netgroup: avoid extraneous LDAP search when retrieving primary key from DN

DNs for netgroup entries can contain either 'cn' or 'ipauniqueid' attribute in
their leaf RDN depending on their origin. Since 'cn' is the primary key, we
can return it in `get_primary_key_from_dn` right away and avoid any extraneous
LDAP search.

https://fedorahosted.org/freeipa/ticket/5855

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>

ipaserver/plugins/netgroup.py

@@ -237,6 +237,26 @@ class netgroup(LDAPObject):
         external_host_param,
     )
 
+    def get_primary_key_from_dn(self, dn):
+        assert isinstance(dn, DN)
+        if not dn.rdns:
+            return u''
+
+        first_ava = dn.rdns[0][0]
+        if first_ava[0] == self.primary_key.name:
+            return unicode(first_ava[1])
+
+        try:
+            entry_attrs = self.backend.get_entry(
+                dn, [self.primary_key.name]
+            )
+            try:
+                return entry_attrs[self.primary_key.name][0]
+            except (KeyError, IndexError):
+                return u''
+        except errors.NotFound:
+            return unicode(dn)
+
 
 @register()
 class netgroup_add(LDAPCreate):