Add reproducer for NSS callback issue

The new test case simply creates 10 connections and calls start_tls_s()
after OPT_X_TLS_NEWCTX.

https://github.com/python-ldap/python-ldap/pull/134
See: https://github.com/python-ldap/python-ldap/issues/60
Signed-off-by: Christian Heimes <cheimes@redhat.com>

Tests/t_ldapobject.py

@@ -23,7 +23,7 @@
 import warnings
 import pickle
 import warnings
-from slapdtest import SlapdTestCase, requires_sasl
+from slapdtest import SlapdTestCase, requires_sasl, requires_tls
 
 # Switch off processing .ldaprc or ldap.conf before importing _ldap
 os.environ['LDAPNOINIT'] = '1'
@@ -418,6 +418,20 @@ def test_byteswarning_initialize(self):
         self._check_byteswarning(
             w[0], u"Under Python 2, python-ldap uses bytes by default.")
 
+    @requires_tls()
+    def test_multiple_starttls(self):
+        # Test for openldap does not re-register nss shutdown callbacks
+        # after nss_Shutdown is called
+        # https://github.com/python-ldap/python-ldap/issues/60
+        # https://bugzilla.redhat.com/show_bug.cgi?id=1520990
+        for _ in range(10):
+            l = self.ldap_object_class(self.server.ldap_uri)
+            l.set_option(ldap.OPT_X_TLS_CACERTFILE, self.server.cafile)
+            l.set_option(ldap.OPT_X_TLS_NEWCTX, 0)
+            l.start_tls_s()
+            l.simple_bind_s(self.server.root_dn, self.server.root_pw)
+            self.assertEqual(l.whoami_s(), 'dn:' + self.server.root_dn)
+
 
 class Test01_ReconnectLDAPObject(Test00_SimpleLDAPObject):
     """