bigquery: retry idempotent RPCs

Add retry logic to every RPC for which it makes sense.

Following the BigQuery team, we ignore the error code and
use the "reason" field of the error to determine whether
to retry.

Outstanding issues:

- Resumable upload consists of an initial call to get a URL,
  followed by posts to that URL. Getting the retry right on that
  initial call requires modifying the ResumableUpload class. At the
  same time, the num_retries argument should be removed.

- Users can't modify the retry behavior of Job.result(), because
  PollingFuture.result() does not accept a retry argument.

bigquery/google/cloud/bigquery/__init__.py

@@ -27,6 +27,7 @@
 __version__ = get_distribution('google-cloud-bigquery').version
 
 from google.cloud.bigquery._helpers import Row
+from google.cloud.bigquery._helpers import DEFAULT_RETRY
 from google.cloud.bigquery.client import Client
 from google.cloud.bigquery.dataset import AccessEntry
 from google.cloud.bigquery.dataset import Dataset
@@ -61,4 +62,5 @@
     'Table',
     'TableReference',
     'UDFResource',
+    'DEFAULT_RETRY',
 ]

bigquery/google/cloud/bigquery/_helpers.py

@@ -20,6 +20,7 @@
 
 import six
 
+from google.api.core import retry
 from google.cloud._helpers import UTC
 from google.cloud._helpers import _date_from_iso8601_date
 from google.cloud._helpers import _datetime_from_microseconds
@@ -520,3 +521,28 @@ def _rows_page_start(iterator, page, response):
         total_rows = int(total_rows)
     iterator.total_rows = total_rows
 # pylint: enable=unused-argument
+
+
+def _should_retry(exc):
+    """Predicate for determining when to retry.
+
+    We retry if and only if the 'reason' is 'backendError'
+    or 'rateLimitExceeded'.
+    """
+    if not hasattr(exc, 'errors'):
+        return False
+    if len(exc.errors) == 0:
+        return False
+    reason = exc.errors[0]['reason']
+    return reason == 'backendError' or reason == 'rateLimitExceeded'
+
+
+DEFAULT_RETRY = retry.Retry(predicate=_should_retry)
+"""The default retry object.
+
+Any method with a ``retry`` parameter will be retried automatically,
+with reasonable defaults. To disable retry, pass ``retry=None``.
+To modify the default retry behavior, call a ``with_XXX`` method
+on ``DEFAULT_RETRY``. For example, to change the deadline to 30 seconds,
+pass ``retry=bigquery.DEFAULT_RETRY.with_deadline(30)``.
+"""