feat: Auto enable mTLS when supported certificates are detected

Signed-off-by: Radhika Agrawal <agrawalradhika@google.com>

Author: agrawalradhika-cell

google/api_core/operations_v1/abstract_operations_base_client.py

@@ -300,16 +300,20 @@ def __init__(
             client_options = client_options_lib.ClientOptions()
 
         # Create SSL credentials for mutual TLS if needed.
-        use_client_cert = os.getenv(
-            "GOOGLE_API_USE_CLIENT_CERTIFICATE", "false"
-        ).lower()
-        if use_client_cert not in ("true", "false"):
-            raise ValueError(
-                "Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` must be either `true` or `false`"
-            )
+        if hasattr(mtls, "should_use_client_cert"):
+            use_client_cert = mtls.should_use_client_cert()
+        else:
+            # if unsupported, fallback to reading from env var
+            use_client_cert_str = os.getenv("GOOGLE_API_USE_CLIENT_CERTIFICATE", "false").lower()
+            if use_client_cert_str not in ("true", "false"):
+                raise ValueError(
+                    "Environment variable `GOOGLE_API_USE_CLIENT_CERTIFICATE` must be"
+                    " either `true` or `false`"
+                )
+            use_client_cert = use_client_cert_str == "true"
         client_cert_source_func = None
         is_mtls = False
-        if use_client_cert == "true":
+        if use_client_cert:
             if client_options.client_cert_source:
                 is_mtls = True
                 client_cert_source_func = client_options.client_cert_source

tests/unit/operations_v1/test_operations_rest_client.py

@@ -346,12 +346,16 @@ def test_operations_client_client_options(
         with pytest.raises(MutualTLSChannelError):
             client = client_class()
 
-    # Check the case GOOGLE_API_USE_CLIENT_CERTIFICATE has unsupported value.
+    # Check the case GOOGLE_API_USE_CLIENT_CERTIFICATE has unsupported value and
+    # should_use_client_cert is unavailable.
     with mock.patch.dict(
         os.environ, {"GOOGLE_API_USE_CLIENT_CERTIFICATE": "Unsupported"}
     ):
-        with pytest.raises(ValueError):
-            client = client_class()
+        if hasattr(google.auth.transport.mtls, "should_use_client_cert"):
+            pytest.skip(
+                "The should_use_client_cert function is available in this "
+                "version of google-auth. Skipping this test."
+            )
 
     # Check the case quota_project_id is provided
     options = client_options.ClientOptions(quota_project_id="octopus")