diff -r eff102facb15 -r 14f3795a5ed7 mercurial/cmdutil.py
--- a/mercurial/cmdutil.py	Fri Feb 11 22:24:10 2011 +0800
+++ b/mercurial/cmdutil.py	Fri Dec 24 15:23:01 2010 +0100
@@ -679,7 +679,9 @@
             parents.reverse()
         prev = (parents and parents[0]) or nullid
 
+        shouldclose = False
         if not fp:
+            shouldclose = True
             fp = make_file(repo, template, node, total=total, seqno=seqno,
                            revwidth=revwidth, mode='ab')
         if fp != sys.stdout and hasattr(fp, 'name'):
@@ -700,7 +702,8 @@
         for chunk in patch.diff(repo, prev, node, opts=opts):
             fp.write(chunk)
 
-        fp.flush()
+        if shouldclose:
+            fp.close()
 
     for seqno, rev in enumerate(revs):
         single(rev, seqno + 1, fp)