Dev server detection workaround (correct PR) (#2898)

Author: crawler

lib/webpacker/dev_server_runner.rb

@@ -64,6 +64,7 @@ def detect_port!
       def execute_cmd
         env = Webpacker::Compiler.env
         env["WEBPACKER_CONFIG"] = @webpacker_config
+        env["WEBPACK_DEV_SERVER"] = "true"
 
         cmd = if node_modules_bin_exist?
           ["#{@node_modules_bin_path}/webpack", "serve"]

test/dev_server_runner_test.rb

@@ -43,19 +43,27 @@ def dev_server.https?; true; end
     end
   end
 
+  def test_environment_variables
+    cmd = ["#{test_app_path}/node_modules/.bin/webpack", "serve", "--config", "#{test_app_path}/config/webpack/development.js"]
+    env = Webpacker::Compiler.env.dup
+    env["WEBPACKER_CONFIG"] = "#{test_app_path}/config/webpacker.yml"
+    env["WEBPACK_DEV_SERVER"] = "true"
+    verify_command(cmd, env: env)
+  end
+
   private
     def test_app_path
       File.expand_path("test_app", __dir__)
     end
 
-    def verify_command(cmd, use_node_modules: true, argv: [])
+    def verify_command(cmd, use_node_modules: true, argv: [], env: Webpacker::Compiler.env)
       cwd = Dir.pwd
       Dir.chdir(test_app_path)
 
       klass = Webpacker::DevServerRunner
       instance = klass.new(argv)
       mock = Minitest::Mock.new
-      mock.expect(:call, nil, [Webpacker::Compiler.env, *cmd])
+      mock.expect(:call, nil, [env, *cmd])
 
       klass.stub(:new, instance) do
         instance.stub(:node_modules_bin_exist?, use_node_modules) do