[nextjs] Fix proxy chain clobbering redirects in multi-language sites

packages/nextjs/src/proxy/proxy.test.ts

@@ -70,7 +70,7 @@ describe('ProxyBase', () => {
     Object.defineProperties(res.headers, {
       set: {
         value: (key, value) => {
-        res.headers[key] = value;
+          res.headers[key] = value;
         },
         enumerable: false,
       },
@@ -677,6 +677,42 @@ describe('defineProxy', () => {
     });
   });
 
+  it('should stop executing proxies when a previous proxy sets location header', async () => {
+    const firstProxySpy = sinon.spy();
+    const skippedProxySpy = sinon.spy();
+
+    const proxy1: ProxyHandler = {
+      handle: (_req, res) => {
+        firstProxySpy();
+        return Promise.resolve(res);
+      },
+    };
+
+    const redirectingProxy: ProxyHandler = {
+      handle: (_req, res) => {
+        res.headers.set('location', '/redirect-target');
+        return Promise.resolve(res);
+      },
+    };
+
+    const proxyAfterRedirect: ProxyHandler = {
+      handle: (_req, res) => {
+        skippedProxySpy();
+        res.headers.set('after-redirect', 'true');
+        return Promise.resolve(res);
+      },
+    };
+
+    const req = {} as NextRequest;
+
+    const result = await defineProxy(proxy1, redirectingProxy, proxyAfterRedirect).exec(req);
+
+    expect(firstProxySpy).to.have.been.calledOnce;
+    expect(skippedProxySpy).to.not.have.been.called;
+    expect(result.headers.get('location')).to.equal('/redirect-target');
+    expect(result.headers.get('after-redirect')).to.equal(null);
+  });
+
   it('should execute proxies with empty response', async () => {
     class SampleProxy extends ProxyBase {
       handle(_req: NextRequest, res: NextResponse) {

packages/nextjs/src/proxy/proxy.ts

@@ -289,8 +289,7 @@ export const defineProxy = (...proxies: ProxyHandler[]) => {
           p.then((res) => {
             // Short-circuit the remaining proxies once a previous one
             // denied the request (e.g. PreviewProxy returning 403).
-            if (res.status === 403) return res;
-
+            if (res.headers?.get('location') || res.status === 403) return res;
             return proxy.handle(req, res);
           }),
         Promise.resolve(response)