diff -r c38a62af000e -r cd4c75200206 mercurial/branchmap.py
--- a/mercurial/branchmap.py	Fri Jan 04 01:07:25 2013 +0100
+++ b/mercurial/branchmap.py	Fri Jan 04 04:52:57 2013 +0100
@@ -139,7 +139,8 @@
                 for node in nodes:
                     f.write("%s %s\n" % (hex(node), encoding.fromlocal(label)))
             f.close()
-        except (IOError, OSError):
+        except (IOError, OSError, util.Abort):
+            # Abort may be raise by read only opener
             pass
 
     def update(self, repo, ctxgen):