Backport of ee/9235: Add LDAP integration to smartcard authentication

app/views/devise/sessions/_new_ldap.html.haml

+- server = local_assigns.fetch(:server)
+
 = form_tag(omniauth_callback_path(:user, server['provider_name']), id: 'new_ldap_user', class: "gl-show-field-errors") do
   .form-group
     = label_tag :username, "#{server['label']} Username"

lib/gitlab/auth/ldap/adapter.rb

@@ -30,14 +30,7 @@ module Gitlab
 
         def users(fields, value, limit = nil)
           options = user_options(Array(fields), value, limit)
-
-          entries = ldap_search(options).select do |entry|
-            entry.respond_to? config.uid
-          end
-
-          entries.map do |entry|
-            Gitlab::Auth::LDAP::Person.new(entry, provider)
-          end
+          users_search(options)
         end
 
         def user(*args)
@@ -90,6 +83,16 @@ module Gitlab
           SEARCH_RETRY_FACTOR[retry_number] * config.timeout
         end
 
+        def users_search(options)
+          entries = ldap_search(options).select do |entry|
+            entry.respond_to? config.uid
+          end
+
+          entries.map do |entry|
+            Gitlab::Auth::LDAP::Person.new(entry, provider)
+          end
+        end
+
         def user_options(fields, value, limit)
           options = {
             attributes: Gitlab::Auth::LDAP::Person.ldap_attributes(config),