fix(api-core): raise ValueError on missing cert source in get_client_cert_source

Author: hebaalazzeh

packages/google-api-core/google/api_core/gapic_v1/client_cert.py

@@ -67,13 +67,16 @@ def get_client_cert_source(
     Returns:
         Callable[[], Tuple[bytes, bytes]] or None: The client cert source to be used by the client.
     """
-    client_cert_source = None
     if use_cert_flag:
         if provided_cert_source:
-            client_cert_source = provided_cert_source
+            return provided_cert_source
         elif (
             hasattr(mtls, "has_default_client_cert_source")
             and mtls.has_default_client_cert_source()
         ):
-            client_cert_source = mtls.default_client_cert_source()
-    return client_cert_source
+            return mtls.default_client_cert_source()
+        else:
+            raise ValueError(
+                "Client certificate is required for mTLS, but no client certificate source was provided or found."
+            )
+    return None

packages/google-api-core/tests/unit/gapic/test_client_cert.py

@@ -91,7 +91,7 @@ def custom_hasattr(obj, name):
         (b"provided", False, True, b"default", None),
         (b"provided", True, True, b"default", b"provided"),
         (None, True, True, b"default", b"default"),
-        (None, True, False, b"default", None),
+        (None, True, False, b"default", "value_error"),
     ],
     ids=[
         "use_cert_false_no_provided",
@@ -122,4 +122,10 @@ def custom_hasattr(obj, name):
                 create=True,
                 return_value=default_val,
             ):
-                assert get_client_cert_source(provided, use_cert) == expected
+                if expected == "value_error":
+                    with pytest.raises(
+                        ValueError, match="Client certificate is required for mTLS"
+                    ):
+                        get_client_cert_source(provided, use_cert)
+                else:
+                    assert get_client_cert_source(provided, use_cert) == expected