api: do not log a traceback if error is not expected

Currently, when using the retry decorator on a function that raises an error on
the last retry, oslo.db always log the following:

ERROR oslo_db.api [-] DB exceeded retry limit.
ERROR oslo_db.api Traceback (most recent call last):
ERROR oslo_db.api File "/usr/local/lib/python2.7/dist-packages/oslo_db/api.py", line 138, in wrapper
ERROR oslo_db.api return f(*args, **kwargs)
ERROR oslo_db.api File "/opt/stack/gnocchi/gnocchi/indexer/sqlalchemy.py", line 686, in list_resources
ERROR oslo_db.api raise indexer.InvalidPagination("Invalid sort keys")
ERROR oslo_db.api InvalidPagination: Invalid pagination: `Invalid sort keys'

Which is wrong, since in this case the `InvalidPagination' is totally unrelated to the retry code.

This patch fixes that by only calling LOG.exception if the last try is really
about an expected retry condition.

Change-Id: Iacc6d3aeac41ddfff76f081428678e2e0688055a
Closes-Bug: #1587637

oslo_db/api.py

@@ -139,10 +139,12 @@ class wrap_db_retry(object):
                     return f(*args, **kwargs)
                 except Exception as e:
                     with excutils.save_and_reraise_exception() as ectxt:
+                        expected = self._is_exception_expected(e)
                         if remaining > 0:
-                            ectxt.reraise = not self._is_exception_expected(e)
+                            ectxt.reraise = not expected
                         else:
-                            LOG.exception(_LE('DB exceeded retry limit.'))
+                            if expected:
+                                LOG.exception(_LE('DB exceeded retry limit.'))
                             # if it's a RetryRequest, we need to unpack it
                             if isinstance(e, exception.RetryRequest):
                                 ectxt.type_ = type(e.inner_exc)