Added a lock on the adapter

To prevent modifying the set of instances while iterating over them.

src/ZODB/mvccadapter.py

@@ -61,11 +61,13 @@ class MVCCAdapter(Base):
     def __init__(self, storage):
         Base.__init__(self, storage)
         self._instances = set()
+        self._lock = threading.Lock()
         if hasattr(storage, 'registerDB'):
             storage.registerDB(self)
 
     def new_instance(self):
         instance = MVCCAdapterInstance(self)
+        with self._lock:
             self._instances.add(instance)
         return instance
 
@@ -76,6 +78,7 @@ class MVCCAdapter(Base):
         return UndoAdapterInstance(self)
 
     def _release(self, instance):
+        with self._lock:
             self._instances.remove(instance)
 
     closed = False
@@ -87,21 +90,24 @@ class MVCCAdapter(Base):
             del self._storage
 
     def invalidateCache(self):
+        with self._lock:
             for instance in self._instances:
                 instance._invalidateCache()
 
     def invalidate(self, transaction_id, oids, version=''):
+        with self._lock:
             for instance in self._instances:
                 instance._invalidate(oids)
 
-    references = serialize.referencesf
-    transform_record_data = untransform_record_data = lambda self, data: data
-
     def _invalidate_finish(self, oids, committing_instance):
+        with self._lock:
             for instance in self._instances:
                 if instance is not committing_instance:
                     instance._invalidate(oids)
 
+    references = serialize.referencesf
+    transform_record_data = untransform_record_data = lambda self, data: data
+
     def pack(self, pack_time, referencesf):
         return self._storage.pack(pack_time, referencesf)