diff -r 17941fc53ae9 -r 593f6359681d mercurial/util.py
--- a/mercurial/util.py	Wed Jan 16 16:49:15 2019 -0800
+++ b/mercurial/util.py	Thu Jan 10 14:57:01 2019 +0100
@@ -2045,7 +2045,7 @@
     function if need.'''
     return path.split(pycompat.ossep)
 
-def mktempcopy(name, emptyok=False, createmode=None):
+def mktempcopy(name, emptyok=False, createmode=None, enforcewritable=False):
     """Create a temporary file with the same contents from name
 
     The permission bits are copied from the original file.
@@ -2061,7 +2061,8 @@
     # Temporary files are created with mode 0600, which is usually not
     # what we want.  If the original file already exists, just copy
     # its mode.  Otherwise, manually obey umask.
-    copymode(name, temp, createmode)
+    copymode(name, temp, createmode, enforcewritable)
+
     if emptyok:
         return temp
     try:
@@ -2204,7 +2205,9 @@
     def __init__(self, name, mode='w+b', createmode=None, checkambig=False):
         self.__name = name      # permanent name
         self._tempname = mktempcopy(name, emptyok=('w' in mode),
-                                    createmode=createmode)
+                                    createmode=createmode,
+                                    enforcewritable=('w' in mode))
+
         self._fp = posixfile(self._tempname, mode)
         self._checkambig = checkambig