Let others listen to SQLAlchemy errors

SQLAlchemy has 2 options of implementing error event listener. It can
either raise exception directly or it can return exception into chain.
With the first option all other handlers are ignored (ref:
http://docs.sqlalchemy.org/en/latest/core/events.html?highlight=context#sqlalchemy.events.ConnectionEvents.handle_error)

OSProfiler is interested to subscribe to SQLAlchemy error events.
However with the current approach only Oslo.DB gets the events.

Change-Id: Ia6bccd5af5f2e38f4333f1cc435f6bb9c12bc09f
Related-Bug: 1706299

oslo_db/sqlalchemy/exc_filters.py

@@ -475,11 +475,11 @@ def handler(context):
                                 if isinstance(
                                         dbe, exception.DBConnectionError):
                                     context.is_disconnect = True
-                                raise
+                                return dbe
 
 
 def register_engine(engine):
-    event.listen(engine, "handle_error", handler)
+    event.listen(engine, "handle_error", handler, retval=True)
 
     @event.listens_for(engine, "rollback_savepoint")
     def rollback_savepoint(conn, name, context):

oslo_db/tests/sqlalchemy/test_exc_filters.py

@@ -1496,3 +1496,36 @@ class TestDBConnectPingWrapping(TestsExceptionFilter):
                 self.OperationalError('%d MySQL server has gone away' % code),
                 is_disconnect=False
             )
+
+
+class TestsErrorHandler(TestsExceptionFilter):
+    def test_multiple_error_handlers(self):
+        handler = mock.MagicMock(return_value=None)
+        sqla.event.listen(self.engine, "handle_error", handler, retval=True)
+
+        # cause an error in DB API
+        self._run_test(
+            "mysql", "select you_made_a_programming_error",
+            self.ProgrammingError("Error 123, you made a mistake"),
+            exception.DBError
+        )
+
+        # expect custom handler to be called together with oslo.db's one
+        self.assertEqual(1, handler.call_count,
+                         'Custom handler should be called')
+
+    def test_chained_exceptions(self):
+        class CustomError(Exception):
+            pass
+
+        def handler(context):
+            return CustomError('Custom Error')
+
+        sqla.event.listen(self.engine, "handle_error", handler, retval=True)
+
+        # cause an error in DB API, expect exception from custom handler
+        self._run_test(
+            "mysql", "select you_made_a_programming_error",
+            self.ProgrammingError("Error 123, you made a mistake"),
+            CustomError
+        )