.

zrace.py

@@ -40,8 +40,7 @@ Below is timing diagram on how the bug happens on ZODB5:
                                                         # which T1 queues in its _storage._invalidations
 
             # T1 retrieves queued invalidations which _includes_
-            # invalidation for transaction that T2 just has committed,
-            # that goes past @head.
+            # invalidation for transaction that T2 just has committed past @head.
             ._storage._lock.acquire()
                 r = _storage._invalidations
             ._storage._lock.release()
@@ -53,6 +52,14 @@ Below is timing diagram on how the bug happens on ZODB5:
         ._cache.invalidate(invalidated)
 
 
+The program simulates two clients: one (T2) constantly modifies two integer
+objects preserving invariant that their values stay equal. The other client
+(T1) constatly opens the database and verifies the invariant. T1 forces access
+to one of the object to always go through loading from the database, and this
+way if live cache becomes stale the bug is observed as invariant breakage.
+
+XXX ZODB4 and ZODB3
+
 [1] https://github.com/zopefoundation/ZODB/blob/5.5.1-29-g0b3db5aee/src/ZODB/Connection.py#L734-L742
 [2] https://github.com/zopefoundation/ZODB/blob/5.5.1-29-g0b3db5aee/src/ZODB/mvccadapter.py#L130-L139
 """