Ensure fetch cache TTL is updated properly

packages/next/src/server/lib/incremental-cache/fetch-cache.ts

@@ -337,26 +337,6 @@ export default class FetchCache implements CacheHandler {
   public async set(...args: Parameters<CacheHandler['set']>) {
     const [key, data, ctx] = args
 
-    const newValue =
-      data?.kind === CachedRouteKind.FETCH ? data.data : undefined
-    const existingCache = memoryCache?.get(key)
-    const existingValue = existingCache?.value
-    if (
-      existingValue?.kind === CachedRouteKind.FETCH &&
-      Object.keys(existingValue.data).every(
-        (field) =>
-          JSON.stringify(
-            (existingValue.data as Record<string, string | Object>)[field]
-          ) ===
-          JSON.stringify((newValue as Record<string, string | Object>)[field])
-      )
-    ) {
-      if (DEBUG) {
-        console.log(`skipping cache set for ${key} as not modified`)
-      }
-      return
-    }
-
     const { fetchCache, fetchIdx, fetchUrl, tags } = ctx
     if (!fetchCache) return
 

test/production/app-dir/fetch-cache/app/not-changed/page.tsx

@@ -0,0 +1,33 @@
+import { unstable_cache } from 'next/cache'
+
+export const dynamic = 'force-dynamic'
+export const fetchCache = 'default-cache'
+
+// this is ensuring we still update TTL even
+// if the cache value didn't change during revalidate
+const stableCacheItem = unstable_cache(
+  async () => {
+    return 'consistent value'
+  },
+  [],
+  {
+    revalidate: 3,
+    tags: ['thankyounext'],
+  }
+)
+
+export default async function Page() {
+  const data = await fetch('https://example.vercel.sh', {
+    next: { tags: ['thankyounext'], revalidate: 3 },
+  }).then((res) => res.text())
+
+  const cachedValue = stableCacheItem()
+
+  return (
+    <>
+      <p>hello world</p>
+      <p id="data">{data}</p>
+      <p id="cache">{cachedValue}</p>
+    </>
+  )
+}

test/production/app-dir/fetch-cache/fetch-cache.test.ts

@@ -237,4 +237,34 @@ describe('fetch-cache', () => {
       fetchGetShouldError = false
     }
   })
+
+  it('should update cache TTL even if cache data does not change', async () => {
+    const fetchCacheRequestsIndex = fetchCacheRequests.length
+
+    for (let i = 0; i < 3; i++) {
+      const res = await fetchViaHTTP(appPort, '/not-changed')
+      expect(res.status).toBe(200)
+      // give time for revalidate period to pass
+      await new Promise((resolve) => setTimeout(resolve, 3_000))
+    }
+
+    const newCacheGets = []
+    const newCacheSets = []
+
+    for (
+      let i = fetchCacheRequestsIndex - 1;
+      i < fetchCacheRequests.length;
+      i++
+    ) {
+      const requestItem = fetchCacheRequests[i]
+      if (requestItem.method === 'get') {
+        newCacheGets.push(requestItem)
+      }
+      if (requestItem.method === 'post') {
+        newCacheSets.push(requestItem)
+      }
+    }
+    expect(newCacheGets.length).toBeGreaterThanOrEqual(2)
+    expect(newCacheSets.length).toBeGreaterThanOrEqual(2)
+  })
 })