Update retry.py

Added debug level logging to respond to comment and cleaned up some comments.

Author: chalmerlowe

google/cloud/bigquery/retry.py

@@ -12,12 +12,15 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import logging
+
 from google.api_core import exceptions
 from google.api_core import retry
 import google.api_core.future.polling
 from google.auth import exceptions as auth_exceptions  # type: ignore
 import requests.exceptions
 
+_LOGGER = logging.getLogger(__name__)
 
 _RETRYABLE_REASONS = frozenset(
     ["rateLimitExceeded", "backendError", "internalError", "badGateway"]
@@ -61,14 +64,15 @@
 def _should_retry(exc):
     """Predicate for determining when to retry.
 
-    We retry if and only if the 'reason' is 'backendError'
-    or 'rateLimitExceeded'.
+    We retry if and only if the 'reason' is in _RETRYABLE_REASONS or is
+    in _UNSTRUCTURED_RETRYABLE_TYPES.
     """
     try:
         reason = exc.errors[0]["reason"]
     except (AttributeError, IndexError, TypeError, KeyError):
         # Fallback for when errors attribute is missing, empty, or not a dict
         # or doesn't contain "reason" (e.g. gRPC exceptions).
+        _LOGGER.debug("Inspecting unstructured error for retry: %r", exc)
         return isinstance(exc, _UNSTRUCTURED_RETRYABLE_TYPES)
 
     return reason in _RETRYABLE_REASONS