X wcfs/xbtreetest/treegen.py: Fix it on ZODB4

There is no TransactionMetaData on ZODB4.

wcfs/internal/xbtree/xbtreetest/treegen.py

@@ -129,7 +129,19 @@ import sys
 from golang import func, defer, panic
 from golang import time
 from ZODB import DB
-from ZODB.Connection import Connection, TransactionMetaData
+from ZODB.Connection import Connection
+
+# XXX   vvv way interferes with recover / sys.exc_info on py2
+# TODO  -> fix recover on py2
+"""
+try:
+    from ZODB.Connection import TransactionMetaData
+except ImportError: # ZODB4
+    TransactionMetaData = None
+"""
+import ZODB.Connection
+TransactionMetaData = getattr(ZODB.Connection, 'TransactionMetaData', None)
+
 from ZODB.MappingStorage import MappingStorage
 import transaction
 import itertools
@@ -477,7 +489,10 @@ def commitDelete(obj, description): # -> tid
 
     # deleteObject works only at IStorage level, and at that low level
     # zstor requires ZODB.IStorageTransactionMetaData not txn (ITransaction)
+    if TransactionMetaData is not None:
         txn_stormeta = TransactionMetaData(txn.user, description, txn.extension)
+    else:
+        txn_stormeta = txn # ZODB4
     zstor.tpc_begin(txn_stormeta)
     zstor.deleteObject(obj._p_oid, obj._p_serial, txn_stormeta)
     zstor.tpc_vote(txn_stormeta)