feat(server): allow async bypass function for proxying (#2696)

lib/Server.js

@@ -254,7 +254,7 @@ class Server {
         this.websocketProxies.push(proxyMiddleware);
       }
 
-      const handle = (req, res, next) => {
+      const handle = async (req, res, next) => {
         if (typeof proxyConfigOrCallback === 'function') {
           const newProxyConfig = proxyConfigOrCallback();
 
@@ -269,7 +269,7 @@ class Server {
         // bypassUrl from it otherwise bypassUrl would be null
         const isByPassFuncDefined = typeof proxyConfig.bypass === 'function';
         const bypassUrl = isByPassFuncDefined
-          ? proxyConfig.bypass(req, res, proxyConfig)
+          ? await proxyConfig.bypass(req, res, proxyConfig)
           : null;
 
         if (typeof bypassUrl === 'boolean') {

test/server/proxy-option.test.js

@@ -36,6 +36,18 @@ const proxyOptionPathsAsProperties = {
       }
     },
   },
+  '/proxy/async': {
+    bypass(req, res) {
+      if (/\/proxy\/async$/.test(req.path)) {
+        return new Promise((resolve) => {
+          setTimeout(() => {
+            res.end('proxy async response');
+            resolve(true);
+          }, 10);
+        });
+      }
+    },
+  },
 };
 
 const proxyOption = {
@@ -135,6 +147,10 @@ describe('proxy option', () => {
       it('should not pass through a proxy when a bypass function returns false', (done) => {
         req.get('/proxyfalse').expect(404, done);
       });
+
+      it('should wait if bypass returns promise', (done) => {
+        req.get('/proxy/async').expect(200, 'proxy async response', done);
+      });
     });
   });