {{ field.mock_value }} 
-        {% endfor %} 
-
-
-def test_{{ method.name|snake_case }}_flattened_error(): 
-    client = {{ service.client_name }}( 
-        credentials=credentials.AnonymousCredentials(), 
-    ) 
-
-    # Attempting to call a method with both a request object and flattened 
-    # fields is an error. 
-    with pytest.raises(ValueError): 
-        client.{{ method.name|snake_case }}( 
-            {{ method.input.ident }}(), 
-            {%- for field in method.flattened_fields.values() %} 
-            {{ field.name }}={{ field.mock_value }}, 
-            {%- endfor %} 
-        ) 
+        assert args[0].{{ key }} == {{ field.mock_value }}
+        {% endfor %}
+
+
+def test_{{ method.name|snake_case }}_flattened_error():
+    client = {{ service.client_name }}(
+        credentials=credentials.AnonymousCredentials(),
+    )
+
+    # Attempting to call a method with both a request object and flattened
+    # fields is an error.
+    with pytest.raises(ValueError):
+        client.{{ method.name|snake_case }}(
+            {{ method.input.ident }}(),
+            {%- for field in method.flattened_fields.values() %}
+            {{ field.name }}={{ field.mock_value }},
+            {%- endfor %}
+        )
 {% endif %}
 
 
@@ -509,10 +569,95 @@ def test_{{ service.name|snake_case }}_host_with_port():
 
 def test_{{ service.name|snake_case }}_grpc_transport_channel():
     channel = grpc.insecure_channel('http://localhost/')
+
+    # Check that if channel is provided, mtls endpoint and client_cert_source
+    # won't be used.
+    callback = mock.MagicMock()
     transport = transports.{{ service.name }}GrpcTransport(
+        host="squid.clam.whelk",
         channel=channel,
+        api_mtls_endpoint="mtls.squid.clam.whelk",
+        client_cert_source=callback,
+    )
+    assert transport.grpc_channel == channel
+    assert transport._host == "squid.clam.whelk:443"
+    assert not callback.called
+
+
+@mock.patch("grpc.ssl_channel_credentials", autospec=True)
+@mock.patch("google.api_core.grpc_helpers.create_channel", autospec=True)
+def test_{{ service.name|snake_case }}_grpc_transport_channel_mtls_with_client_cert_source(
+    grpc_create_channel, grpc_ssl_channel_cred
+):
+    # Check that if channel is None, but api_mtls_endpoint and client_cert_source
+    # are provided, then a mTLS channel will be created.
+    mock_cred = mock.Mock()
+
+    mock_ssl_cred = mock.Mock()
+    grpc_ssl_channel_cred.return_value = mock_ssl_cred
+
+    mock_grpc_channel = mock.Mock()
+    grpc_create_channel.return_value = mock_grpc_channel
+
+    transport = transports.{{ service.name }}GrpcTransport(
+        host="squid.clam.whelk",
+        credentials=mock_cred,
+        api_mtls_endpoint="mtls.squid.clam.whelk",
+        client_cert_source=client_cert_source_callback,
     )
-    assert transport.grpc_channel is channel
+    grpc_ssl_channel_cred.assert_called_once_with(
+        certificate_chain=b"cert bytes", private_key=b"key bytes"
+    )
+    grpc_create_channel.assert_called_once_with(
+        "mtls.squid.clam.whelk:443",
+        credentials=mock_cred,
+        ssl_credentials=mock_ssl_cred,
+        scopes=(
+            {%- for scope in service.oauth_scopes %}
+            '{{ scope }}',
+            {%- endfor %}
+        ),
+    )
+    assert transport.grpc_channel == mock_grpc_channel
+
+
+@pytest.mark.parametrize(
+    "api_mtls_endpoint", ["mtls.squid.clam.whelk", "mtls.squid.clam.whelk:443"]
+)
+@mock.patch("google.api_core.grpc_helpers.create_channel", autospec=True)
+def test_{{ service.name|snake_case }}_grpc_transport_channel_mtls_with_adc(
+    grpc_create_channel, api_mtls_endpoint
+):
+    # Check that if channel and client_cert_source are None, but api_mtls_endpoint
+    # is provided, then a mTLS channel will be created with SSL ADC.
+    mock_grpc_channel = mock.Mock()
+    grpc_create_channel.return_value = mock_grpc_channel
+
+    # Mock google.auth.transport.grpc.SslCredentials class.
+    mock_ssl_cred = mock.Mock()
+    with mock.patch.multiple(
+        "google.auth.transport.grpc.SslCredentials",
+        __init__=mock.Mock(return_value=None),
+        ssl_credentials=mock.PropertyMock(return_value=mock_ssl_cred),
+    ):
+        mock_cred = mock.Mock()
+        transport = transports.{{ service.name }}GrpcTransport(
+            host="squid.clam.whelk",
+            credentials=mock_cred,
+            api_mtls_endpoint=api_mtls_endpoint,
+            client_cert_source=None,
+        )
+        grpc_create_channel.assert_called_once_with(
+            "mtls.squid.clam.whelk:443",
+            credentials=mock_cred,
+            ssl_credentials=mock_ssl_cred,
+            scopes=(
+                {%- for scope in service.oauth_scopes %}
+                '{{ scope }}',
+                {%- endfor %}
+            ),
+        )
+        assert transport.grpc_channel == mock_grpc_channel
 
 
 {% if service.has_lro -%}
diff --git a/noxfile.py b/noxfile.py
index cfee241ad2..691e88163d 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -118,6 +118,10 @@ def showcase_unit(
     """Run the generated unit tests against the Showcase library."""
 
     # Install pytest and gapic-generator-python
+    session.install(
+        "-e",
+        "git+https://github.com/googleapis/python-api-core.git@ca6c41cf460e505e6b228263170927270626222a#egg=google-api-core",
+    )
     session.install('coverage', 'pytest', 'pytest-cov', 'pytest-xdist',)
     session.install('.')