fix: fix the merge issue between 1424 and 1373 (#1461)

* fix: fix the merge issue between 1424 and 1373

* Update _function_session.py

Author: jialuoo

bigframes/functions/_function_session.py

@@ -47,6 +47,7 @@
 )
 
 from bigframes import clients
+from bigframes import version as bigframes_version
 import bigframes.core.compile.ibis_types
 import bigframes.exceptions as bfe
 import bigframes.series as bf_series
@@ -265,6 +266,13 @@ def remote_function(
         .. deprecated:: 0.0.1
         This is an internal method. Please use :func:`bigframes.pandas.remote_function` instead.
 
+        .. warning::
+            To use remote functions with Bigframes 2.0 and onwards, please (preferred)
+            set an explicit user-managed ``cloud_function_service_account`` or (discouraged)
+            set ``cloud_function_service_account`` to use the Compute Engine service account
+            by setting it to `"default"`.
+            See, https://cloud.google.com/functions/docs/securing/function-identity.
+
         .. note::
             Please make sure following is setup before using this API:
 
@@ -445,6 +453,26 @@ def remote_function(
         # Some defaults may be used from the session if not provided otherwise.
         session = self._resolve_session(session)
 
+        # raise a UserWarning if user does not explicitly set cloud_function_service_account to a
+        # user-managed cloud_function_service_account of to default
+        msg = bfe.format_message(
+            "You have not explicitly set a user-managed `cloud_function_service_account`. "
+            "Using the default Compute Engine service account. "
+            "To use Bigframes 2.0, please explicitly set `cloud_function_service_account` "
+            'either to a user-managed service account (preferred) or to `"default"` '
+            "to use the Compute Engine service account (discouraged). "
+            "See, https://cloud.google.com/functions/docs/securing/function-identity."
+        )
+
+        if (
+            bigframes_version.__version__.startswith("1.")
+            and cloud_function_service_account is None
+        ):
+            warnings.warn(msg, stacklevel=2, category=FutureWarning)
+
+        if cloud_function_service_account == "default":
+            cloud_function_service_account = None
+
         # A BigQuery client is required to perform BQ operations.
         bigquery_client = self._resolve_bigquery_client(session, bigquery_client)