Add userinfo oauth endpoint (#32548)

config/initializers/cors.rb

@@ -23,6 +23,7 @@ Rails.application.config.middleware.insert_before 0, Rack::Cors do
                methods: %i(post put delete get patch options)
       resource '/oauth/token', methods: [:post]
       resource '/oauth/revoke', methods: [:post]
+      resource '/oauth/userinfo', methods: [:get, :post]
     end
   end
 end

config/routes.rb

@@ -64,6 +64,13 @@ Rails.application.routes.draw do
                 tokens: 'oauth/tokens'
   end
 
+  namespace :oauth do
+    # As this is borrowed from OpenID, the specification says we must also support
+    # POST for the userinfo endpoint:
+    # https://openid.net/specs/openid-connect-core-1_0.html#UserInfo
+    match 'userinfo', via: [:get, :post], to: 'userinfo#show', defaults: { format: 'json' }
+  end
+
   scope path: '.well-known' do
     scope module: :well_known do
       get 'oauth-authorization-server', to: 'oauth_metadata#show', as: :oauth_metadata, defaults: { format: 'json' }