post: skip sticky disk cleanup when this step reused an existing buildkitd - #136

Summary

When setup-docker-builder runs more than once in the same job (e.g. a composite action that retries a build and calls setup + build-push per attempt), only the first instance starts buildkitd and mounts /var/lib/buildkit. Since dc5a171, later instances detect the running daemon and reuse it, and their post step correctly skips shutdown via maybeShutdownBuildkitd() — but it then falls straight into the sync / integrity check / umount /var/lib/buildkit block while buildkitd is still alive. That unmount fails 3× with target is busy and is reported as BUILDER_CLEANUP, feeding blacksmith_stickydisk_failures_total and the "Docker Setup High Failure Count" alert. Post steps run in reverse order, so this happens on every reuse instance before the owning instance's post finally shuts down and unmounts successfully. Net effect: the disk was being cleaned up, but each reuse instance emitted a spurious cleanup failure (N-1 false reports per job with N setup invocations, which only happens when earlier attempts failed).

Fix: record reuse in action state and have the post step exit early in that case.

// startBlacksmithBuilder, existing-buildkitd branch
stateHelper.setReusedExistingBuilder(true);
return { addr: null, exposeId: "" };

// post
if (stateHelper.getReusedExistingBuilder()) {
  core.info("This instance reused a builder started by an earlier setup-docker-builder step; skipping cleanup ...");
  removeTmpDir();
  reporter.closeBlacksmithAgentClient();
  return;
}

A dedicated state key is used rather than !getBuildkitdAddr() because the fallback path (sticky disk mounted, buildkitd failed to start) also has no addr but does still need the unmount.

dist/ regenerated via the pre-commit hook.

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Requested by: @bruce-y

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