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

.changeset/seven-ducks-breathe.md

@@ -0,0 +1,5 @@
+---
+"@opennextjs/aws": patch
+---
+
+fix: Avoid merging Location header on response when its an array

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

@@ -1,4 +1,5 @@
 import type http from "node:http";
+import logger from "../logger";
 
 export const parseHeaders = (
   headers?: http.OutgoingHttpHeader[] | http.OutgoingHttpHeaders,
@@ -12,7 +13,27 @@ export const parseHeaders = (
     if (value === undefined) {
       continue;
     }
-    result[key.toLowerCase()] = convertHeader(value);
+    const keyLower = key.toLowerCase();
+    /**
+     * Next can return an Array for the Location header
+     * We dont want to merge that into a comma-separated string
+     * If they are the same just return one of them
+     * Otherwise return the last one
+     * See: https://github.com/opennextjs/opennextjs-cloudflare/issues/875#issuecomment-3258248276
+     * and https://github.com/opennextjs/opennextjs-aws/pull/977#issuecomment-3261763114
+     */
+    if (keyLower === "location" && Array.isArray(value)) {
+      if (value[0] === value[1]) {
+        result[keyLower] = value[0];
+      } else {
+        logger.warn(
+          "Multiple different values for Location header found. Using the last one",
+        );
+        result[keyLower] = value[value.length - 1];
+      }
+      continue;
+    }
+    result[keyLower] = convertHeader(value);
   }
 
   return result;

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

@@ -1,4 +1,9 @@
-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 +48,21 @@ 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",
+    });
+  });
+});