fix(fetch-cache): fix additional typo, add type & data validation (#64799)

## Why?

This PR fixes the fetch cache, which currently is not using the fetch
cache when it exists (the tags were not getting set properly), and tags
have not updated.

(Before)


https://github.com/vercel/next.js/assets/28912696/74d8f592-0698-4ae4-be4b-30cffb1ffe11

(After)


https://github.com/vercel/next.js/assets/28912696/af12b13a-46c6-41c3-9ac3-20e1ec44a865

- #64786
- #63547

Closes NEXT-3173

Author: samcx

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

@@ -1,6 +1,14 @@
 import type { CacheHandler, CacheHandlerContext, CacheHandlerValue } from './'
+import type {
+  CachedFetchValue,
+  IncrementalCacheValue,
+} from '../../response-cache'
 
 import LRUCache from 'next/dist/compiled/lru-cache'
+
+import { z } from 'next/dist/compiled/zod'
+import type zod from 'next/dist/compiled/zod'
+
 import {
   CACHE_ONE_YEAR,
   NEXT_CACHE_SOFT_TAGS_HEADER,
@@ -23,6 +31,18 @@ const CACHE_REVALIDATE_HEADER = 'x-vercel-revalidate' as const
 const CACHE_FETCH_URL_HEADER = 'x-vercel-cache-item-name' as const
 const CACHE_CONTROL_VALUE_HEADER = 'x-vercel-cache-control' as const
 
+const zCachedFetchValue: zod.ZodType<CachedFetchValue> = z.object({
+  kind: z.literal('FETCH'),
+  data: z.object({
+    headers: z.record(z.string()),
+    body: z.string(),
+    url: z.string(),
+    status: z.number().optional(),
+  }),
+  tags: z.array(z.string()).optional(),
+  revalidate: z.number(),
+})
+
 export default class FetchCache implements CacheHandler {
   private headers: Record<string, string>
   private cacheEndpoint?: string
@@ -233,19 +253,22 @@ export default class FetchCache implements CacheHandler {
           throw new Error(`invalid response from cache ${res.status}`)
         }
 
-        const cached = await res.json()
+        const json: IncrementalCacheValue = await res.json()
+        const parsed = zCachedFetchValue.safeParse(json)
 
-        if (!cached || cached.kind !== 'FETCH') {
-          this.debug && console.log({ cached })
-          throw new Error(`invalid cache value`)
+        if (!parsed.success) {
+          this.debug && console.log({ json })
+          throw new Error('invalid cache value')
         }
 
+        const { data: cached } = parsed
+
         // if new tags were specified, merge those tags to the existing tags
         if (cached.kind === 'FETCH') {
           cached.tags ??= []
           for (const tag of tags ?? []) {
             if (!cached.tags.includes(tag)) {
-              cached.tag.push(tag)
+              cached.tags.push(tag)
             }
           }
         }