ipaldap: avoid invalid modlist when attribute encoding differs

ipaldap does not take into account the possibility of the attribute
encoding returned by python-ldap differing from the attribute
encoding produced by FreeIPA.  In particular this can occur with DNs
with special characters that require escaping.  For example,
python-ldap (or the underlying LDAP library) escapes special
characters using hex encoding:

  CN=Test Sub-CA 201604041620,OU=ftweedal,O=Red Hat\2C Inc.,L=Brisbane,C=AU

Whereas FreeIPA, when encoding the DN, escapes the character
directly:

  CN=Test Sub-CA 201604041620,OU=ftweedal,O=Red Hat\, Inc.,L=Brisbane,C=AU

Therefore it is possible to generate an invalid modlist. For
example, during external CA certificate renewal, if the issuer DN
includes a comma in one of the attribute values (as above), an
invalid modlist will be generated:

  [ (ldap.MOD_ADD, 'ipacaissuerdn',
      [b'CN=Test Sub-CA 201604041620,OU=ftweedal,O=Red Hat\, Inc.,L=Brisbane,C=AU'])
  , (ldap.MOD_DELETE, 'ipacaissuerdn',
      [b'CN=Test Sub-CA 201604041620,OU=ftweedal,O=Red Hat\2C Inc.,L=Brisbane,C=AU'])
  ]

Although encoded differently, these are the same value.  If this
modification is applied to the object, attributeOrValueExists (error
20) occurs.

To avoid the issue, put deletes before adds in the modlist.  If a
value is present (with different encodings) as both an addition and
a deletion, it must be because the original object contained the
value with a different encoding.  Therefore it is safe to delete it,
then add it back.

Note that the modlist is not optimal.  In the simplest case (like
above example), there should be no modification to perform.  It is
considerably more complex (and more computation) to implement this
because the raw attribute values must be decoded before comparison.

Fixes: https://pagure.io/freeipa/issue/7750
Reviewed-By: Christian Heimes <cheimes@redhat.com>

ipapython/ipaldap.py

@@ -562,10 +562,13 @@ class LDAPEntry(MutableMapping):
                     raise errors.OnlyOneValueAllowed(attr=name)
                 modlist.append((ldap.MOD_REPLACE, name, adds))
             else:
-                if adds:
-                    modlist.append((ldap.MOD_ADD, name, adds))
+                # dels before adds, in case the same value occurs in
+                # both due to encoding differences
+                # (https://pagure.io/freeipa/issue/7750)
                 if dels:
                     modlist.append((ldap.MOD_DELETE, name, dels))
+                if adds:
+                    modlist.append((ldap.MOD_ADD, name, adds))
 
         # Usually the modlist order does not matter.
         # However, for schema updates, we want 'attributetypes' before