Refactor sqlerror tween to add support for pyramid_retry

hopefully this doesn't break anything else..

tailbone/tweens.py

@@ -29,32 +29,46 @@ from __future__ import unicode_literals, absolute_import
 import six
 from sqlalchemy.exc import OperationalError
 
-from transaction.interfaces import TransientError
-
 
 def sqlerror_tween_factory(handler, registry):
     """
     Produces a tween which will convert ``sqlalchemy.exc.OperationalError``
-    instances (caused by database server restart) into a retryable
-    ``transaction.interfaces.TransientError`` instance, so that a second
-    attempt may be made to connect to the database before really giving up.
+    instances (caused by database server restart) into a retryable error
+    instance, so that a second attempt may be made to connect to the database
+    before really giving up.
 
     .. note::
        This tween alone is not enough to cause the transaction to be retried;
        it only marks the error as being *retryable*.  If you wish more than one
-       attempt to be made, you must define the ``tm.attempts`` setting within
-       your Pyramid app configuration.  For more info see `Retrying`_.
+       attempt to be made, you must define the ``retry.attempts`` (or
+       ``tm.attempts`` if running pyramid<1.9) setting within your Pyramid app
+       configuration.  For more info see `Retrying`_.
 
        .. _Retrying: http://docs.pylonsproject.org/projects/pyramid_tm/en/latest/#retrying
     """
 
     def sqlerror_tween(request):
+        try:
+            from pyramid_retry import mark_error_retryable
+        except ImportError:
+            mark_error_retryable = None
+            from transaction.interfaces import TransientError
+
         try:
             response = handler(request)
         except OperationalError as error:
+
+            # if connection is invalid, allow retry
             if error.connection_invalidated:
-                raise TransientError(six.text_type(error))
+                if mark_error_retryable:
+                    mark_error_retryable(error)
+                    raise error
+                else:
+                    raise TransientError(six.text_type(error))
+
+            # if connection was *not* invalid, raise original error
             raise
+
         return response
 
     return sqlerror_tween