fix: auto reply to OPTIONS requests only when unhandled

Prior to this change the internal options middleware always responsed to such requests. This is because the middleware was registered too early and was not being used as a fallback. With this change we register this middleware as the last middleware to ensure that this is only used as a fallback when OPTIONS requests are not handled.

Closes webpack#4551

Author: alan-agius4

lib/Server.js

@@ -2131,32 +2131,6 @@ class Server {
       });
     }
 
-    {
-      /**
-       *
-       * @param {Request} req
-       * @param {Response} res
-       * @param {NextFunction} next
-       * @returns {void}
-       *
-       */
-      const optionsRequestResponseMiddleware = (req, res, next) => {
-        if (req.method === "OPTIONS") {
-          res.statusCode = 204;
-          res.setHeader("Content-Length", "0");
-          res.end();
-          return;
-        }
-        next();
-      };
-
-      middlewares.push({
-        name: "options-middleware",
-        path: "*",
-        middleware: optionsRequestResponseMiddleware,
-      });
-    }
-
     middlewares.push({
       name: "webpack-dev-middleware",
       middleware:
@@ -2411,6 +2385,28 @@ class Server {
       });
     }
 
+    // Register this middleware always as the last one so that it's only used as a
+    // fallback when no other middleware responses.
+    middlewares.push({
+      name: "options-middleware",
+      path: "*",
+      /**
+       * @param {Request} req
+       * @param {Response} res
+       * @param {NextFunction} next
+       * @returns {void}
+       */
+      middleware: (req, res, next) => {
+        if (req.method === "OPTIONS") {
+          res.statusCode = 204;
+          res.setHeader("Content-Length", "0");
+          res.end();
+          return;
+        }
+        next();
+      },
+    });
+
     if (typeof this.options.setupMiddlewares === "function") {
       middlewares = this.options.setupMiddlewares(middlewares, this);
     }