fix: revalidation with file-system-cache

[gtjamesa]

What?

When using the file system cache with isrMemoryCacheSize: 0, time-based revalidation is not working, and the file is constantly updated. I have also added some debug logging to mirror that in the fetch-cache handler

Detailed explanation in #58507

Why?

The cached object's tags are incorrectly accessed, causing the cache to be rewritten every hit. This is catastrophic for a caching system that relies on file modification timestamps. The tags are one level up in the object from where they are currently being accessed.

Below shows a cached fetch representation on disk. When written, the tags reside at obj.tags instead of obj.data.tags

{
  "kind": "FETCH",
  "data": {
    "headers": {
      "connection": "keep-alive",
      "content-encoding": "br",
      "content-type": "application/json; charset=utf-8",
      "date": "Wed, 15 Nov 2023 21:17:42 GMT",
      "server": "nginx/1.18.0 (Ubuntu)",
      "transfer-encoding": "chunked",
      "vary": "Accept-Encoding"
    },
    "body": "[SNIP]",
    "status": 200,
    "url": "https://timeapi.io/api/Time/current/zone?timeZone=UTC"
    # this is where the current code is trying to pull the tags
    # "tags": [ "time-with-fetch" ]
  },
  "revalidate": 20,
  # tags actually live here
  "tags": [
    "time-with-fetch"
  ]
}

Fixes #58507

[ijjk]

Stats from current PR

Default Build

General Overall increase ⚠️

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
buildDuration	10.1s	10.3s	⚠️ +231ms
buildDurationCached	5.9s	6.5s	⚠️ +579ms
nodeModulesSize	199 MB	199 MB	⚠️ +9.15 kB
nextStartRea..uration (ms)	415ms	403ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
199-HASH.js gzip	28.8 kB	28.8 kB	N/A
3f784ff6-HASH.js gzip	53.3 kB	53.3 kB	✓
494.HASH.js gzip	180 B	181 B	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	241 B	240 B	N/A
main-HASH.js gzip	31.7 kB	31.8 kB	N/A
webpack-HASH.js gzip	1.7 kB	1.7 kB	✓
Overall change	100 kB	100 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
_app-HASH.js gzip	194 B	195 B	N/A
_error-HASH.js gzip	182 B	181 B	N/A
amp-HASH.js gzip	504 B	506 B	N/A
css-HASH.js gzip	322 B	323 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	✓
edge-ssr-HASH.js gzip	253 B	255 B	N/A
head-HASH.js gzip	348 B	347 B	N/A
hooks-HASH.js gzip	369 B	368 B	N/A
image-HASH.js gzip	4.3 kB	4.3 kB	N/A
index-HASH.js gzip	256 B	256 B	✓
link-HASH.js gzip	2.63 kB	2.63 kB	N/A
routerDirect..HASH.js gzip	311 B	311 B	✓
script-HASH.js gzip	384 B	383 B	N/A
withRouter-HASH.js gzip	307 B	308 B	N/A
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	3.17 kB	3.17 kB	✓

Client Build Manifests

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
_buildManifest.js gzip	484 B	483 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
index.html gzip	528 B	527 B	N/A
link.html gzip	539 B	540 B	N/A
withRouter.html gzip	525 B	522 B	N/A
Overall change	0 B	0 B	✓

Edge SSR bundle Size

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
edge-ssr.js gzip	92.6 kB	92.6 kB	N/A
page.js gzip	145 kB	145 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
middleware-b..fest.js gzip	627 B	624 B	N/A
middleware-r..fest.js gzip	150 B	151 B	N/A
middleware.js gzip	24.8 kB	24.8 kB	N/A
edge-runtime..pack.js gzip	1.92 kB	1.92 kB	✓
Overall change	1.92 kB	1.92 kB	✓

Next Runtimes

	vercel/next.js canary	gtjamesa/next.js fs-cache-fix	Change
app-page-exp...dev.js gzip	167 kB	167 kB	✓
app-page-exp..prod.js gzip	93.4 kB	93.4 kB	✓
app-page-tur..prod.js gzip	94.1 kB	94.1 kB	✓
app-page-tur..prod.js gzip	88.7 kB	88.7 kB	✓
app-page.run...dev.js gzip	137 kB	137 kB	✓
app-page.run..prod.js gzip	88 kB	88 kB	✓
app-route-ex...dev.js gzip	23.8 kB	23.8 kB	✓
app-route-ex..prod.js gzip	16.4 kB	16.4 kB	✓
app-route-tu..prod.js gzip	16.4 kB	16.4 kB	✓
app-route-tu..prod.js gzip	16 kB	16 kB	✓
app-route.ru...dev.js gzip	23.2 kB	23.2 kB	✓
app-route.ru..prod.js gzip	16 kB	16 kB	✓
pages-api-tu..prod.js gzip	9.37 kB	9.37 kB	✓
pages-api.ru...dev.js gzip	9.64 kB	9.64 kB	✓
pages-api.ru..prod.js gzip	9.37 kB	9.37 kB	✓
pages-turbo...prod.js gzip	21.8 kB	21.8 kB	✓
pages.runtim...dev.js gzip	22.5 kB	22.5 kB	✓
pages.runtim..prod.js gzip	21.8 kB	21.8 kB	✓
server.runti..prod.js gzip	49 kB	49.1 kB	N/A
Overall change	875 kB	875 kB	✓

Diff details

Diff for page.js

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Diff for edge-ssr.js

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Diff for server.runtime.prod.js

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Commit: f159a28

[ijjk]

Tests Passed

[ztanner]

thanks for the PR & for deep diving into this!