diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -26,6 +26,7 @@
 import gc
 import hashlib
 import imp
+import mmap
 import os
 import platform as pyplatform
 import re as remod
@@ -407,6 +408,12 @@
             self._lenbuf += len(data)
             self._buffer.append(data)
 
+def mmapread(fp):
+    try:
+        return mmap.mmap(fp.fileno(), 0, access=mmap.ACCESS_READ)
+    except ValueError: # cannot mmap an empty file
+        return ''
+
 def popen2(cmd, env=None, newlines=False):
     # Setting bufsize to -1 lets the system decide the buffer size.
     # The default for bufsize is 0, meaning unbuffered. This leads to