Added interface declaration and explanation for why afterCompletion only rolls back the load connection.

buildout.cfg

@@ -2,12 +2,12 @@
 develop = .
 base-parts = test py python omelette coverage-test coverage-report
 parts = ${buildout:base-parts}
-eggs = relstorage
+eggs = relstorage [${buildout:db}]
 db =
 
 [test]
 recipe = zc.recipe.testrunner
-eggs = relstorage [test, ${buildout:db}] 
+eggs = relstorage [test, ${buildout:db}]
 
 [py]
 recipe = zc.recipe.egg

relstorage/storage.py

@@ -97,6 +97,13 @@ class _DummyLock(object):
     def release(self):
         return
 
+def optional_interfaces():
+    try:
+        from ZODB.interfaces import IMVCCAfterCompletionStorage
+        return (IMVCCAfterCompletionStorage,)
+    except ImportError:
+        return ()
+
 @implementer(ZODB.interfaces.IStorage,
              ZODB.interfaces.IMVCCStorage,
              ZODB.interfaces.IMultiCommitStorage,
@@ -104,7 +111,8 @@ class _DummyLock(object):
              ZODB.interfaces.IStorageIteration,
              ZODB.interfaces.IStorageUndoable,
              ZODB.interfaces.IBlobStorage,
-             ZODB.interfaces.IBlobStorageRestoreable)
+             ZODB.interfaces.IBlobStorageRestoreable,
+             *optional_interfaces())
 class RelStorage(UndoLogCompatible,
                  ConflictResolution.ConflictResolvingStorage):
     """Storage to a relational database, based on invalidation polling"""
@@ -1074,6 +1082,12 @@ class RelStorage(UndoLogCompatible,
             self.blobhelper.abort()
 
     def afterCompletion(self):
+        # Note that this method exists mainly to deal with read-only
+        # transactions that don't go through 2-phase commit (although
+        # it's called for all transactions).  For this reason, we only
+        # have to roll back the load connection.  The store connection
+        # is completed during normal write-transaction commit or
+        # abort.
         self._rollback_load_connection()
 
     def lastTransaction(self):

relstorage/tests/reltestbase.py

@@ -852,12 +852,20 @@ class GenericRelStorageTests(
         # outside of 2-phase commit is otherise equivalent to calling
         # tpc_abort.
         self._storage = self.make_storage(revert_when_stale=False)
+
         import mock
         with mock.patch.object(
             self._storage, '_rollback_load_connection') as rb:
             self._storage.afterCompletion()
             rb.assert_called_with()
 
+        try:
+            from ZODB.interfaces import IMVCCAfterCompletionStorage
+        except ImportError:
+            pass
+        else:
+            self.assertTrue(
+                IMVCCAfterCompletionStorage.providedBy(self._storage))
 
 from .test_zodbconvert import FSZODBConvertTests