fix(websocket): handle errors in handleUpgrade

src/http-proxy-middleware.ts

@@ -93,10 +93,16 @@ export class HttpProxyMiddleware {
   };
 
   private handleUpgrade = async (req: Request, socket, head) => {
-    if (this.shouldProxy(this.proxyOptions.pathFilter, req)) {
-      const activeProxyOptions = await this.prepareProxyRequest(req);
-      this.proxy.ws(req, socket, head, activeProxyOptions);
-      debug('server upgrade event received. Proxying WebSocket');
+    try {
+      if (this.shouldProxy(this.proxyOptions.pathFilter, req)) {
+        const activeProxyOptions = await this.prepareProxyRequest(req);
+        this.proxy.ws(req, socket, head, activeProxyOptions);
+        debug('server upgrade event received. Proxying WebSocket');
+      }
+    } catch (err) {
+      // This error does not include the URL as the fourth argument as we won't
+      // have the URL if `this.prepareProxyRequest` throws an error.
+      this.proxy.emit('error', err, req, socket);
     }
   };
 

src/plugins/default/error-response-plugin.ts

@@ -1,19 +1,32 @@
+import type { Socket } from 'net';
 import { getStatusCode } from '../../status-code';
 import { Plugin, Response } from '../../types';
 
+function isResponseLike(obj: any): obj is Response {
+  return obj && typeof obj.writeHead === 'function';
+}
+
+function isSocketLike(obj: any): obj is Socket {
+  return obj && typeof obj.write === 'function' && !('writeHead' in obj);
+}
+
 export const errorResponsePlugin: Plugin = (proxyServer, options) => {
-  proxyServer.on('error', (err, req, res: Response, target?) => {
+  proxyServer.on('error', (err, req, res: Response | Socket, target?) => {
     // Re-throw error. Not recoverable since req & res are empty.
     if (!req && !res) {
       throw err; // "Error: Must provide a proper URL as target"
     }
 
-    if (res.writeHead && !res.headersSent) {
-      const statusCode = getStatusCode((err as unknown as any).code);
-      res.writeHead(statusCode);
-    }
+    if (isResponseLike(res)) {
+      if (!res.headersSent) {
+        const statusCode = getStatusCode((err as unknown as any).code);
+        res.writeHead(statusCode);
+      }
 
-    const host = req.headers && req.headers.host;
-    res.end(`Error occurred while trying to proxy: ${host}${req.url}`);
+      const host = req.headers && req.headers.host;
+      res.end(`Error occurred while trying to proxy: ${host}${req.url}`);
+    } else if (isSocketLike(res)) {
+      res.destroy();
+    }
   });
 };

test/e2e/websocket.spec.ts

@@ -97,15 +97,14 @@ describe('E2E WebSocket proxy', () => {
 
   describe('with router and pathRewrite', () => {
     beforeEach(() => {
-      // override
-      proxyServer = createApp(
+      const proxyMiddleware = createProxyMiddleware({
         // cSpell:ignore notworkinghost
-        createProxyMiddleware({
-          target: 'ws://notworkinghost:6789',
-          router: { '/socket': `ws://localhost:${WS_SERVER_PORT}` },
-          pathRewrite: { '^/socket': '' },
-        })
-      ).listen(SERVER_PORT);
+        target: 'ws://notworkinghost:6789',
+        router: { '/socket': `ws://localhost:${WS_SERVER_PORT}` },
+        pathRewrite: { '^/socket': '' },
+      });
+
+      proxyServer = createApp(proxyMiddleware).listen(SERVER_PORT);
 
       proxyServer.on('upgrade', proxyMiddleware.upgrade);
     });
@@ -124,4 +123,29 @@ describe('E2E WebSocket proxy', () => {
       ws.send('foobar');
     });
   });
+
+  describe('with error in router', () => {
+    beforeEach(() => {
+      const proxyMiddleware = createProxyMiddleware({
+        // cSpell:ignore notworkinghost
+        target: `http://notworkinghost:6789`,
+        router: async () => {
+          throw new Error('error');
+        },
+      });
+
+      proxyServer = createApp(proxyMiddleware).listen(SERVER_PORT);
+
+      proxyServer.on('upgrade', proxyMiddleware.upgrade);
+    });
+
+    it('should handle error', (done) => {
+      ws = new WebSocket(`ws://localhost:${SERVER_PORT}/socket`);
+
+      ws.on('error', (err) => {
+        expect(err).toBeTruthy();
+        done();
+      });
+    });
+  });
 });