Include LDAP UID attr in default attrs for LDAP lookups

In cases where the configured LDAP `uid` attribute is non-standard
(not `uid`, `samaccountname`, `userid`) the `username` value in
the LDAP auth hash will not contain a value. By including the
configured `uid` attribute in the `username` default attributes
array we ensure that the value will be present in the auth hash.

changelogs/unreleased/dblessing-ldap-username-uid-fix.yml

+---
+title: Include LDAP UID attribute in default attributes for all LDAP lookups
+merge_request: 28148
+author:
+type: fixed

lib/gitlab/auth/ldap/config.rb

@@ -178,7 +178,7 @@ module Gitlab
 
         def default_attributes
           {
-            'username'    => %w(uid sAMAccountName userid),
+            'username'    => %W(#{uid} uid sAMAccountName userid).uniq,
             'email'       => %w(mail email userPrincipalName),
             'name'        => 'cn',
             'first_name'  => 'givenName',

spec/lib/gitlab/auth/ldap/config_spec.rb

@@ -502,6 +502,20 @@ AtlErSqafbECNDSwS5BX8yDpu5yRBJ4xegO/rNlmb8ICRYkuJapD1xXicFOsmfUK
     end
   end
 
+  describe '#default_attributes' do
+    it 'includes the configured uid attribute in the username attributes' do
+      stub_ldap_config(options: { 'uid' => 'my_uid_attr' })
+
+      expect(config.default_attributes['username']).to include('my_uid_attr')
+    end
+
+    it 'only includes unique values for username attributes' do
+      stub_ldap_config(options: { 'uid' => 'uid' })
+
+      expect(config.default_attributes['username']).to contain_exactly('uid', 'sAMAccountName', 'userid')
+    end
+  end
+
   describe '#base' do
     context 'when the configured base is not normalized' do
       it 'returns the normalized base' do