app/controllers/users_controller.rb

@@ -26,7 +26,10 @@ class UsersController < ApplicationController
     @user = User.new(user_params)
 
     respond_to do |format|
-      if @user.save
+      if ENV["REGISTRATION_ALLOWED"].blank?
+        format.html { redirect_to new_session_url, alert: "Registration disabled." }
+        format.json { render json: @user.errors, status: :unprocessable_entity }
+      elsif @user.save
         @session = Session.new(session_params).save
         session[:current_user_id] = @session.user_id
 

test/controllers/users_controller_test.rb

@@ -17,6 +17,7 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
   end
 
   test "should create user" do
+    stub_environment(REGISTRATION_ALLOWED: "true") do
       assert_difference("User.count") do
         params = {
           user: {
@@ -27,6 +28,7 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
         }
         post users_url, params: params
       end
+    end
 
     assert_redirected_to user_url(User.last)
   end

test/system/users_test.rb

@@ -15,12 +15,14 @@ class UsersTest < ApplicationSystemTestCase
     visit users_url
     click_on "Sign up"
 
+    stub_environment(REGISTRATION_ALLOWED: "true") do
       fill_in "Email", with: "userthree@example.local"
       fill_in "Password", with: "secret"
       fill_in "Password confirmation", with: "secret"
       click_on "Create User"
 
       assert_text "User was successfully created"
+    end
     click_on "Back"
   end
 

test/test_helper.rb

@@ -14,4 +14,15 @@ class ActiveSupport::TestCase
     post sessions_url, params: { session: { email: email, password: password } }
     assert session[:current_user_id].present?
   end
+
+  def stub_environment(env)
+    old_env = ENV.to_hash
+    ENV.update(env.stringify_keys)
+
+    begin
+      yield
+    ensure
+      ENV.replace(old_env)
+    end
+  end
 end