Implement OAuth Userinfo Endpoint

app/controllers/oauth/userinfo_controller.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+class Oauth::UserinfoController < Api::BaseController
+  before_action -> { doorkeeper_authorize! :profile }, only: [:show]
+  before_action :require_user!
+
+  def show
+    @account = current_account
+    render json: @account, serializer: OauthUserinfoSerializer
+  end
+end

app/serializers/oauth_userinfo_serializer.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+class OauthUserinfoSerializer < ActiveModel::Serializer
+  include RoutingHelper
+
+  attributes :iss, :sub, :name, :preferred_username, :profile, :picture
+
+  def iss
+    root_url
+  end
+
+  def sub
+    ActivityPub::TagManager.instance.uri_for(object)
+  end
+
+  def name
+    object.unavailable? ? '' : object.display_name
+  end
+
+  def preferred_username
+    object.username
+  end
+
+  def profile
+    ActivityPub::TagManager.instance.url_for(object)
+  end
+
+  def picture
+    full_asset_url(object.unavailable? ? object.avatar.default_url : object.avatar_original_url)
+  end
+end

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]
     end
   end
 end

config/routes.rb

@@ -64,6 +64,10 @@ Rails.application.routes.draw do
                 tokens: 'oauth/tokens'
   end
 
+  namespace :oauth do
+    get 'userinfo', 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' }

spec/requests/oauth/userinfo_spec.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Oauth Userinfo Endpoint' do
+  include RoutingHelper
+
+  describe 'GET /oauth/userinfo' do
+    subject do
+      get '/oauth/userinfo', headers: headers
+    end
+
+    let(:user)     { Fabricate(:user) }
+    let(:account)  { user.account }
+    let(:token)    { Fabricate(:accessible_access_token, resource_owner_id: user.id, scopes: scopes) }
+    let(:scopes)   { 'profile' }
+    let(:headers)  { { 'Authorization' => "Bearer #{token.token}" } }
+
+    it_behaves_like 'forbidden for wrong scope', 'read:accounts'
+
+    it 'returns http success' do
+      subject
+
+      expect(response).to have_http_status(:success)
+      expect(response.content_type).to start_with('application/json')
+      expect(response.parsed_body).to include({
+        iss: root_url,
+        sub: account_url(account),
+        name: account.display_name,
+        preferred_username: account.username,
+        profile: short_account_url(account),
+        picture: full_asset_url(account.unavailable? ? account.avatar.default_url : account.avatar_original_url),
+      })
+    end
+  end
+end