feat: add GOOGLE_API_USE_MTLS support

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/client.py.j2

@@ -2,6 +2,7 @@
 
 {% block content %}
 from collections import OrderedDict
+import os
 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
@@ -11,6 +12,8 @@ from google.api_core import exceptions                 # type: ignore
 from google.api_core import gapic_v1                   # type: ignore
 from google.api_core import retry as retries           # type: ignore
 from google.auth import credentials                    # type: ignore
+from google.auth.transport import mtls                 # type: ignore
+from google.auth.exceptions import MutualTLSChannelError  # type: ignore
 from google.oauth2 import service_account              # type: ignore
 
 {% filter sort_lines -%}
@@ -144,21 +147,47 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
             transport (Union[str, ~.{{ service.name }}Transport]): The
                 transport to use. If set to None, a transport is chosen
                 automatically.
-            client_options (ClientOptions): Custom options for the client.
+            client_options (ClientOptions): Custom options for the client. It
+                won't take effect unless ``transport`` is None.
                 (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``.
+                default endpoint provided by the client. GOOGLE_API_USE_MTLS
+                environment variable can also be used to override the endpoint:
+                "Always" (always use the default mTLS endpoint), "Never" (always
+                use the default regular endpoint, this is the default value for
+                the environment variable) and "Auto" (auto switch to the default
+                mTLS endpoint if client SSL credentials is present). However,
+                the ``api_endpoint`` property takes precedence if provided.
+                (2) The ``client_cert_source`` property is used to provide client
+                SSL credentials for mutual TLS transport. If not provided, the
+                default SSL credentials will be used if present.
 
         Raises:
-            google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+            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)
+        if client_options is None:
+            client_options = ClientOptions.ClientOptions()
+
+        if transport is None and client_options.api_endpoint is None:
+            use_mtls_env = os.getenv("GOOGLE_API_USE_MTLS", "Never")
+            if use_mtls_env == "Never":
+                client_options.api_endpoint = self.DEFAULT_ENDPOINT
+            elif use_mtls_env == "Always":
+                client_options.api_endpoint = self.DEFAULT_MTLS_ENDPOINT
+            elif use_mtls_env == "Auto":
+                has_client_cert_source = (
+                    client_options.client_cert_source is not None
+                    or mtls.has_default_client_cert_source()
+                )
+                client_options.api_endpoint = (
+                    self.DEFAULT_MTLS_ENDPOINT if has_client_cert_source else self.DEFAULT_ENDPOINT
+                )
+            else:
+                raise MutualTLSChannelError(
+                    "Unsupported GOOGLE_API_USE_MTLS value. Accepted values: Never, Auto, Always"
+                )
 
         # Save or instantiate the transport.
         # Ordinarily, we provide the transport, but allowing a custom transport
@@ -169,38 +198,16 @@ class {{ service.client_name }}(metaclass={{ service.client_name }}Meta):
                 raise ValueError('When providing a transport instance, '
                                  'provide its credentials directly.')
             self._transport = transport
-        elif client_options is None or (
-            client_options.api_endpoint is None
-            and client_options.client_cert_source is None
-        ):
-            # Don't trigger mTLS if we get an empty ClientOptions.
+        elif isinstance(transport, str):
             Transport = type(self).get_transport_class(transport)
             self._transport = Transport(
                 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,
+                host=client_options.api_endpoint,
+                api_mtls_endpoint=client_options.api_endpoint,
                 client_cert_source=client_options.client_cert_source,
               )
 

gapic/ads-templates/%namespace/%name/%version/%sub/services/%service/transports/grpc.py.j2

@@ -7,6 +7,7 @@ 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 import auth                    # type: ignore
 from google.auth import credentials        # type: ignore
 from google.auth.transport.grpc import SslCredentials  # type: ignore
 
@@ -63,7 +64,7 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
                 is None.
 
         Raises:
-          google.auth.exceptions.MutualTlsChannelError: If mutual TLS transport
+          google.auth.exceptions.MutualTLSChannelError: If mutual TLS transport
               creation failed for any reason.
         """
         if channel:
@@ -76,6 +77,9 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
         elif api_mtls_endpoint:
             host = api_mtls_endpoint if ":" in api_mtls_endpoint else api_mtls_endpoint + ":443"
 
+            if credentials is None:
+                credentials, _ = auth.default(scopes=self.AUTH_SCOPES)
+
             # Create SSL credentials with client_cert_source or application
             # default SSL credentials.
             if client_cert_source:
@@ -96,7 +100,7 @@ class {{ service.name }}GrpcTransport({{ service.name }}Transport):
 
         # Run the base constructor.
         super().__init__(host=host, credentials=credentials)
-        self._stubs = {}  # type: Dict[str, Callable]    
+        self._stubs = {}  # type: Dict[str, Callable]
 
 
     @classmethod

gapic/ads-templates/tests/unit/%name_%version/%sub/test_%service.py.j2

@@ -1,6 +1,7 @@
 {% extends "_base.py.j2" %}
 
 {% block content %}
+import os
 from unittest import mock
 
 import grpc
@@ -11,6 +12,7 @@ import pytest
 {% filter sort_lines -%}
 from google import auth
 from google.auth import credentials
+from google.auth.exceptions import MutualTLSChannelError
 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
@@ -63,6 +65,14 @@ def test_{{ service.client_name|snake_case }}_from_service_account_file():
         {% if service.host %}assert client._transport._host == '{{ service.host }}{% if ":" not in service.host %}:443{% endif %}'{% endif %}
 
 
+def test_{{ service.client_name|snake_case }}_get_transport_class():
+    transport = {{ service.client_name }}.get_transport_class()
+    assert transport == transports.{{ service.name }}GrpcTransport
+
+    transport = {{ service.client_name }}.get_transport_class("grpc")
+    assert transport == transports.{{ service.name }}GrpcTransport
+
+
 def test_{{ service.client_name|snake_case }}_client_options():
     # 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:
@@ -72,58 +82,99 @@ def test_{{ service.client_name|snake_case }}_client_options():
         client = {{ service.client_name }}(transport=transport)
         gtc.assert_not_called()
 
-    # Check mTLS is not triggered with empty client options.
-    options = client_options.ClientOptions()
+    # Check that if channel is provided via str we will 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 = gtc.return_value = mock.MagicMock()
-        client = {{ service.client_name }}(client_options=options)
-        transport.assert_called_once_with(
-            credentials=None,
-            host=client.DEFAULT_ENDPOINT,
-        )
+        client = {{ service.client_name }}(transport="grpc")
+        gtc.assert_called()
 
-    # Check mTLS is not triggered if api_endpoint is provided but
-    # client_cert_source is None.
+    # Check the case api_endpoint is provided.
     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=None,
+            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
-    )
+    # Check the case api_endpoint is not provided and GOOGLE_API_USE_MTLS is
+    # "Never".
+    os.environ["GOOGLE_API_USE_MTLS"] = "Never"
     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)
+        client = {{ service.client_name }}()
         grpc_transport.assert_called_once_with(
-            api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
-            client_cert_source=client_cert_source_callback,
+            api_mtls_endpoint=client.DEFAULT_ENDPOINT,
+            client_cert_source=None,
             credentials=None,
             host=client.DEFAULT_ENDPOINT,
         )
 
-    # Check mTLS is triggered if api_endpoint and client_cert_source are provided.
-    options = client_options.ClientOptions(
-        api_endpoint="squid.clam.whelk",
-        client_cert_source=client_cert_source_callback
-    )
+    # Check the case api_endpoint is not provided and GOOGLE_API_USE_MTLS is
+    # "Always".
+    os.environ["GOOGLE_API_USE_MTLS"] = "Always"
+    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 }}()
+        grpc_transport.assert_called_once_with(
+            api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
+            client_cert_source=None,
+            credentials=None,
+            host=client.DEFAULT_MTLS_ENDPOINT,
+        )
+
+    # Check the case api_endpoint is not provided, GOOGLE_API_USE_MTLS is
+    # "Auto", and client_cert_source is provided.
+    os.environ["GOOGLE_API_USE_MTLS"] = "Auto"
+    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="squid.clam.whelk",
+            api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
             client_cert_source=client_cert_source_callback,
             credentials=None,
-            host="squid.clam.whelk",
+            host=client.DEFAULT_MTLS_ENDPOINT,
         )
 
+    # Check the case api_endpoint is not provided, GOOGLE_API_USE_MTLS is
+    # "Auto", and default_client_cert_source is provided.
+    os.environ["GOOGLE_API_USE_MTLS"] = "Auto"
+    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:
+        with mock.patch('google.auth.transport.mtls.has_default_client_cert_source', return_value=True):
+            grpc_transport.return_value = None
+            client = {{ service.client_name }}()
+            grpc_transport.assert_called_once_with(
+                api_mtls_endpoint=client.DEFAULT_MTLS_ENDPOINT,
+                client_cert_source=None,
+                credentials=None,
+                host=client.DEFAULT_MTLS_ENDPOINT,
+            )
+
+    # Check the case api_endpoint is not provided, GOOGLE_API_USE_MTLS is
+    # "Auto", but client_cert_source and default_client_cert_source are None.
+    os.environ["GOOGLE_API_USE_MTLS"] = "Auto"
+    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:
+        with mock.patch('google.auth.transport.mtls.has_default_client_cert_source', return_value=False):
+            grpc_transport.return_value = None
+            client = {{ service.client_name }}()
+            grpc_transport.assert_called_once_with(
+                api_mtls_endpoint=client.DEFAULT_ENDPOINT,
+                client_cert_source=None,
+                credentials=None,
+                host=client.DEFAULT_ENDPOINT,
+            )
+
+    # Check the case api_endpoint is not provided and GOOGLE_API_USE_MTLS has
+    # unsupported value.
+    os.environ["GOOGLE_API_USE_MTLS"] = "Unsupported"
+    with pytest.raises(MutualTLSChannelError):
+        client = {{ service.client_name }}()
+
+    del os.environ["GOOGLE_API_USE_MTLS"]
+
 
 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 }}.transports.{{ service.name }}GrpcTransport.__init__') as grpc_transport:
@@ -132,7 +183,7 @@ def test_{{ service.client_name|snake_case }}_client_options_from_dict():
             client_options={'api_endpoint': 'squid.clam.whelk'}
         )
         grpc_transport.assert_called_once_with(
-            api_mtls_endpoint=None,
+            api_mtls_endpoint="squid.clam.whelk",
             client_cert_source=None,
             credentials=None,
             host="squid.clam.whelk",
@@ -490,12 +541,24 @@ def test_{{ service.name|snake_case }}_auth_adc():
         ))
 
 
+def test_{{ service.name|snake_case }}_transport_auth_adc():
+    # If credentials and host are not provided, the transport class should use
+    # ADC credentials.
+    with mock.patch.object(auth, 'default') as adc:
+        adc.return_value = (credentials.AnonymousCredentials(), None)
+        transports.{{ service.name }}GrpcTransport(host="squid.clam.whelk")
+        adc.assert_called_once_with(scopes=(
+            {%- for scope in service.oauth_scopes %}
+            '{{ scope }}',
+            {%- endfor %}
+        ))
+
+
 def test_{{ service.name|snake_case }}_host_no_port():
     {% with host = (service.host|default('localhost', true)).split(':')[0] -%}
     client = {{ service.client_name }}(
         credentials=credentials.AnonymousCredentials(),
         client_options=client_options.ClientOptions(api_endpoint='{{ host }}'),
-        transport='grpc',
     )
     assert client._transport._host == '{{ host }}:443'
     {% endwith %}
@@ -506,7 +569,6 @@ def test_{{ service.name|snake_case }}_host_with_port():
     client = {{ service.client_name }}(
         credentials=credentials.AnonymousCredentials(),
         client_options=client_options.ClientOptions(api_endpoint='{{ host }}:8000'),
-        transport='grpc',
     )
     assert client._transport._host == '{{ host }}:8000'
     {% endwith %}