diff --git a/mercurial/changelog.py b/mercurial/changelog.py
--- a/mercurial/changelog.py
+++ b/mercurial/changelog.py
@@ -59,11 +59,12 @@ def stripdesc(desc):
 class appender(object):
     '''the changelog index must be updated last on disk, so we use this class
     to delay writes to it'''
-    def __init__(self, fp, buf):
+    def __init__(self, vfs, name, mode, buf):
         self.data = buf
+        fp = vfs(name, mode)
         self.fp = fp
         self.offset = fp.tell()
-        self.size = util.fstat(fp).st_size
+        self.size = vfs.fstat(fp).st_size
 
     def end(self):
         return self.size + len("".join(self.data))
@@ -114,7 +115,7 @@ def delayopener(opener, target, divert, 
         if divert:
             return opener(name + ".a", mode.replace('a', 'w'))
         # otherwise, divert to memory
-        return appender(opener(name, mode), buf)
+        return appender(opener, name, mode, buf)
     return o
 
 class changelog(revlog.revlog):
diff --git a/mercurial/scmutil.py b/mercurial/scmutil.py
--- a/mercurial/scmutil.py
+++ b/mercurial/scmutil.py
@@ -245,6 +245,9 @@ class abstractvfs(object):
     def exists(self, path=None):
         return os.path.exists(self.join(path))
 
+    def fstat(self, fp):
+        return util.fstat(fp)
+
     def isdir(self, path=None):
         return os.path.isdir(self.join(path))