fix: don't cache preloaded redirect results

.changeset/preload-redirect-cache.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/kit': patch
+---
+
+fix: don't cache preloaded redirect results, so navigation re-runs `load` instead of replaying a stale redirect

packages/kit/src/runtime/client/client.js

@@ -622,8 +622,11 @@ async function _preload_data(intent) {
 			token: preload,
 			promise: load_route({ ...intent, preload }).then((result) => {
 				preload_tokens.delete(preload);
-				if (result.type === 'loaded' && result.state.error) {
-					// Don't cache errors, because they might be transient
+				if (result.type === 'redirect' || (result.type === 'loaded' && result.state.error)) {
+					// Don't cache errors or redirects, because they might be transient.
+					// A cached redirect would be replayed without re-running `load`
+					// until a navigation commits, which can turn a resolvable
+					// redirect chain into a redirect loop
 					discard_load_cache();
 				}
 				return result;

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/dashboard/+page.server.js

@@ -0,0 +1,9 @@
+import { redirect } from '@sveltejs/kit';
+import { state } from '../state.js';
+
+export function load() {
+	if (!state.selected) {
+		redirect(303, '/data-sveltekit/preload-data/redirect-gate/select');
+	}
+	return {};
+}

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/dashboard/+page.svelte

@@ -0,0 +1 @@
+<h1>dashboard</h1>

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/select/+page.server.js

@@ -0,0 +1,7 @@
+import { redirect } from '@sveltejs/kit';
+import { state } from '../state.js';
+
+export function load() {
+	state.selected = true;
+	redirect(303, '/data-sveltekit/preload-data/redirect-gate/dashboard');
+}

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/start/+page.server.js

@@ -0,0 +1,6 @@
+import { state } from '../state.js';
+
+export function load() {
+	// reset so the test can run repeatedly against the same server
+	state.selected = false;
+}

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/start/+page.svelte

@@ -0,0 +1,5 @@
+<a
+	id="gated"
+	href="/data-sveltekit/preload-data/redirect-gate/dashboard"
+	data-sveltekit-preload-data="hover">go to dashboard</a
+>

packages/kit/test/apps/basics/src/routes/data-sveltekit/preload-data/redirect-gate/state.js

@@ -0,0 +1 @@
+export const state = { selected: false };

packages/kit/test/apps/basics/test/client.test.js

@@ -1071,6 +1071,24 @@ test.describe('data-sveltekit attributes', () => {
 		await expect(page).toHaveURL(offline_url);
 	});
 
+	test('preloaded redirect is not replayed on navigation', async ({ page }) => {
+		await page.goto('/data-sveltekit/preload-data/redirect-gate/start');
+
+		// preload /dashboard while its load redirects to /select
+		await page.locator('#gated').hover();
+		await page.locator('#gated').dispatchEvent('touchstart');
+		await Promise.all([
+			page.waitForTimeout(100), // wait for preloading to start
+			page.waitForLoadState('networkidle') // wait for preloading to finish
+		]);
+
+		// navigating must re-run load rather than replay the cached redirect,
+		// otherwise the mutual redirect never resolves and loops
+		await page.locator('#gated').click();
+		await expect(page.locator('h1')).toHaveText('dashboard');
+		await expect(page).toHaveURL(/redirect-gate\/dashboard/);
+	});
+
 	test('data-sveltekit-preload-data error does not block user navigation', async ({
 		page,
 		context,