post-step: keep committing the sticky disk when buildkitd is already gone; disable in-use pruning

[piob-io]

Summary

Follow-up to useblacksmith/buildkit#34 for the v2 action. Fixes the failure mode where a job builds and pushes fine but the post-step skips the sticky-disk commit ("Skipping sticky disk commit due to cleanup error"), leaving the cache frozen on the previous snapshot.

Chain being broken: user runs buildx prune --all --keep-storage N in-job → forked buildkitd prunes a parent layer whose children survive → the post-step's diagnostic buildctl du nil-derefs in DiskUsage and buildkitd dies → pkill -TERM buildkitd exits 1 → cleanupError set → commit skipped.

buildkitd.toml (writeBuildkitdTomlFile): add pruneInUse = false under [worker.oci]. With buildkit#34 this selects upstream leaf-first prune semantics for v2, so a size-bounded prune can no longer remove a still-referenced parent. buildkitd builds that don't know the key ignore it. v1 (main) is untouched and keeps the current in-use pruning via the daemon default.

Commit gating (shutdownBuildkitd, maybeShutdownBuildkitd):

// shutdown.ts
export async function shutdownBuildkitd(): Promise<boolean>
  // pkill -TERM exit 1 (no process)  -> warn, return false   (was: throw)
  // pkill -KILL exit 1 (raced exit)  -> return true          (was: throw)
  // graceful / real SIGKILL          -> return true

// main.ts post-step
const wasRunning = await shutdownBuildkitd();
if (!wasRunning) { warn; await logBuildkitdLogTail(); return; }  // cleanup continues

buildctl du was already diagnostic-only (logBuildCacheContents catches). Only sync / unmount / post-unmount flush failures set cleanupError now, i.e. the commit is gated on filesystem safety, not on whether the daemon happened to still be alive.

Cleanup: remove the dead pruneBuildkitCache helper (+ its tests) and the stale max-cache-size-mb input docs — v2's action.yml has no such input; README now points users at buildkitd GC instead of in-job prune.

Tests: shutdown.test.ts (graceful → true, already-gone → false without throwing, other pkill errors rethrown); writeBuildkitdTomlFile asserts pruneInUse = false + GC in the generated TOML. dist/ rebuilt.

Link to Devin session: https://app.devin.ai/sessions/5c8deaae10f14cf49877e6f03ff873f9
Open in Devin Desktop: https://app.devin.ai/desktop/session/5c8deaae10f14cf49877e6f03ff873f9?variant=devin
Requested by: @piob-io

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