fix: Avoid merging Location header on response when its an array

Author: sommeeeer

packages/open-next/src/http/util.ts

@@ -12,6 +12,12 @@ export const parseHeaders = (
     if (value === undefined) {
       continue;
     }
+    // Next can sometimes return an Array for the Location header
+    // See: https://github.com/opennextjs/opennextjs-cloudflare/issues/875#issuecomment-3258248276
+    if (key.toLowerCase() === "location" && Array.isArray(value)) {
+      result[key.toLowerCase()] = value[0];
+      continue;
+    }
     result[key.toLowerCase()] = convertHeader(value);
   }
 

packages/tests-unit/tests/http/utils.test.ts

@@ -1,4 +1,6 @@
-import { parseSetCookieHeader } from "@opennextjs/aws/http/util.js";
+import type http from "node:http";
+
+import { parseHeaders, parseSetCookieHeader } from "@opennextjs/aws/http/util.js";
 
 describe("parseSetCookieHeader", () => {
   it("returns an empty list if cookies is emptyish", () => {
@@ -43,3 +45,21 @@ describe("parseSetCookieHeader", () => {
     ]);
   });
 });
+
+describe("parseHeaders", () => {
+  it("parses headers correctly", () => {
+    const headers = parseHeaders({
+      location: ["/target", "/target"],
+      "x-custom-header": "customValue",
+      "x-multiple-values": ["value1", "value2"],
+      "x-undefined-header": undefined,
+      "x-opennext": "is-so-cool",
+    } as unknown as http.OutgoingHttpHeaders);
+    expect(headers).toEqual({
+      location: "/target",
+      "x-custom-header": "customValue",
+      "x-multiple-values": "value1,value2",
+      "x-opennext": "is-so-cool",
+    });
+  });
+});