add ldap filter validation to ldap group link and update controller

app/controllers/groups/ldap_group_links_controller.rb

@@ -43,6 +43,6 @@ class Groups::LdapGroupLinksController < Groups::ApplicationController
   end
 
   def ldap_group_link_params
-    params.require(:ldap_group_link).permit(:cn, :group_access, :provider)
+    params.require(:ldap_group_link).permit(:cn, :filter, :group_access, :provider)
   end
 end

app/models/ldap_group_link.rb

@@ -2,8 +2,11 @@ class LdapGroupLink < ActiveRecord::Base
   include Gitlab::Access
   belongs_to :group
 
-  validates :cn, :group_access, :group_id, presence: true
-  validates :cn, uniqueness: { scope: [:group_id, :provider] }
+  validates :cn, :group_access, :group_id, presence: true, unless: :filter
+  validates :cn, uniqueness: { scope: [:group_id, :provider] }, unless: :filter
+  validates :filter, :group_access, :group_id, presence: true, unless: :cn
+  validates :filter, uniqueness: { scope: [:group_id, :provider] }, unless: :cn
+  validates :filter, ldap_filter: true
   validates :group_access, inclusion: { in: Gitlab::Access.all_values }
   validates :provider, presence: true