Merge pull request #29498 from nBerg/support-sse-connections-closed

proxy-backend support sse connections closing

.changeset/thin-trams-work.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-proxy-backend': patch
+---
+
+Fixed handling of proxied SSE connections when the upstream server closes the connection

plugins/proxy-backend/src/service/router.test.ts

@@ -479,6 +479,7 @@ describe('buildMiddleware', () => {
         pragma: 'value',
         'set-cookie': ['value'],
       },
+      on: jest.fn(),
     } as Partial<http.IncomingMessage>;
 
     expect(config).toBeDefined();
@@ -522,6 +523,7 @@ describe('buildMiddleware', () => {
         'set-cookie': [],
         'x-auth-request-user': 'asd',
       },
+      on: jest.fn(),
     } as Partial<http.IncomingMessage>;
 
     expect(config).toBeDefined();
@@ -603,4 +605,49 @@ describe('buildMiddleware', () => {
       ),
     ).toThrow(/Proxy target is not a valid URL/);
   });
+
+  it('closes SSE connections when backend closes the connection', async () => {
+    buildMiddleware(
+      '/proxy',
+      logger,
+      '/test',
+      {
+        target: 'http://mocked',
+      },
+      httpRouterService,
+    );
+
+    expect(createProxyMiddleware).toHaveBeenCalledTimes(1);
+
+    const config = mockCreateProxyMiddleware.mock.calls[0][1] as Options;
+
+    const testServerResponse = {
+      headers: {},
+      on: jest.fn((event, callback) => {
+        if (event === 'close') {
+          callback();
+        }
+        return testServerResponse;
+      }),
+    } as Partial<http.IncomingMessage>;
+
+    const testClientResponse = {
+      writableEnded: false,
+      end: jest.fn(),
+    } as Partial<Response>;
+
+    expect(config).toBeDefined();
+    expect(config.onProxyRes).toBeDefined();
+
+    config.onProxyRes!(
+      testServerResponse as http.IncomingMessage,
+      {} as Request,
+      testClientResponse as Response,
+    );
+    expect(testServerResponse.on).toHaveBeenCalledWith(
+      'close',
+      expect.any(Function),
+    );
+    expect(testClientResponse.end).toHaveBeenCalled();
+  });
 });

plugins/proxy-backend/src/service/router.ts

@@ -214,8 +214,12 @@ export function buildMiddleware(
     ].map(h => h.toLocaleLowerCase()),
   );
 
-  // only forward the allowed headers in backend->client
-  fullConfig.onProxyRes = (proxyRes: http.IncomingMessage) => {
+  fullConfig.onProxyRes = (
+    proxyRes: http.IncomingMessage,
+    _,
+    res: express.Response,
+  ) => {
+    // only forward the allowed headers in backend->client
     const headerNames = Object.keys(proxyRes.headers);
 
     headerNames.forEach(h => {
@@ -223,6 +227,14 @@ export function buildMiddleware(
         delete proxyRes.headers[h];
       }
     });
+
+    // handle SSE connections closed by the backend
+    // https://github.com/chimurai/http-proxy-middleware/discussions/765
+    proxyRes.on('close', () => {
+      if (!res.writableEnded) {
+        res.end();
+      }
+    });
   };
 
   if (reviveConsumedRequestBodies) {