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Cache replay restores outputs with their archived modification times, so mtime-based tools skip changed files

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Bewertung

Schwierigkeit
3/5
Geschätzter Aufwand
1-2 Tage
Anfängerfreundlichkeit
72/100
Issue-Typ
Bug
Klarheit
Klar beschrieben
Aktivitätsstatus
Aktiv
Tech-Stack
rust, typescript, vite
Bereich
build-system, tooling

Rechercherichtung

Start with crates/vt/src/session/cache/archive.rs, especially the metadata recording and unpacking lines linked in the issue; check how the tar crate handles modification times during extraction. The issue suggests disabling mtime preservation before unpacking. Confirm the behavior with the provided vp run gen reproduction and check that replayed outputs get fresh modification times while cache fingerprints remain content based.

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Beschreibung

Written by an AI agent (Claude) on behalf of @jasonkuhrt.

A cache hit restores each output file with the modification time it had when it was archived, not the time it was restored. A tool downstream that decides staleness by modification time then treats a replayed file with new content as older than its own last build, and skips it.

We hit this as a green type check on stale code. A generate task's outputs were replayed from an entry archived earlier, under an incremental TypeScript build whose build info stayed in place. The build reads an input only when it is newer than its build info, so it skipped the projects whose generated sources had just changed.

Repro with vite-plus 1.4.0:

// vite.config.ts
export default {
  run: {
    tasks: {
      gen: {
        command: 'cp in.txt out.txt',
        cache: { input: ['in.txt'], output: ['out.txt'] },
      },
    },
  },
};

echo a > in.txt
vp run gen            # cache miss, executing
stat -f %m out.txt    # 1792806461
rm out.txt
sleep 3
vp run gen            # cache hit, replaying
stat -f %m out.txt    # 1792806461, while `date +%s` is 1792806475

The archive records each file's metadata, its time included (header.set_metadata, archive.rs:39). The restore unpacks with tar's defaults (archive.rs:62-64), and tar 0.4.45 defaults preserve_mtime to true (src/archive.rs:62). Nothing in the cache code or docs says the times are meant to survive a restore.

The harm comes when the entry was archived before the downstream tool last ran. Within one checkout that's rare, because the local entry for a task is the latest one. Across machines and branches it's routine: the remote tier, or a cache directory carried between CI runs, replays content another run archived hours earlier, onto a checkout whose downstream build ran since.

A possible fix is archive.set_preserve_mtime(false) before unpack, so a restored file is dated when it lands. Vite Task's own fingerprints hash content (crates/vt/src/session/execute/fingerprint.rs), so its cache shouldn't be affected. A tool that compares two outputs of the same archive by time would see them restored at nearly the same moment, which seems the safer error.

Our workaround: before an incremental build, re-date each input whose status-change time is after its build info but whose modification time is before it. That's the signature of an extraction that kept an old time.

Vorherrschende Sprache
Rust
Sterne
480
Forks
50
Ø Merge
1 T. 3 Std.
Gemergte PRs (30 T.)
70

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