feat: add mTLS to ads template

gapic/ads-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 +58,39 @@ 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
+    )
 
     @classmethod
     def from_service_account_file(cls, filename: str, *args, **kwargs):
@@ -92,7 +125,7 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
     def __init__(self, *,
             credentials: credentials.Credentials = None,
             transport: Union[str, {{ service.name }}Transport] = None,
-            client_options: ClientOptions = DEFAULT_OPTIONS,
+            client_options: ClientOptions = None,
             ) -> None:
         """Instantiate the {{ (service.client_name|snake_case).replace('_', ' ') }}.
 
@@ -106,6 +139,17 @@ 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 ``client_cert_source``
+                is provided, mutual TLS transport will be created with the given
+                ``api_endpoint`` or the default mTLS endpoint, and the client
+                SSL credentials obtained from ``client_cert_source``.
+
+        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)
@@ -114,16 +158,45 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
         # 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 client_options is None or (
+            client_options.api_endpoint == None
+            and client_options.client_cert_source is None
+        ):
+            # Don't trigger mTLS if we get an empty ClientOptions.
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
-                credentials=credentials,
-                host=client_options.api_endpoint{% if service.host %} or '{{ service.host }}'{% endif %},
+                credentials=credentials, host=self.DEFAULT_ENDPOINT
             )
+        else:
+            # We have a non-empty ClientOptions. If client_cert_source is
+            # provided, trigger mTLS with user provided endpoint or the default
+            # mTLS endpoint.
+            if client_options.client_cert_source:
+                api_mtls_endpoint = (
+                    client_options.api_endpoint
+                    if client_options.api_endpoint
+                    else self.DEFAULT_MTLS_ENDPOINT
+                )
+            else:
+                api_mtls_endpoint = None
+
+            api_endpoint = (
+                client_options.api_endpoint
+                if client_options.api_endpoint
+                else self.DEFAULT_ENDPOINT
+            )
+
+            self._transport = {{ service.name }}GrpcTransport(
+                credentials=credentials,
+                host=api_endpoint,
+                api_mtls_endpoint=api_mtls_endpoint,
+                client_cert_source=client_options.client_cert_source,
+              )
 
     {% for method in service.methods.values() -%}
     def {{ method.name|snake_case }}(self,

gapic/ads-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]
+        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,