chore: serverMiddleware HMR improvements (#6896)

* chore(server): fallback route for resolveMiddleware

* chore(server): support express HMR

[release]

packages/server/src/server.js

@@ -206,6 +206,12 @@ export default class Server {
       }
     }
 
+    // SubApp (Express)
+    if (typeof middleware.handle.handle === 'function') {
+      const server = middleware.handle
+      middleware.handle = server.handle.bind(server)
+    }
+
     return middleware
   }
 
@@ -222,10 +228,7 @@ export default class Server {
       consola.error('ServerMiddleware Error:', error)
 
       // Placeholder for error
-      middleware = {
-        route: '#error',
-        handle: (req, res, next) => { next(error) }
-      }
+      middleware = (req, res, next) => { next(error) }
     }
 
     // Normalize
@@ -237,10 +240,15 @@ export default class Server {
     return middleware
   }
 
-  resolveMiddleware (middleware) {
+  resolveMiddleware (middleware, fallbackRoute = '/') {
     // Ensure middleware is normalized
     middleware = this._normalizeMiddleware(middleware)
 
+    // Fallback route
+    if (!middleware.route) {
+      middleware.route = fallbackRoute
+    }
+
     // Resolve final route
     middleware.route = (
       (middleware.prefix !== false ? this.options.router.base : '') +
@@ -255,7 +263,7 @@ export default class Server {
 
   useMiddleware (middleware) {
     const { route, handle } = this.resolveMiddleware(middleware)
-    this.app.use(route.includes('#error') ? '/' : route, handle)
+    this.app.use(route, handle)
   }
 
   replaceMiddleware (query, middleware) {
@@ -275,7 +283,7 @@ export default class Server {
     }
 
     // Resolve middleware
-    const { route, handle } = this.resolveMiddleware(middleware)
+    const { route, handle } = this.resolveMiddleware(middleware, serverStackItem.route)
 
     // Update serverStackItem
     serverStackItem.handle = handle