Work around a ZEO4 server bug in an edge case.

src/ZEO/tests/zeo-fan-out.test

@@ -25,6 +25,22 @@ Now, let's create some client storages that connect to these:
     >>> import os, ZEO, ZODB.blob, ZODB.POSException, transaction
 
     >>> db0 = ZEO.DB(port0, blob_dir='cb0')
+
+XXX Work around a ZEO4 server bug (that was fixed in the ZEO5 server)
+which prevents sending invalidations, and thus tids for transactions
+that only add objects.  This doesn't matter for ZODB4/ZEO4 but does
+for ZODB5/ZEO5, with it's greater reliance on MVCC.  This mainly
+affects creation of the database root object, which is typically the
+only transaction that only adds objects.  Exacerbating this further is
+that previously, databases didn't really use MVCC when checking for
+the root object, but now they do because they go through a connection.
+
+    >>> with db0.transaction() as conn:
+    ...     conn.root.work_around_zeo4_server_bug = 1
+    >>> with db0.transaction() as conn:
+    ...     del conn.root.work_around_zeo4_server_bug
+
+
     >>> db1 = ZEO.DB(addr1, blob_dir='cb1')
     >>> tm1 = transaction.TransactionManager()
     >>> c1 = db1.open(transaction_manager=tm1)