diff --git a/mercurial/chgserver.py b/mercurial/chgserver.py
--- a/mercurial/chgserver.py
+++ b/mercurial/chgserver.py
@@ -528,7 +528,7 @@ class chgcmdserver(commandserver.server)
     def _setumask(self, data):
         mask = struct.unpack(b'>I', data)[0]
         self.ui.log(b'chgserver', b'setumask %r\n', mask)
-        os.umask(mask)
+        util.setumask(mask)
 
     def runcommand(self):
         # pager may be attached within the runcommand session, which should
diff --git a/mercurial/util.py b/mercurial/util.py
--- a/mercurial/util.py
+++ b/mercurial/util.py
@@ -130,6 +130,16 @@ umask = platform.umask
 unlink = platform.unlink
 username = platform.username
 
+
+def setumask(val):
+    ''' updates the umask. used by chg server '''
+    if pycompat.iswindows:
+        return
+    os.umask(val)
+    global umask
+    platform.umask = umask = val & 0o777
+
+
 # small compat layer
 compengines = compression.compengines
 SERVERROLE = compression.SERVERROLE