Merge pull request #22471 from lidizheng/auth-meta

Correct the docstring and implementation of AuthMetadataPlugin

src/python/grpcio/grpc/__init__.py

@@ -628,7 +628,7 @@ class AuthMetadataPlugin(six.with_metaclass(abc.ABCMeta)):
     def __call__(self, context, callback):
         """Implements authentication by passing metadata to a callback.
 
-        Implementations of this method must not block.
+        This method will be invoked asynchronously in a separate thread.
 
         Args:
           context: An AuthMetadataContext providing information on the RPC that

src/python/grpcio/grpc/_auth.py

@@ -14,7 +14,6 @@
 """GRPCAuthMetadataPlugins for standard authentication."""
 
 import inspect
-from concurrent import futures
 
 import grpc
 
@@ -24,43 +23,29 @@ def _sign_request(callback, token, error):
     callback(metadata, error)
 
 
-def _create_get_token_callback(callback):
-
-    def get_token_callback(future):
-        try:
-            access_token = future.result().access_token
-        except Exception as exception:  # pylint: disable=broad-except
-            _sign_request(callback, None, exception)
-        else:
-            _sign_request(callback, access_token, None)
-
-    return get_token_callback
-
-
 class GoogleCallCredentials(grpc.AuthMetadataPlugin):
     """Metadata wrapper for GoogleCredentials from the oauth2client library."""
 
     def __init__(self, credentials):
         self._credentials = credentials
-        self._pool = futures.ThreadPoolExecutor(max_workers=1)
-
         # Hack to determine if these are JWT creds and we need to pass
         # additional_claims when getting a token
         self._is_jwt = 'additional_claims' in inspect.getargspec(  # pylint: disable=deprecated-method
             credentials.get_access_token).args
 
     def __call__(self, context, callback):
-        # MetadataPlugins cannot block (see grpc.beta.interfaces.py)
-        if self._is_jwt:
-            future = self._pool.submit(
-                self._credentials.get_access_token,
-                additional_claims={'aud': context.service_url})
+        try:
+            if self._is_jwt:
+                access_token = self._credentials.get_access_token(
+                    additional_claims={
+                        'aud': context.service_url
+                    }).access_token
+            else:
+                access_token = self._credentials.get_access_token().access_token
+        except Exception as exception:  # pylint: disable=broad-except
+            _sign_request(callback, None, exception)
         else:
-            future = self._pool.submit(self._credentials.get_access_token)
-        future.add_done_callback(_create_get_token_callback(callback))
-
-    def __del__(self):
-        self._pool.shutdown(wait=False)
+            _sign_request(callback, access_token, None)
 
 
 class AccessTokenAuthMetadataPlugin(grpc.AuthMetadataPlugin):