Add userinfo oauth endpoint (#32548)

spec/requests/oauth/userinfo_spec.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe 'Oauth Userinfo Endpoint' do
+  include RoutingHelper
+
+  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}" } }
+
+  shared_examples 'returns successfully' do
+    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.avatar_original_url),
+      })
+    end
+  end
+
+  describe 'GET /oauth/userinfo' do
+    subject do
+      get '/oauth/userinfo', headers: headers
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'read:accounts'
+    it_behaves_like 'returns successfully'
+  end
+
+  # 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
+  describe 'POST /oauth/userinfo' do
+    subject do
+      post '/oauth/userinfo', headers: headers
+    end
+
+    it_behaves_like 'forbidden for wrong scope', 'read:accounts'
+    it_behaves_like 'returns successfully'
+  end
+end