feat: define useful properties on `google.auth.external_account.Credentials` (#770)

This includes the following properties:

- `info`: This is the reverse of `from_info` defined on subclasses and useful to
  serialize external account credentials.
- `service_account_email`: This is the corresponding service account email if impersonation is used.
- `is_user`: This is `False` for workload identity pools and `True` for workforce pools (not yet supported).
  This can be mainly determined from the STS audience.

While the properties will primarily facilitate integration with gcloud, they are publicly useful for other contexts.

4 files changed, 231 insertions, 8 deletions

diff --git a/google/auth/external_account.py b/google/auth/external_account.py
index 0429ee0..e40c652 100644
--- a/google/auth/external_account.py
+++ b/google/auth/external_account.py
@@ -28,8 +28,10 @@ token exchange endpoint following the `OAuth 2.0 Token Exchange`_ spec.
 """
 
 import abc
+import copy
 import datetime
 import json
+import re
 
 import six
 
@@ -40,6 +42,8 @@ from google.auth import impersonated_credentials
 from google.oauth2 import sts
 from google.oauth2 import utils
 
+# External account JSON type identifier.
+_EXTERNAL_ACCOUNT_JSON_TYPE = "external_account"
 # The token exchange grant_type used for exchanging credentials.
 _STS_GRANT_TYPE = "urn:ietf:params:oauth:grant-type:token-exchange"
 # The token exchange requested_token_type. This is always an access_token.
@@ -118,6 +122,76 @@ class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
         self._project_id = None
 
     @property
+    def info(self):
+        """Generates the dictionary representation of the current credentials.
+
+        Returns:
+            Mapping: The dictionary representation of the credentials. This is the
+                reverse of "from_info" defined on the subclasses of this class. It is
+                useful for serializing the current credentials so it can deserialized
+                later.
+        """
+        config_info = {
+            "type": _EXTERNAL_ACCOUNT_JSON_TYPE,
+            "audience": self._audience,
+            "subject_token_type": self._subject_token_type,
+            "token_url": self._token_url,
+            "service_account_impersonation_url": self._service_account_impersonation_url,
+            "credential_source": copy.deepcopy(self._credential_source),
+            "quota_project_id": self._quota_project_id,
+            "client_id": self._client_id,
+            "client_secret": self._client_secret,
+        }
+        # Remove None fields in the info dictionary.
+        for k, v in dict(config_info).items():
+            if v is None:
+                del config_info[k]
+
+        return config_info
+
+    @property
+    def service_account_email(self):
+        """Returns the service account email if service account impersonation is used.
+
+        Returns:
+            Optional[str]: The service account email if impersonation is used. Otherwise
+                None is returned.
+        """
+        if self._service_account_impersonation_url:
+            # Parse email from URL. The formal looks as follows:
+            # https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/name@project-id.iam.gserviceaccount.com:generateAccessToken
+            url = self._service_account_impersonation_url
+            start_index = url.rfind("/")
+            end_index = url.find(":generateAccessToken")
+            if start_index != -1 and end_index != -1 and start_index < end_index:
+                start_index = start_index + 1
+                return url[start_index:end_index]
+        return None
+
+    @property
+    def is_user(self):
+        """Returns whether the credentials represent a user (True) or workload (False).
+        Workloads behave similarly to service accounts. Currently workloads will use
+        service account impersonation but will eventually not require impersonation.
+        As a result, this property is more reliable than the service account email
+        property in determining if the credentials represent a user or workload.
+
+        Returns:
+            bool: True if the credentials represent a user. False if they represent a
+                workload.
+        """
+        # If service account impersonation is used, the credentials will always represent a
+        # service account.
+        if self._service_account_impersonation_url:
+            return False
+        # Workforce pools representing users have the following audience format:
+        # //iam.googleapis.com/locations/$location/workforcePools/$poolId/providers/$providerId
+        p = re.compile(r"//iam\.googleapis\.com/locations/[^/]+/workforcePools/")
+        if p.match(self._audience):
+            return True
+        return False
+
+    @property
     def requires_scopes(self):
         """Checks if the credentials requires scopes.
 
@@ -282,14 +356,8 @@ class Credentials(credentials.Scoped, credentials.CredentialsWithQuotaProject):
         )
 
         # Determine target_principal.
-        start_index = self._service_account_impersonation_url.rfind("/")
-        end_index = self._service_account_impersonation_url.find(":generateAccessToken")
-        if start_index != -1 and end_index != -1 and start_index < end_index:
-            start_index = start_index + 1
-            target_principal = self._service_account_impersonation_url[
-                start_index:end_index
-            ]
-        else:
+        target_principal = self.service_account_email
+        if not target_principal:
             raise exceptions.RefreshError(
                 "Unable to determine target principal from service account impersonation URL."
             )
diff --git a/tests/test_aws.py b/tests/test_aws.py
index 7c7ee36..9ca08d5 100644
--- a/tests/test_aws.py
+++ b/tests/test_aws.py
@@ -919,6 +919,19 @@ class TestCredentials(object):
 
         assert excinfo.match(r"aws version '3' is not supported in the current build.")
 
+    def test_info(self):
+        credentials = self.make_credentials(
+            credential_source=self.CREDENTIAL_SOURCE.copy()
+        )
+
+        assert credentials.info == {
+            "type": "external_account",
+            "audience": AUDIENCE,
+            "subject_token_type": SUBJECT_TOKEN_TYPE,
+            "token_url": TOKEN_URL,
+            "credential_source": self.CREDENTIAL_SOURCE,
+        }
+
     def test_retrieve_subject_token_missing_region_url(self):
         # When AWS_REGION envvar is not available, region_url is required for
         # determining the current AWS region.
diff --git a/tests/test_external_account.py b/tests/test_external_account.py
index 8f8d980..7390fb9 100644
--- a/tests/test_external_account.py
+++ b/tests/test_external_account.py
@@ -31,6 +31,12 @@ CLIENT_SECRET = "password"
 # Base64 encoding of "username:password"
 BASIC_AUTH_ENCODING = "dXNlcm5hbWU6cGFzc3dvcmQ="
 SERVICE_ACCOUNT_EMAIL = "service-1234@service-name.iam.gserviceaccount.com"
+# List of valid workforce pool audiences.
+TEST_USER_AUDIENCES = [
+    "//iam.googleapis.com/locations/global/workforcePools/pool-id/providers/provider-id",
+    "//iam.googleapis.com/locations/eu/workforcePools/pool-id/providers/provider-id",
+    "//iam.googleapis.com/locations/eu/workforcePools/workloadIdentityPools/providers/provider-id",
+]
 
 
 class CredentialsImpl(external_account.Credentials):
@@ -342,6 +348,116 @@ class TestCredentials(object):
             r"Unable to determine target principal from service account impersonation URL."
         )
 
+    def test_info(self):
+        credentials = self.make_credentials()
+
+        assert credentials.info == {
+            "type": "external_account",
+            "audience": self.AUDIENCE,
+            "subject_token_type": self.SUBJECT_TOKEN_TYPE,
+            "token_url": self.TOKEN_URL,
+            "credential_source": self.CREDENTIAL_SOURCE.copy(),
+        }
+
+    def test_info_with_full_options(self):
+        credentials = self.make_credentials(
+            client_id=CLIENT_ID,
+            client_secret=CLIENT_SECRET,
+            quota_project_id=self.QUOTA_PROJECT_ID,
+            service_account_impersonation_url=self.SERVICE_ACCOUNT_IMPERSONATION_URL,
+        )
+
+        assert credentials.info == {
+            "type": "external_account",
+            "audience": self.AUDIENCE,
+            "subject_token_type": self.SUBJECT_TOKEN_TYPE,
+            "token_url": self.TOKEN_URL,
+            "service_account_impersonation_url": self.SERVICE_ACCOUNT_IMPERSONATION_URL,
+            "credential_source": self.CREDENTIAL_SOURCE.copy(),
+            "quota_project_id": self.QUOTA_PROJECT_ID,
+            "client_id": CLIENT_ID,
+            "client_secret": CLIENT_SECRET,
+        }
+
+    def test_service_account_email_without_impersonation(self):
+        credentials = self.make_credentials()
+
+        assert credentials.service_account_email is None
+
+    def test_service_account_email_with_impersonation(self):
+        credentials = self.make_credentials(
+            service_account_impersonation_url=self.SERVICE_ACCOUNT_IMPERSONATION_URL
+        )
+
+        assert credentials.service_account_email == SERVICE_ACCOUNT_EMAIL
+
+    @pytest.mark.parametrize(
+        "audience",
+        # Workload identity pool audiences or invalid workforce pool audiences.
+        [
+            # Legacy K8s audience format.
+            "identitynamespace:1f12345:my_provider",
+            (
+                "//iam.googleapis.com/projects/123456/locations/"
+                "global/workloadIdentityPools/pool-id/providers/"
+                "provider-id"
+            ),
+            (
+                "//iam.googleapis.com/projects/123456/locations/"
+                "eu/workloadIdentityPools/pool-id/providers/"
+                "provider-id"
+            ),
+            # Pool ID with workforcePools string.
+            (
+                "//iam.googleapis.com/projects/123456/locations/"
+                "global/workloadIdentityPools/workforcePools/providers/"
+                "provider-id"
+            ),
+            # Unrealistic / incorrect workforce pool audiences.
+            "//iamgoogleapis.com/locations/eu/workforcePools/pool-id/providers/provider-id",
+            "//iam.googleapiscom/locations/eu/workforcePools/pool-id/providers/provider-id",
+            "//iam.googleapis.com/locations/workforcePools/pool-id/providers/provider-id",
+            "//iam.googleapis.com/locations/eu/workforcePool/pool-id/providers/provider-id",
+            "//iam.googleapis.com/locations//workforcePool/pool-id/providers/provider-id",
+        ],
+    )
+    def test_is_user_with_non_users(self, audience):
+        credentials = CredentialsImpl(
+            audience=audience,
+            subject_token_type=self.SUBJECT_TOKEN_TYPE,
+            token_url=self.TOKEN_URL,
+            credential_source=self.CREDENTIAL_SOURCE,
+        )
+
+        assert credentials.is_user is False
+
+    @pytest.mark.parametrize("audience", TEST_USER_AUDIENCES)
+    def test_is_user_with_users(self, audience):
+        credentials = CredentialsImpl(
+            audience=audience,
+            subject_token_type=self.SUBJECT_TOKEN_TYPE,
+            token_url=self.TOKEN_URL,
+            credential_source=self.CREDENTIAL_SOURCE,
+        )
+
+        assert credentials.is_user is True
+
+    @pytest.mark.parametrize("audience", TEST_USER_AUDIENCES)
+    def test_is_user_with_users_and_impersonation(self, audience):
+        # Initialize the credentials with service account impersonation.
+        credentials = CredentialsImpl(
+            audience=audience,
+            subject_token_type=self.SUBJECT_TOKEN_TYPE,
+            token_url=self.TOKEN_URL,
+            credential_source=self.CREDENTIAL_SOURCE,
+            service_account_impersonation_url=self.SERVICE_ACCOUNT_IMPERSONATION_URL,
+        )
+
+        # Even though the audience is for a workforce pool, since service account
+        # impersonation is used, the credentials will represent a service account and
+        # not a user.
+        assert credentials.is_user is False
+
     @mock.patch("google.auth._helpers.utcnow", return_value=datetime.datetime.min)
     def test_refresh_without_client_auth_success(self, unused_utcnow):
         response = self.SUCCESS_RESPONSE.copy()
diff --git a/tests/test_identity_pool.py b/tests/test_identity_pool.py
index 90a0e25..b529268 100644
--- a/tests/test_identity_pool.py
+++ b/tests/test_identity_pool.py
@@ -430,6 +430,32 @@ class TestCredentials(object):
             r"Missing subject_token_field_name for JSON credential_source format"
         )
 
+    def test_info_with_file_credential_source(self):
+        credentials = self.make_credentials(
+            credential_source=self.CREDENTIAL_SOURCE_TEXT_URL.copy()
+        )
+
+        assert credentials.info == {
+            "type": "external_account",
+            "audience": AUDIENCE,
+            "subject_token_type": SUBJECT_TOKEN_TYPE,
+            "token_url": TOKEN_URL,
+            "credential_source": self.CREDENTIAL_SOURCE_TEXT_URL,
+        }
+
+    def test_info_with_url_credential_source(self):
+        credentials = self.make_credentials(
+            credential_source=self.CREDENTIAL_SOURCE_JSON_URL.copy()
+        )
+
+        assert credentials.info == {
+            "type": "external_account",
+            "audience": AUDIENCE,
+            "subject_token_type": SUBJECT_TOKEN_TYPE,
+            "token_url": TOKEN_URL,
+            "credential_source": self.CREDENTIAL_SOURCE_JSON_URL,
+        }
+
     def test_retrieve_subject_token_missing_subject_token(self, tmpdir):
         # Provide empty text file.
         empty_file = tmpdir.join("empty.txt")