fix: auto reply to OPTIONS requests only when unhandled (#4559)

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 #4551

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);
     }