diff -r 644adf9c20fb -r 03127e580980 mercurial/loggingutil.py
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/mercurial/loggingutil.py	Sun Nov 18 18:25:37 2018 +0900
@@ -0,0 +1,59 @@
+# loggingutil.py - utility for logging events
+#
+# Copyright 2010 Nicolas Dumazet
+# Copyright 2013 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+
+from __future__ import absolute_import
+
+import errno
+
+from . import (
+    pycompat,
+)
+
+def openlogfile(ui, vfs, name, maxfiles=0, maxsize=0):
+    def rotate(oldpath, newpath):
+        try:
+            vfs.unlink(newpath)
+        except OSError as err:
+            if err.errno != errno.ENOENT:
+                ui.debug("warning: cannot remove '%s': %s\n" %
+                         (newpath, err.strerror))
+        try:
+            if newpath:
+                vfs.rename(oldpath, newpath)
+        except OSError as err:
+            if err.errno != errno.ENOENT:
+                ui.debug("warning: cannot rename '%s' to '%s': %s\n" %
+                         (newpath, oldpath, err.strerror))
+
+    if maxsize > 0:
+        try:
+            st = vfs.stat(name)
+        except OSError:
+            pass
+        else:
+            if st.st_size >= maxsize:
+                path = vfs.join(name)
+                for i in pycompat.xrange(maxfiles - 1, 1, -1):
+                    rotate(oldpath='%s.%d' % (path, i - 1),
+                           newpath='%s.%d' % (path, i))
+                rotate(oldpath=path,
+                       newpath=maxfiles > 0 and path + '.1')
+    return vfs(name, 'a', makeparentdirs=False)
+
+class proxylogger(object):
+    """Forward log events to another logger to be set later"""
+
+    def __init__(self):
+        self.logger = None
+
+    def tracked(self, event):
+        return self.logger is not None and self.logger.tracked(event)
+
+    def log(self, ui, event, msg, opts):
+        assert self.logger is not None
+        self.logger.log(ui, event, msg, opts)