Replaces the phase-0 resolution probe with the real actions, merging the two
independent per-branch Cargo cache implementations on this forge into the
design neither of them had.
## The merge
- zemyna seeds a PR branch by `cp -al` hardlink clone (near-free: cost scales
with inode count, not bytes) from the base branch's LIVE target dir — a
torn read waiting for a second job slot (its own #911).
- emowheel seeds from a PUBLISHED IMMUTABLE SNAPSHOT (no race by
construction) but with `cp -a`, duplicating ~35 GB per branch.
This ships hardlink-clone FROM a published snapshot: zemyna's cost profile,
emowheel's soundness, and #911 closed structurally rather than by the runner
happening to have one execution slot.
## The bug both implementations have
A build inside a `cp -al` clone DOES mutate the directory it was cloned from.
Cargo replaces real artifacts, but writes its metadata — and build scripts
write their OUT_DIR — with a plain truncating write, straight through the
shared inode. Measured set: `.fingerprint/<unit>/dep-<target>` (under
CARGO_UNSTABLE_CHECKSUM_FRESHNESS), `build/<pkg>/{output,root-output,out/**}`,
`deps/*.d` and `<profile>/*.d`.
The checksum-freshness case is a wrong answer, not a slow build: a PR clone
rewrites the base's dep-info to describe the PR's sources while the base's
cache still holds the artifact built from the base's; once the PR merges, the
base's next run finds the checksums match, reports `Fresh`, and links a binary
built from the pre-merge code. Reproduced end to end.
Fix: hardlink the artifacts (the GB), real-copy the metadata (the MB) — about
3.7% of a 6.9 GB Bevy target dir, against 100% for a full copy.
## Contents
- `cargo-cache/action.yml` — consume: resolve keys, seed from the base's
snapshot via staging + one atomic rename, strip Cargo lock files, unshare
the mutable paths, restore mtimes from git history, lock, prune.
- `cargo-cache-publish/action.yml` — publish: record the build watermark,
atomically republish the snapshot on a protected branch, release the lock
(`mode: release-lock` for the `if: always()` step).
- `scripts/` — all logic, so it is testable standalone; the YAML is wiring.
- `scripts/*selftest.sh` + `selftest.sh` — five suites, 63 assertions, every
fix paired with a control that reproduces the bug. All green locally.
Eviction merges emowheel's liveness pass (dead branches pruned
unconditionally, not gated on disk pressure) with LRU-under-pressure, but
inverts the order within the pressure pass: `target-*` before `snapshot-*`,
because a snapshot is hardlinked to everything cloned from it, so evicting one
frees almost no real bytes while costing every future PR its warm start.
restore-mtimes.sh is ported from emowheel (the watermark variant, which closes
the merge hazard zemyna's copy still has) with its provenance de-projectised.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Sqh2vscfzisk83VuPVQX9L
