de-obfuscated some delicate locking code

src/ZODB/MappingStorage.py

@@ -261,15 +261,19 @@ class MappingStorage(object):
         self._commit_lock.release()
 
     # ZODB.interfaces.IStorage
-    @ZODB.utils.locked(opened)
     def tpc_begin(self, transaction, tid=None):
+        with self._lock:
+
+            ZODB.utils.check_precondition(self.opened)
+
             # The tid argument exists to support testing.
             if transaction is self._transaction:
                 raise ZODB.POSException.StorageTransactionError(
                     "Duplicate tpc_begin calls for same transaction")
-        self._lock.release()
+
         self._commit_lock.acquire()
-        self._lock.acquire()
+
+        with self._lock:
             self._transaction = transaction
             self._tdata = {}
             if tid is None:

src/ZODB/utils.py

@@ -268,6 +268,12 @@ def mktemp(dir=None, prefix='tmp'):
     os.close(handle)
     return filename
 
+def check_precondition(precondition):
+    if not precondition():
+        raise AssertionError(
+            "Failed precondition: ",
+            precondition.__doc__.strip())
+
 class Locked(object):
 
     def __init__(self, func, inst=None, class_=None, preconditions=()):