Retry transient errors in 'PollingFuture.result'. (#6305)

Closes #6301.

api_core/google/api_core/future/polling.py

@@ -28,7 +28,12 @@ class _OperationNotComplete(Exception):
     pass
 
 
-RETRY_PREDICATE = retry.if_exception_type(_OperationNotComplete)
+RETRY_PREDICATE = retry.if_exception_type(
+    _OperationNotComplete,
+    exceptions.TooManyRequests,
+    exceptions.InternalServerError,
+    exceptions.BadGateway,
+)
 DEFAULT_RETRY = retry.Retry(predicate=RETRY_PREDICATE)
 
 

api_core/tests/unit/future/test_polling.py

@@ -19,6 +19,7 @@
 import mock
 import pytest
 
+from google.api_core import exceptions
 from google.api_core.future import polling
 
 
@@ -118,6 +119,34 @@ def test_result_timeout():
         future.result(timeout=1)
 
 
+class PollingFutureImplTransient(PollingFutureImplWithPoll):
+    def __init__(self, errors):
+        super(PollingFutureImplTransient, self).__init__()
+        self._errors = errors
+
+    def done(self):
+        if self._errors:
+            error, self._errors = self._errors[0], self._errors[1:]
+            raise error('testing')
+        self.poll_count += 1
+        self.set_result(42)
+        return True
+
+
+def test_result_transient_error():
+    future = PollingFutureImplTransient((
+        exceptions.TooManyRequests,
+        exceptions.InternalServerError,
+        exceptions.BadGateway,
+    ))
+    result = future.result()
+    assert result == 42
+    assert future.poll_count == 1
+    # Repeated calls should not cause additional polling
+    assert future.result() == result
+    assert future.poll_count == 1
+
+
 def test_callback_background_thread():
     future = PollingFutureImplWithPoll()
     callback = mock.Mock()