From d65ec54f23a17b98452e691b9263a59dfca5ec98 Mon Sep 17 00:00:00 2001
From: Nina Berg <nikb100@gmail.com>
Date: Mon, 7 Apr 2025 11:30:24 -0400
Subject: [PATCH 1/3] proxy-backend support sse connections closing

Signed-off-by: Nina Berg <nikb100@gmail.com>
---
 plugins/proxy-backend/src/service/router.ts | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/plugins/proxy-backend/src/service/router.ts b/plugins/proxy-backend/src/service/router.ts
index a6a52db63a..e58bccd7af 100644
--- a/plugins/proxy-backend/src/service/router.ts
+++ b/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) {

From 9b5b744e240e1b2aa76e3b1837b312f7ff8d97b1 Mon Sep 17 00:00:00 2001
From: Nina Berg <nikb100@gmail.com>
Date: Mon, 7 Apr 2025 14:35:58 -0400
Subject: [PATCH 2/3] add changeset and tests

Signed-off-by: Nina Berg <nikb100@gmail.com>
---
 .changeset/thin-trams-work.md                 |  5 ++
 .../proxy-backend/src/service/router.test.ts  | 47 +++++++++++++++++++
 2 files changed, 52 insertions(+)
 create mode 100644 .changeset/thin-trams-work.md

diff --git a/.changeset/thin-trams-work.md b/.changeset/thin-trams-work.md
new file mode 100644
index 0000000000..ddc31631d1
--- /dev/null
+++ b/.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
diff --git a/plugins/proxy-backend/src/service/router.test.ts b/plugins/proxy-backend/src/service/router.test.ts
index 66d6de2f28..7d9a3dadbc 100644
--- a/plugins/proxy-backend/src/service/router.test.ts
+++ b/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();
+  });
 });

From d903b9228430422c226f41bbd40a02b3b6ca8daa Mon Sep 17 00:00:00 2001
From: Nina Berg <nikb100@gmail.com>
Date: Mon, 7 Apr 2025 14:47:52 -0400
Subject: [PATCH 3/3] capitalize SSE for vale

Signed-off-by: Nina Berg <nikb100@gmail.com>
---
 .changeset/thin-trams-work.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/.changeset/thin-trams-work.md b/.changeset/thin-trams-work.md
index ddc31631d1..26accde189 100644
--- a/.changeset/thin-trams-work.md
+++ b/.changeset/thin-trams-work.md
@@ -2,4 +2,4 @@
 '@backstage/plugin-proxy-backend': patch
 ---
 
-Fixed handling of proxied sse connections when the upstream server closes the connection
+Fixed handling of proxied SSE connections when the upstream server closes the connection