Make OpenID Connect work without requiring a name

If there is no name argument given, OmniAuth will try to guess the name
by the class name. In
https://github.com/omniauth/omniauth/blob/v1.9.0/lib/omniauth/strategy.rb#L139,
`OmniAuth::Strategies::OpenIDConnect` gets translated to
`openidconnect`.

This leads to an immediate 404 error after clicking the login button
because OmniAuth can't match the current route (/users/auth/openid_connect)
against the expected one (/users/auth/openidconnect).

Other providers, such as Google OAuth2, set this name as the default
option within the OmniAuth Strategy. Until a fix is merged upstream,
let's just set the parameter ourselves.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/62208

changelogs/unreleased/sh-fix-openid-connect-defaults.yml

+---
+title: Make OpenID Connect work without requiring a name
+merge_request: 29312
+author:
+type: fixed

lib/gitlab/omniauth_initializer.rb

@@ -63,6 +63,12 @@ module Gitlab
         { remote_sign_out_handler: authentiq_signout_handler }
       when 'shibboleth'
         { fail_with_empty_uid: true }
+      when 'openid_connect'
+        # If a name argument is omitted, OmniAuth will expect that the
+        # matching route is /auth/users/openidconnect instead of
+        # /auth/users/openid_connect because of
+        # https://gitlab.com/gitlab-org/gitlab-ce/issues/62208#note_178780341.
+        { name: 'openid_connect' }
       else
         {}
       end

spec/lib/gitlab/omniauth_initializer_spec.rb

@@ -83,5 +83,13 @@ describe Gitlab::OmniauthInitializer do
 
       subject.execute([cas3_config])
     end
+
+    it 'configures name for openid_connect' do
+      openid_connect_config = { 'name' => 'openid_connect', 'args' => {} }
+
+      expect(devise_config).to receive(:omniauth).with(:openid_connect, name: 'openid_connect')
+
+      subject.execute([openid_connect_config])
+    end
   end
 end