cache: "default" shouldn't get cached in route handlers (#70937)

In Next 15, route handlers should no longer cache any `fetch` requests
by default. We do this by bailing out of caching if no explicit cache
config is defined. However, this misses the case where `cache:
"default"` is specified. Since the default caching behavior is to not
cache, this should also opt into the automatic no cache behavior.

This adds a test for the route handler and still validates that it
doesn't impact page-level ISR cache.

packages/next/src/server/lib/patch-fetch.ts

@@ -409,6 +409,7 @@ export function createPatchedFetcher(
          * - Fetch cache configs are not set. Specifically:
          *    - A page fetch cache mode is not set (export const fetchCache=...)
          *    - A fetch cache mode is not set in the fetch call (fetch(url, { cache: ... }))
+         *      or the fetch cache mode is set to 'default'
          *    - A fetch revalidate value is not set in the fetch call (fetch(url, { revalidate: ... }))
          * - OR the fetch comes after a configuration that triggered dynamic rendering (e.g., reading cookies())
          *   and the fetch was considered uncacheable (e.g., POST method or has authorization headers)
@@ -417,7 +418,10 @@ export function createPatchedFetcher(
           // eslint-disable-next-line eqeqeq
           pageFetchCacheMode == undefined &&
           // eslint-disable-next-line eqeqeq
-          currentFetchCacheConfig == undefined &&
+          (currentFetchCacheConfig == undefined ||
+            // when considering whether to opt into the default "no-cache" fetch semantics,
+            // a "default" cache config should be treated the same as no cache config
+            currentFetchCacheConfig === 'default') &&
           // eslint-disable-next-line eqeqeq
           currentFetchRevalidate == undefined
         const autoNoCache =

test/e2e/app-dir/app-static/app-static.test.ts

@@ -66,6 +66,44 @@ describe('app-dir static/dynamic handling', () => {
     }
   })
 
+  it('should correctly handle "default" cache statuses', async () => {
+    const res = await next.fetch('/default-config-fetch')
+    expect(res.status).toBe(200)
+
+    const html = await res.text()
+    const $ = cheerio.load(html)
+    const data = $('#data').text()
+
+    expect(data).toBeTruthy()
+
+    const res2 = await next.fetch('/default-config-fetch')
+    const html2 = await res2.text()
+    const data2 = cheerio.load(html2)('#data').text()
+
+    if (isNextDev) {
+      expect(data).not.toBe(data2)
+    } else {
+      // "default" cache does not impact ISR handling on a page, similar to the above test
+      // case for no fetch config
+      const pageCache = (
+        res.headers.get('x-vercel-cache') || res.headers.get('x-nextjs-cache')
+      ).toLowerCase()
+
+      expect(pageCache).toBeTruthy()
+      expect(pageCache).not.toBe('MISS')
+      expect(data).toBe(data2)
+    }
+
+    // route handlers should not automatically cache fetches with "default" cache
+    const routeRes = await next.fetch('/default-config-fetch/api')
+    const initialRouteData = (await routeRes.json()).data
+
+    const nextRes = await next.fetch('/default-config-fetch/api')
+    const newRouteData = (await nextRes.json()).data
+
+    expect(initialRouteData).not.toEqual(newRouteData)
+  })
+
   it('should still cache even though the `traceparent` header was different', async () => {
     const res = await next.fetch('/strip-header-traceparent')
     expect(res.status).toBe(200)
@@ -715,6 +753,9 @@ describe('app-dir static/dynamic handling', () => {
         [
           "(new)/custom/page.js",
           "(new)/custom/page_client-reference-manifest.js",
+          "(new)/default-config-fetch/api/route.js",
+          "(new)/default-config-fetch/page.js",
+          "(new)/default-config-fetch/page_client-reference-manifest.js",
           "(new)/no-config-fetch/page.js",
           "(new)/no-config-fetch/page_client-reference-manifest.js",
           "_not-found.html",
@@ -753,6 +794,8 @@ describe('app-dir static/dynamic handling', () => {
           "default-cache-search-params/page_client-reference-manifest.js",
           "default-cache/page.js",
           "default-cache/page_client-reference-manifest.js",
+          "default-config-fetch.html",
+          "default-config-fetch.rsc",
           "dynamic-error/[id]/page.js",
           "dynamic-error/[id]/page_client-reference-manifest.js",
           "dynamic-no-gen-params-ssr/[slug]/page.js",
@@ -1186,6 +1229,22 @@ describe('app-dir static/dynamic handling', () => {
             "initialRevalidateSeconds": false,
             "srcRoute": "/blog/[author]/[slug]",
           },
+          "/default-config-fetch": {
+            "dataRoute": "/default-config-fetch.rsc",
+            "experimentalBypassFor": [
+              {
+                "key": "Next-Action",
+                "type": "header",
+              },
+              {
+                "key": "content-type",
+                "type": "header",
+                "value": "multipart/form-data;.*",
+              },
+            ],
+            "initialRevalidateSeconds": false,
+            "srcRoute": "/default-config-fetch",
+          },
           "/force-cache": {
             "dataRoute": "/force-cache.rsc",
             "experimentalBypassFor": [

test/e2e/app-dir/app-static/app/(new)/default-config-fetch/api/route.js

@@ -0,0 +1,10 @@
+import { NextResponse } from 'next/server'
+
+export async function GET() {
+  const data = await fetch(
+    'https://next-data-api-endpoint.vercel.app/api/random?default-config-fetch',
+    { cache: 'default' }
+  ).then((res) => res.text())
+
+  return NextResponse.json({ data })
+}

test/e2e/app-dir/app-static/app/(new)/default-config-fetch/page.js

@@ -0,0 +1,13 @@
+export default async function Page() {
+  const data = await fetch(
+    `https://next-data-api-endpoint.vercel.app/api/random`,
+    { cache: 'default' }
+  ).then((res) => res.text())
+
+  return (
+    <>
+      <p>/default-config-fetch</p>
+      <p id="data">{data}</p>
+    </>
+  )
+}