Avoid fetcher 404 when lazy route discovery is racing against a navigation

.changeset/purple-boats-bake.md

@@ -0,0 +1,5 @@
+---
+"react-router": patch
+---
+
+Avoid initial fetcher execution 404 error when Lazy Route Discovery is interrupted by a navigation

integration/fetcher-test.ts

@@ -525,3 +525,76 @@ test.describe("fetcher aborts and adjacent forms", () => {
     await page.waitForSelector("#idle", { timeout: 2000 });
   });
 });
+
+test.describe("fetcher lazy route discovery", () => {
+  let fixture: Fixture;
+  let appFixture: AppFixture;
+
+  test.afterAll(() => {
+    appFixture.close();
+  });
+
+  test("skips revalidation of initial load fetchers performing lazy route discovery", async ({
+    page,
+  }) => {
+    fixture = await createFixture({
+      files: {
+        "app/routes/parent.tsx": js`
+          import * as React from "react";
+          import { useFetcher, useNavigate, Outlet } from "react-router";
+
+          export default function Index() {
+            const fetcher = useFetcher();
+            const navigate = useNavigate();
+
+            React.useEffect(() => {
+              fetcher.load('/api');
+            }, []);
+
+            React.useEffect(() => {
+              navigate('/parent/child');
+            }, []);
+
+            return (
+              <>
+                <h1>Parent</h1>
+                {fetcher.data ?
+                  <pre data-fetcher>{fetcher.data}</pre> :
+                  null}
+                <Outlet/>
+              </>
+            );
+          }
+        `,
+        "app/routes/parent.child.tsx": js`
+          export default function Index() {
+            return <h2>Child</h2>;
+          }
+        `,
+        "app/routes/api.tsx": js`
+          export async function loader() {
+            return "FETCHED!"
+          }
+        `,
+      },
+    });
+
+    // Slow down the fetcher discovery a tiny bit so it doesn't resolve prior
+    // to the navigation
+    page.route(/\/__manifest/, async (route) => {
+      console.log(route.request().url());
+      if (route.request().url().includes(encodeURIComponent("/api"))) {
+        await new Promise((r) => setTimeout(r, 100));
+      }
+      route.continue();
+    });
+
+    appFixture = await createAppFixture(fixture);
+    let app = new PlaywrightFixture(appFixture, page);
+    await app.goto("/parent");
+    await page.waitForSelector("h2", { timeout: 3000 });
+    await expect(page.locator("h2")).toHaveText("Child");
+    await page.waitForSelector("[data-fetcher]", { timeout: 3000 });
+    await expect(page.locator("[data-fetcher]")).toHaveText("FETCHED!");
+  });
+});

packages/react-router/lib/router/router.ts

@@ -1995,6 +1995,7 @@ export function createRouter(init: RouterInit): Router {
       fetchRedirectIds,
       routesToUse,
       basename,
+      init.patchRoutesOnNavigation != null,
       pendingActionResult
     );
 
@@ -2441,6 +2442,7 @@ export function createRouter(init: RouterInit): Router {
       fetchRedirectIds,
       routesToUse,
       basename,
+      init.patchRoutesOnNavigation != null,
       [match.route.id, actionResult]
     );
 
@@ -4583,6 +4585,7 @@ function getMatchesToLoad(
   fetchRedirectIds: Set<string>,
   routesToUse: AgnosticDataRouteObject[],
   basename: string | undefined,
+  hasPatchRoutesOnNavigation: boolean,
   pendingActionResult?: PendingActionResult
 ): {
   dsMatches: DataStrategyMatch[];
@@ -4717,13 +4720,23 @@ function getMatchesToLoad(
       return;
     }
 
+    let fetcher = state.fetchers.get(key);
+    let isMidInitialLoad =
+      fetcher && fetcher.state !== "idle" && fetcher.data === undefined;
     let fetcherMatches = matchRoutes(routesToUse, f.path, basename);
 
     // If the fetcher path no longer matches, push it in with null matches so
     // we can trigger a 404 in callLoadersAndMaybeResolveData.  Note this is
     // currently only a use-case for Remix HMR where the route tree can change
     // at runtime and remove a route previously loaded via a fetcher
     if (!fetcherMatches) {
+      // If this fetcher is still in it's initial loading state, then this is
+      // most likely not a 404 and the fetcher is still in the middle of lazy
+      // route discovery so we can just skip revalidation and let it finish
+      // it's initial load
+      if (hasPatchRoutesOnNavigation && isMidInitialLoad) {
+        return;
+      }
       revalidatingFetchers.push({
         key,
         routeId: f.routeId,
@@ -4744,7 +4757,6 @@ function getMatchesToLoad(
     // Revalidating fetchers are decoupled from the route matches since they
     // load from a static href.  They revalidate based on explicit revalidation
     // (submission, useRevalidator, or X-Remix-Revalidate)
-    let fetcher = state.fetchers.get(key);
     let fetcherMatch = getTargetMatch(fetcherMatches, f.path);
 
     let fetchController = new AbortController();
@@ -4768,11 +4780,7 @@ function getMatchesToLoad(
         lazyRoutePropertiesToSkip,
         scopedContext
       );
-    } else if (
-      fetcher &&
-      fetcher.state !== "idle" &&
-      fetcher.data === undefined
-    ) {
+    } else if (isMidInitialLoad) {
       if (isRevalidationRequired) {
         // If the fetcher hasn't ever completed loading yet, then this isn't a
         // revalidation, it would just be a brand new load if an explicit