diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -270,7 +270,7 @@ class changelog(revlog.revlog):
         self._nodecache = r._nodecache
         self._chunkcache = r._chunkcache
 
-    def _writepending(self):
+    def _writepending(self, tr):
         "create a file containing the unfinalized state for pretxnchangegroup"
         if self._delaybuf:
             # make a temporary copy of the index
diff --git a/mercurial/transaction.py b/mercurial/transaction.py
--- a/mercurial/transaction.py
+++ b/mercurial/transaction.py
@@ -281,6 +281,8 @@ class transaction(object):
     def addpending(self, category, callback):
         """add a callback to be called when the transaction is pending
 
+        The transaction will be given as callback's first argument.
+
         Category is a unique identifier to allow overwriting an old callback
         with a newer callback.
         """
@@ -294,7 +296,7 @@ class transaction(object):
         categories = sorted(self._pendingcallback)
         for cat in categories:
             # remove callback since the data will have been flushed
-            any = self._pendingcallback.pop(cat)()
+            any = self._pendingcallback.pop(cat)(self)
             self._anypending = self._anypending or any
         return self._anypending