feat: Add Pluggable auth support (#988)

* Port identity pool credentials

* access_token retrieved

* -> pluggable

* Update pluggable.py

* Create test_pluggable.py

* Unit tests

* Address pr issues

google/auth/_default.py

@@ -325,6 +325,12 @@ def _get_external_account_credentials(
         credentials = aws.Credentials.from_info(
             info, scopes=scopes, default_scopes=default_scopes
         )
+    elif info.get("credential_source").get("executable") is not None:
+        from google.auth import pluggable
+
+        credentials = pluggable.Credentials.from_info(
+            info, scopes=scopes, default_scopes=default_scopes
+        )
     else:
         try:
             # Check if configuration corresponds to an Identity Pool credentials.

google/auth/pluggable.py

@@ -0,0 +1,273 @@
+# Copyright 2022 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Pluggable Credentials.
+
+This module provides credentials to access Google Cloud resources from on-prem
+or non-Google Cloud platforms which support external credentials (e.g. OIDC ID
+tokens) retrieved from local file locations or local servers. This includes
+Microsoft Azure and OIDC identity providers (e.g. K8s workloads registered with
+Hub with Hub workload identity enabled).
+
+These credentials are recommended over the use of service account credentials
+in on-prem/non-Google Cloud platforms as they do not involve the management of
+long-live service account private keys.
+
+Pluggable Credentials are initialized using external_account arguments which
+are typically loaded from third-party executables. Unlike other
+credentials that can be initialized with a list of explicit arguments, secrets
+or credentials, external account clients use the environment and hints/guidelines
+provided by the external_account JSON file to retrieve credentials and exchange
+them for Google access tokens.
+"""
+
+try:
+    from collections.abc import Mapping
+# Python 2.7 compatibility
+except ImportError:  # pragma: NO COVER
+    from collections import Mapping
+import io
+import json
+import os
+import subprocess
+
+from google.auth import _helpers
+from google.auth import exceptions
+from google.auth import external_account
+
+# External account JSON type identifier.
+EXECUTABLE_SUPPORTED_MAX_VERSION = 1
+
+class Credentials(external_account.Credentials):
+    """External account credentials sourced from executables."""
+
+    def __init__(
+        self,
+        audience,
+        subject_token_type,
+        token_url,
+        credential_source,
+        service_account_impersonation_url=None,
+        client_id=None,
+        client_secret=None,
+        quota_project_id=None,
+        scopes=None,
+        default_scopes=None,
+        workforce_pool_user_project=None,
+    ):
+        """Instantiates an external account credentials object from a executables.
+
+        Args:
+            audience (str): The STS audience field.
+            subject_token_type (str): The subject token type.
+            token_url (str): The STS endpoint URL.
+            credential_source (Mapping): The credential source dictionary used to
+                provide instructions on how to retrieve external credential to be
+                exchanged for Google access tokens.
+
+                Example credential_source for pluggable credential::
+
+                    {
+                        "executable": {
+                            "command": "/path/to/get/credentials.sh --arg1=value1 --arg2=value2",
+                            "timeout_millis": 5000,
+                            "output_file": "/path/to/generated/cached/credentials"
+                        }
+                    }
+
+            service_account_impersonation_url (Optional[str]): The optional service account
+                impersonation getAccessToken URL.
+            client_id (Optional[str]): The optional client ID.
+            client_secret (Optional[str]): The optional client secret.
+            quota_project_id (Optional[str]): The optional quota project ID.
+            scopes (Optional[Sequence[str]]): Optional scopes to request during the
+                authorization grant.
+            default_scopes (Optional[Sequence[str]]): Default scopes passed by a
+                Google client library. Use 'scopes' for user-defined scopes.
+            workforce_pool_user_project (Optona[str]): The optional workforce pool user
+                project number when the credential corresponds to a workforce pool and not
+                a workload Pluggable. The underlying principal must still have
+                serviceusage.services.use IAM permission to use the project for
+                billing/quota.
+
+        Raises:
+            google.auth.exceptions.RefreshError: If an error is encountered during
+                access token retrieval logic.
+            ValueError: For invalid parameters.
+
+        .. note:: Typically one of the helper constructors
+            :meth:`from_file` or
+            :meth:`from_info` are used instead of calling the constructor directly.
+        """
+
+        super(Credentials, self).__init__(
+            audience=audience,
+            subject_token_type=subject_token_type,
+            token_url=token_url,
+            credential_source=credential_source,
+            service_account_impersonation_url=service_account_impersonation_url,
+            client_id=client_id,
+            client_secret=client_secret,
+            quota_project_id=quota_project_id,
+            scopes=scopes,
+            default_scopes=default_scopes,
+            workforce_pool_user_project=workforce_pool_user_project,
+        )
+        if not isinstance(credential_source, Mapping):
+            self._credential_source_executable = None
+            raise ValueError(
+                "Missing credential_source. The credential_source is not a dict."
+            )
+        else:
+            self._credential_source_executable = credential_source.get("executable")
+            if not self._credential_source_executable:
+                raise ValueError(
+                    "Missing credential_source. An 'executable' must be provided."
+                )
+            self._credential_source_executable_command = self._credential_source_executable.get("command")
+            self._credential_source_executable_timeout_millis = self._credential_source_executable.get("timeout_millis")
+            self._credential_source_executable_output_file = self._credential_source_executable.get("output_file")
+
+            # environment_id is only supported in AWS or dedicated future external
+            # account credentials.
+            if "environment_id" in credential_source:
+                raise ValueError(
+                    "Invalid Pluggable credential_source field 'environment_id'"
+                )
+
+        if not self._credential_source_executable_command:
+            raise ValueError(
+                "Missing command. Executable command must be provided."
+            )
+        if not self._credential_source_executable_timeout_millis:
+            raise ValueError(
+                "Missing timeout_millis. Executable timeout millis must be provided."
+            )
+
+    @_helpers.copy_docstring(external_account.Credentials)
+    def retrieve_subject_token(self, request):
+        env_allow_executables = os.environ.get('GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES')
+        if env_allow_executables != '1':
+            raise ValueError(
+                "Executables need to be explicitly allowed (set GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES to '1') to run."
+            )
+
+        # Inject env vars
+        original_audience = os.getenv("GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE")
+        os.environ["GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE"] = self._audience
+        original_subject_token_type = os.getenv("GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE")
+        os.environ["GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE"] = self._subject_token_type
+        original_interactive = os.getenv("GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE")
+        os.environ["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"] = "0" # Always set to 0 until interactive mode is implemented.
+        original_service_account_impersonation_url = os.getenv("GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL")
+        if self._service_account_impersonation_url is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL"] = self._service_account_impersonation_url
+        original_credential_source_executable_output_file = os.getenv("GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE")
+        if self._credential_source_executable_output_file is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE"] = self._credential_source_executable_output_file
+
+        result = subprocess.run(self._credential_source_executable_command.split(), timeout=self._credential_source_executable_timeout_millis/1000, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
+
+        # Reset env vars
+        if original_audience is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE"] = original_audience
+        else:
+            del os.environ["GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE"]
+        if original_subject_token_type is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE"] = self.original_subject_token_type
+        else:
+            del os.environ["GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE"]
+        if original_interactive is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"] = original_interactive
+        else:
+            del os.environ["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"]
+        if original_service_account_impersonation_url is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL"] = original_service_account_impersonation_url
+        elif os.getenv("GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL") is not None:
+            del os.environ["GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL"]
+        if original_credential_source_executable_output_file is not None:
+            os.environ["GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE"] = original_credential_source_executable_output_file
+        elif os.getenv("GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE") is not None:
+            del os.environ["GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE"]
+
+        if result.returncode != 0:
+            raise exceptions.RefreshError(
+                "Executable exited with non-zero return code {}. Error: {}".format(result.returncode, result.stdout)
+            )
+        else:
+            data = result.stdout.decode('utf-8')
+            response = json.loads(data)
+            if not response['success']:
+                raise exceptions.RefreshError(
+                    "Executable returned unsuccessful response: {}.".format(response)
+                )
+            elif response['version'] > EXECUTABLE_SUPPORTED_MAX_VERSION:
+                raise exceptions.RefreshError(
+                    "Executable returned unsupported version {}.".format(response['version'])
+                )
+            elif response["token_type"] == "urn:ietf:params:oauth:token-type:jwt" or response["token_type"] == "urn:ietf:params:oauth:token-type:id_token": # OIDC
+                return response["id_token"]
+            elif response["token_type"] == "urn:ietf:params:oauth:token-type:saml2": # SAML
+                return response["saml_response"]
+            else:
+                raise exceptions.RefreshError(
+                    "Executable returned unsupported token type."
+                ) 
+
+    @classmethod
+    def from_info(cls, info, **kwargs):
+        """Creates a Pluggable Credentials instance from parsed external account info.
+
+        Args:
+            info (Mapping[str, str]): The Pluggable external account info in Google
+                format.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.pluggable.Credentials: The constructed
+                credentials.
+
+        Raises:
+            ValueError: For invalid parameters.
+        """
+        return cls(
+            audience=info.get("audience"),
+            subject_token_type=info.get("subject_token_type"),
+            token_url=info.get("token_url"),
+            service_account_impersonation_url=info.get(
+                "service_account_impersonation_url"
+            ),
+            client_id=info.get("client_id"),
+            client_secret=info.get("client_secret"),
+            credential_source=info.get("credential_source"),
+            quota_project_id=info.get("quota_project_id"),
+            workforce_pool_user_project=info.get("workforce_pool_user_project"),
+            **kwargs
+        )
+
+    @classmethod
+    def from_file(cls, filename, **kwargs):
+        """Creates an Pluggable Credentials instance from an external account json file.
+
+        Args:
+            filename (str): The path to the Pluggable external account json file.
+            kwargs: Additional arguments to pass to the constructor.
+
+        Returns:
+            google.auth.pluggable.Credentials: The constructed
+                credentials.
+        """
+        with io.open(filename, "r", encoding="utf-8") as json_file:
+            data = json.load(json_file)
+            return cls.from_info(data, **kwargs)