Strip leading dot as suggested by @d-maurer.

CHANGES.rst

@@ -28,7 +28,7 @@ https://github.com/zopefoundation/Zope/blob/4.x/CHANGES.rst
   to the complete matrix view when more than 30 roles are defined.
   (`#1039 <https://github.com/zopefoundation/Zope/pull/1039>`_)
 
-- Allow leading ``.`` in cookie domain names.
+- Strip leading ``.`` in cookie domain names.
   (`#1041 <https://github.com/zopefoundation/Zope/pull/1041>`_)
 
 

src/ZPublisher/cookie.py

@@ -242,14 +242,11 @@ def domain_converter(value):
         return value
     from encodings.idna import ToASCII
     from encodings.idna import nameprep
-    add_leading_dot = False
-    if u_value.startswith("."):
-        u_value = u_value[1:]
-        add_leading_dot = True
-    value = ".".join(to_str(ToASCII(nameprep(c))) for c in u_value.split("."))
-    if add_leading_dot:
-        value = f".{value}"
-    return value
+    # According to https://www.rfc-editor.org/rfc/rfc6265#section-4.1.2.3 a
+    # leading dot is ignored. If it is there `ToASCII` breaks on the empty
+    # string:
+    u_value = u_value.lstrip('.')
+    return ".".join(to_str(ToASCII(nameprep(c))) for c in u_value.split("."))
 
 
 registerCookieParameter("Domain", domain_converter)

src/ZPublisher/tests/test_cookie.py

@@ -145,10 +145,10 @@ def test_domain(self):
         _, v = convertCookieParameter("domain",
                                       "Fußball.example".encode())
         self.assertEqual(v, "fussball.example")
-        # allow leading dot according to
+        # a leading dot is stripped as it is ignored according to
         # https://www.rfc-editor.org/rfc/rfc6265#section-4.1.2.3
         _, v = convertCookieParameter("domain", ".zope.dev")
-        self.assertEqual(v, ".zope.dev")
+        self.assertEqual(v, "zope.dev")
 
     def test_path(self):
         # test object