feat: ADC can load an impersonated service account credentials. (#962)

* Make the impersonated credentials to be Scoped so default func will add scopes.

* Add tests for impersonated service account credentials.

Co-authored-by: Anthonios Partheniou <partheniou@google.com>

google/auth/impersonated_credentials.py

@@ -120,7 +120,9 @@ def _make_iam_token_request(
         six.raise_from(new_exc, caught_exc)
 
 
-class Credentials(credentials.CredentialsWithQuotaProject, credentials.Signing):
+class Credentials(
+    credentials.Scoped, credentials.CredentialsWithQuotaProject, credentials.Signing
+):
     """This module defines impersonated credentials which are essentially
     impersonated identities.
 
@@ -309,6 +311,10 @@ def service_account_email(self):
     def signer(self):
         return self
 
+    @property
+    def requires_scopes(self):
+        return not self._target_scopes
+
     @_helpers.copy_docstring(credentials.CredentialsWithQuotaProject)
     def with_quota_project(self, quota_project_id):
         return self.__class__(
@@ -321,6 +327,18 @@ def with_quota_project(self, quota_project_id):
             iam_endpoint_override=self._iam_endpoint_override,
         )
 
+    @_helpers.copy_docstring(credentials.Scoped)
+    def with_scopes(self, scopes, default_scopes=None):
+        return self.__class__(
+            self._source_credentials,
+            target_principal=self._target_principal,
+            target_scopes=scopes or default_scopes,
+            delegates=self._delegates,
+            lifetime=self._lifetime,
+            quota_project_id=self._quota_project_id,
+            iam_endpoint_override=self._iam_endpoint_override,
+        )
+
 
 class IDTokenCredentials(credentials.CredentialsWithQuotaProject):
     """Open ID Connect ID Token-based service account credentials.

tests/test__default.py

@@ -1089,6 +1089,60 @@ def test_default_no_warning_with_quota_project_id_for_user_creds(get_adc_path):
     credentials, project_id = _default.default(quota_project_id="project-foo")
 
 
+@mock.patch(
+    "google.auth._cloud_sdk.get_application_default_credentials_path", autospec=True
+)
+def test_default_impersonated_service_account(get_adc_path):
+    get_adc_path.return_value = IMPERSONATED_SERVICE_ACCOUNT_AUTHORIZED_USER_SOURCE_FILE
+
+    credentials, _ = _default.default()
+
+    assert isinstance(credentials, impersonated_credentials.Credentials)
+    assert isinstance(
+        credentials._source_credentials, google.oauth2.credentials.Credentials
+    )
+    assert credentials.service_account_email == "service-account-target@example.com"
+    assert credentials._delegates == ["service-account-delegate@example.com"]
+    assert not credentials._quota_project_id
+    assert not credentials._target_scopes
+
+
+@mock.patch(
+    "google.auth._cloud_sdk.get_application_default_credentials_path", autospec=True
+)
+def test_default_impersonated_service_account_set_scopes(get_adc_path):
+    get_adc_path.return_value = IMPERSONATED_SERVICE_ACCOUNT_AUTHORIZED_USER_SOURCE_FILE
+    scopes = ["scope1", "scope2"]
+
+    credentials, _ = _default.default(scopes=scopes)
+    assert credentials._target_scopes == scopes
+
+
+@mock.patch(
+    "google.auth._cloud_sdk.get_application_default_credentials_path", autospec=True
+)
+def test_default_impersonated_service_account_set_default_scopes(get_adc_path):
+    get_adc_path.return_value = IMPERSONATED_SERVICE_ACCOUNT_AUTHORIZED_USER_SOURCE_FILE
+    default_scopes = ["scope1", "scope2"]
+
+    credentials, _ = _default.default(default_scopes=default_scopes)
+    assert credentials._target_scopes == default_scopes
+
+
+@mock.patch(
+    "google.auth._cloud_sdk.get_application_default_credentials_path", autospec=True
+)
+def test_default_impersonated_service_account_set_both_scopes_and_default_scopes(
+    get_adc_path
+):
+    get_adc_path.return_value = IMPERSONATED_SERVICE_ACCOUNT_AUTHORIZED_USER_SOURCE_FILE
+    scopes = ["scope1", "scope2"]
+    default_scopes = ["scope3", "scope4"]
+
+    credentials, _ = _default.default(scopes=scopes, default_scopes=default_scopes)
+    assert credentials._target_scopes == scopes
+
+
 def test__get_api_key_credentials_no_env_var():
     cred, project_id = _default._get_api_key_credentials(quota_project_id="project-foo")
     assert cred is None

tests/test_impersonated_credentials.py

@@ -399,6 +399,22 @@ def test_with_quota_project_iam_endpoint_override(
         request_kwargs = request.call_args[1]
         assert request_kwargs["url"] == self.IAM_ENDPOINT_OVERRIDE
 
+    def test_with_scopes(self):
+        credentials = self.make_credentials()
+        credentials._target_scopes = []
+        assert credentials.requires_scopes is True
+        credentials = credentials.with_scopes(["fake_scope1", "fake_scope2"])
+        assert credentials.requires_scopes is False
+        assert credentials._target_scopes == ["fake_scope1", "fake_scope2"]
+
+    def test_with_scopes_provide_default_scopes(self):
+        credentials = self.make_credentials()
+        credentials._target_scopes = []
+        credentials = credentials.with_scopes(
+            ["fake_scope1"], default_scopes=["fake_scope2"]
+        )
+        assert credentials._target_scopes == ["fake_scope1"]
+
     def test_id_token_success(
         self, mock_donor_credentials, mock_authorizedsession_idtoken
     ):