Fix single fetch redirects whena basename is present

.changeset/rich-teachers-warn.md

@@ -0,0 +1,5 @@
+---
+"@remix-run/server-runtime": patch
+---
+
+Single Fetch: Fix redirects when a `basename` is presernt

integration/single-fetch-test.ts

@@ -1463,6 +1463,71 @@ test.describe("single-fetch", () => {
     expect(await app.getHtml("#target")).toContain("Target");
   });
 
+  test("processes redirects when a basename is present", async ({ page }) => {
+    let fixture = await createFixture({
+      compiler: "vite",
+      files: {
+        ...files,
+        "vite.config.ts": js`
+          import { defineConfig } from "vite";
+          import { vitePlugin as remix } from "@remix-run/dev";
+          export default defineConfig({
+            plugins: [
+              remix({
+                basename: '/base',
+                future: {
+                  unstable_singleFetch: true,
+                }
+              }),
+            ],
+          });
+        `,
+        "app/routes/data.tsx": js`
+          import { redirect } from '@remix-run/node';
+          export function loader() {
+            throw redirect('/target');
+          }
+          export default function Component() {
+            return null
+          }
+        `,
+        "app/routes/target.tsx": js`
+          export default function Component() {
+            return <h1 id="target">Target</h1>
+          }
+        `,
+      },
+    });
+
+    console.error = () => {};
+
+    let res = await fixture.requestDocument("/base/data");
+    expect(res.status).toBe(302);
+    expect(res.headers.get("Location")).toBe("/base/target");
+    expect(await res.text()).toBe("");
+
+    let { status, data } = await fixture.requestSingleFetchData(
+      "/base/data.data"
+    );
+    expect(data).toEqual({
+      [SingleFetchRedirectSymbol]: {
+        status: 302,
+        redirect: "/target",
+        reload: false,
+        replace: false,
+        revalidate: false,
+      },
+    });
+    expect(status).toBe(202);
+
+    let appFixture = await createAppFixture(fixture);
+    let app = new PlaywrightFixture(appFixture, page);
+    await app.goto("/base/");
+    await app.clickLink("/base/data");
+    await page.waitForSelector("#target");
+    expect(await app.getHtml("#target")).toContain("Target");
+  });
+
   test("processes thrown loader errors", async ({ page }) => {
     let fixture = await createFixture({
       config: {

packages/remix-server-runtime/server.ts

@@ -214,7 +214,11 @@ export const createRequestHandler: CreateRequestHandlerFunction = (
 
         if (isRedirectResponse(response)) {
           let result: SingleFetchResult | SingleFetchResults =
-            getSingleFetchRedirect(response.status, response.headers);
+            getSingleFetchRedirect(
+              response.status,
+              response.headers,
+              _build.basename
+            );
 
           if (request.method === "GET") {
             result = {

packages/remix-server-runtime/single-fetch.ts

@@ -8,6 +8,7 @@ import {
   isRouteErrorResponse,
   unstable_data as routerData,
   UNSAFE_ErrorResponseImpl as ErrorResponseImpl,
+  stripBasename,
 } from "@remix-run/router";
 import { encode } from "turbo-stream";
 
@@ -111,7 +112,11 @@ export async function singleFetchAction(
     // let non-Response return values through
     if (isResponse(result)) {
       return {
-        result: getSingleFetchRedirect(result.status, result.headers),
+        result: getSingleFetchRedirect(
+          result.status,
+          result.headers,
+          build.basename
+        ),
         headers: result.headers,
         status: SINGLE_FETCH_REDIRECT_STATUS,
       };
@@ -122,7 +127,11 @@ export async function singleFetchAction(
 
     if (isRedirectStatusCode(context.statusCode) && headers.has("Location")) {
       return {
-        result: getSingleFetchRedirect(context.statusCode, headers),
+        result: getSingleFetchRedirect(
+          context.statusCode,
+          headers,
+          build.basename
+        ),
         headers,
         status: SINGLE_FETCH_REDIRECT_STATUS,
       };
@@ -192,7 +201,8 @@ export async function singleFetchLoaders(
         result: {
           [SingleFetchRedirectSymbol]: getSingleFetchRedirect(
             result.status,
-            result.headers
+            result.headers,
+            build.basename
           ),
         },
         headers: result.headers,
@@ -208,7 +218,8 @@ export async function singleFetchLoaders(
         result: {
           [SingleFetchRedirectSymbol]: getSingleFetchRedirect(
             context.statusCode,
-            headers
+            headers,
+            build.basename
           ),
         },
         headers,
@@ -264,10 +275,17 @@ export async function singleFetchLoaders(
 
 export function getSingleFetchRedirect(
   status: number,
-  headers: Headers
+  headers: Headers,
+  basename: string | undefined
 ): SingleFetchRedirectResult {
+  let redirect = headers.get("Location")!;
+
+  if (basename) {
+    redirect = stripBasename(redirect, basename) || redirect;
+  }
+
   return {
-    redirect: headers.get("Location")!,
+    redirect,
     status,
     revalidate:
       // Technically X-Remix-Revalidate isn't needed here - that was an implementation