gitdan-actions/README.md

# gitdan-actions

Shared Gitea Actions composite actions for the gitdan forge.

Currently one thing, done properly: **`cargo-cache`** — a persistent,
per-branch Cargo build cache for self-hosted Gitea runners, where a pull
request's cache is a near-free hardlink clone of an immutable snapshot its
base branch published.

> **Final home:** this repository will live at `daniel/gitdan-actions`. Pin
> that path in `uses:` once the transfer completes.

---

## Why this exists

Two of this forge's Rust projects independently built the same idea and each
got one half right.

| | seeding mechanism | seed source |
|---|---|---|
| project A | `cp -al` hardlink clone — near-free, cost scales with inode count, not bytes | the base branch's **live** target dir — races a build that is still writing |
| project B | `cp -a` full copy — sound, but ~35 GB duplicated per branch | a **published immutable snapshot** — nothing ever writes it while it is read |

This action is the diagonal: **hardlink-clone from a published snapshot.**
Cheap like A, sound like B. It also closes a latent race in A by construction
(the seed is staged and swapped in with one atomic rename) rather than relying
on the runner having a single execution slot.

One thing neither project had, and the reason the clone is not a plain
`cp -al`: **a build inside a hardlink clone does mutate the directory it was
cloned from.** Cargo writes real artifacts by replacing them, but writes its
metadata — and build scripts write their `OUT_DIR` — with a plain truncating
write, straight through the shared inode. Under
`CARGO_UNSTABLE_CHECKSUM_FRESHNESS` the file that gets corrupted is
`.fingerprint/<unit>/dep-<target>`, which holds the per-source checksums that
decide freshness, and the failure is silent stale-artifact reuse rather than a
slow build. `scripts/hardlink-clone-selftest.sh` reproduces it as an explicit
control and asserts the fix. The fix is to hardlink the artifacts (the GB) and
real-copy the metadata (the MB) — about 3.7% of a Bevy-sized target directory,
against 100% for a full copy.

---

## Quick start

```yaml
name: CI
on:
  push:
    branches: [main, dev]
  pull_request:
    branches: [main, dev]

jobs:
  ci:
    runs-on: ubuntu-latest
    # REQUIRED, and it cannot come from the action: `container.volumes` is a
    # job-level property, so the persistent cache volume must be declared
    # here. Use a volume name unique to this repository.
    container:
      volumes:
        - myrepo-ci-target:/cache
    steps:
      - uses: actions/checkout@v4
        with:
          # REQUIRED. The mtime restore walks every commit that ever touched a
          # tracked file; a depth-1 checkout makes every file resolve to the
          # tip commit and the cache stops working. The action fails loudly
          # rather than silently degrading if this is missing.
          fetch-depth: 0

      - uses: https://gitdan.com/daniel/gitdan-actions/cargo-cache@v1
        with:
          protected-branches: 'dev main'

      # ... toolchain, system deps, and the build itself. CARGO_TARGET_DIR is
      # already exported to the job environment by the step above.
      - run: cargo clippy --workspace --all-targets -- -D warnings
      - run: cargo test --workspace

      # After the build succeeds: record the watermark, and publish a snapshot
      # if this run is a push to a protected branch.
      - uses: https://gitdan.com/daniel/gitdan-actions/cargo-cache-publish@v1

      # Release this job's cache lock even when the build failed, so the
      # eviction pass does not have to wait out the staleness grace period.
      - if: always()
        uses: https://gitdan.com/daniel/gitdan-actions/cargo-cache-publish@v1
        with:
          mode: release-lock
```

Recommended alongside it, in the workflow's `env:` block:

```yaml
env:
  CARGO_INCREMENTAL: 0                  # per-run bloat on a persistent volume
  CARGO_PROFILE_DEV_DEBUG: line-tables-only
  CARGO_PROFILE_TEST_DEBUG: line-tables-only
  # Nightly only. Content-addressed freshness instead of mtime-based — a
  # strictly stronger guarantee, complementary to the mtime restore (which
  # still covers directory-form `rerun-if-changed` build-script watches).
  CARGO_UNSTABLE_CHECKSUM_FRESHNESS: "true"
```

---

## How it works

```
/cache/
  target-<key>       one per ref. Where a build actually runs.
  snapshot-<key>     one per publisher ref. Immutable between publishes;
                     the only thing a consumer ever clones from.
```

Transient dot-prefixed entries appear alongside those two — staging trees,
eviction asides, reader markers. Their names are a contract with the host that
owns the volume; see [Scratch names in a cache root](#scratch-names-in-a-cache-root-are-a-cross-repo-contract).

`<key>` is the ref sanitised to a safe path component, capped at 48
characters, plus an 8-hex SHA-1 prefix of the *raw* ref. The hash is not
decoration: `feat/foo` and `feat-foo` sanitise identically and would otherwise
share one directory.

**A pull request run** resolves its own key from `github.head_ref` (not
`ref_name`, which on a `pull_request` event is a synthetic merge ref that
changes on every push) and its base key from `github.base_ref`. If it has no
directory yet, it hardlink-clones `snapshot-<base>` into a staging path, strips
Cargo's lock files, real-copies everything Cargo writes in place, and renames
the staging path into `target-<own>`.

**A push to a protected branch** has no base to layer over. It builds in its
own directory and, if the build goes green, republishes it as
`snapshot-<own>`: stage a clone, rename the old snapshot aside, rename the new
one in, then reclaim the old one once nothing is still reading it. Consumers
only ever observe a complete snapshot or none at all.

**Concurrency, on the destination.** Two jobs sharing one cache key each stage
under their own tag and race on one atomic rename; the loser discards its
staging copy. There is no window in which a partially-populated directory is
visible under the final name. Two jobs then building in the same directory is
Cargo's own `.cargo-lock` territory, which is what that lock is for.

**Concurrency, on the source.** The atomic rename is necessary and not
sufficient, because renaming a *truncated* tree publishes a truncated tree
atomically. A clone reads its source over many seconds, and a publisher
rotating that source unlinks the generation being read — at which point
`cp -al` can silently omit a subtree it never saw, and report success. Two
mechanisms, both required:

- **The publisher does not unlink under a reader.** A consumer publishes a
  `.reading-<snapshot>-<tag>` marker *before* it resolves the snapshot path;
  the publisher scans for markers *after* its first rename. A consumer holding
  the old generation therefore published its marker before that scan and
  cannot be missed, and one that arrives after the scan necessarily resolves
  to the new generation. The publisher waits for readers to drain
  (`read-grace-seconds`, default 300) and, if they do not, **defers** the
  reclamation rather than forcing it — the old generation stays on disk and is
  swept by a later publish.
- **The consumer verifies its own clone.** Every attempt checks `cp -al`'s
  exit status, the source directory's inode before and after (a wholesale
  replacement mid-walk would otherwise splice two generations), and the entry
  count (the only signal for a subtree unlinked before its parent was listed —
  there is no error to read). A tree that fails any of the three is deleted
  and the clone retried; one that fails the last attempt fails the job. A
  partial tree never reaches the final name. The two counts cost one metadata
  walk each: measured on ext4 with a warm cache over a 78,554-entry tree, 44 ms
  per walk against 3,126 ms for the `cp -al` they guard — about 2.8%.

**What this does and does not guarantee.** Four separate claims, deliberately
not collapsed into one:

- **A publisher rotating a snapshot cannot tear a clone of it — by
  construction.** This is the case zemyna #911 is about, and the marker
  ordering above is what closes it: the publisher's scan cannot miss a
  consumer that resolved the old generation, and on timeout it defers the
  unlink rather than forcing it. On this path the consumer's own verification
  is a redundant second check, not the thing holding the guarantee up.
- **Nor can the eviction pass — by the same construction.** A cache chosen for
  eviction is renamed aside and only then re-examined for readers, so the scan
  the unlink rests on happens strictly after that rename, exactly as the
  publisher's does. A consumer that resolved the directory published its
  marker before the scan and so cannot be missed; one arriving after the
  rename cannot resolve the path at all and starts cold instead. A cache
  claimed inside that window is put back under its own name, and one whose
  name a concurrent seed has taken in the meantime is left aside and reclaimed
  by a later pass once its readers drain. That later pass leaves an aside
  directory alone until it has been set aside for a minute — not for the
  unlink's sake, which the ordering proof above already covers, but so that a
  pass still deciding about one is never mistaken for a pass that died holding
  it. That settle window is a bound rather than a construction, and it is the
  only part of this that is. Until the rest of it was structural it was merely
  policy — snapshots belong to protected refs, protected refs are never
  eviction candidates — a property held by vigilance rather than by
  construction.
- **Every other way the source can change mid-clone is detected, not
  prevented.** A `seed-fallback-dir` pointing at a directory something else
  writes has no interlock at all. There, the per-attempt verification is what
  stands between a torn read and a corrupt cache: the clone is retried
  (`CACHE_CLONE_ATTEMPTS`, default 4) and then **fails the job loudly** —
  never seeded partially, and never degraded to a silent cold build.
- **Disk reclamation is bounded, not immediate.** A consumer slower than the
  grace period leaves one extra snapshot generation of directory entries on
  the volume until a later publish sweeps it; a consumer whose job was killed
  outright holds it until its marker passes `reader-stale-seconds`. The
  residual is capped at one deferred generation per publisher ref, and its
  real cost is close to inode count rather than byte count, since the
  artifacts are hardlinked to whatever cloned them. A declined eviction is
  never unlinked under the job that claimed it; what it costs meanwhile is
  disk, normally as the cache restored under its own name and otherwise as one
  set aside for a later pass to reclaim.

**Eviction** runs three passes: caches for branches that no longer exist on
origin are removed unconditionally; then, only if free space is under the
threshold, live caches are evicted oldest-first; then, as a last resort, this
run's own cache. Protected refs and any cache held open by a running job are
never candidates. Within the pressure pass, `target-*` directories are evicted
before `snapshot-*` ones — the reverse of the obvious order, because a
snapshot is hardlinked to everything cloned from it, so removing one frees
almost no real bytes while costing every future PR its warm start.

**File mtimes.** `actions/checkout` stamps every file with "now", which makes
every crate look changed to Cargo's mtime-based freshness check — a persistent
target directory buys nothing without fixing that. Each tracked file is
restored to the timestamp of the most recent commit that touched it, plus a
watermark override: for any file that changed since *this cache's own last
successful build*, "now" is stamped instead. That override is what makes a
merge safe, since a merge can introduce a commit authored before this cache's
last build, where the historically-correct mtime is exactly the wrong answer.

---

## Scratch names in a cache root are a cross-repo contract

Read this before adding a dot-prefixed directory under a cache root.

The volume is not swept by these scripts alone. A host-level arbiter —
daniel/gitdan's `scripts/ci-cache-reclaim.sh`, which runs outside any job —
reclaims the scratch trees a killed job strands here, and reads the reader
markers to decide whether a tree is still live. Its `LEFTOVER NAMING CONTRACT`
block is the canonical description; this side owns the names.

| name | produced by | if the job dies holding it |
|---|---|---|
| `.stage-<tag>` | `cache-lib.sh`, `hardlink_clone_into()` | stranded; only the arbiter reclaims it |
| `.publish-new-<tag>` | `publish-snapshot.sh` | stranded; see `daniel/gitdan#30` |
| `.publish-old-<key>-<tag>` | `publish-snapshot.sh` | swept by the next publish of that key, if there is one |
| `.evicting-<name>-<pid>` | `prune-cache.sh`, `evict_dir()` | swept at the start of the next prune pass |
| `.reading-<source>-<tag>` | `cache-lib.sh`, `reader_lock_acquire()` | not garbage — see below |

A `.reading-*` marker is protective, not scratch: it is how both this repo's
prune pass and the arbiter tell an in-flight clone from an abandoned one, and
the arbiter never deletes one. Delete a live marker and the tree it covers
becomes eligible for an unlink underneath the walk that is reading it, which is
the silent truncation the whole interlock exists to prevent.

**The rule: no new dot-prefixed entry under a cache root without a matching
prefix in `ci-cache-reclaim.sh`.** Adding a shape counts exactly as much as
renaming one. That script enumerates by explicit prefix rather than by dotglob
— deliberately, because a dotglob would pull reader markers into the candidate
stream alongside the trees they protect — so a name it has not been told about
is not handled conservatively, it is invisible, and an unreclaimed staging tree
is a full clone of a multi-GB target dir on the one volume whose entire problem
is disk. `.publish-new-` is the worked example: the two `.publish-*` names were
outside the contract when it was written, and `.publish-new-` strands exactly
as `.stage-` does, so it is the one that needed catching. Bringing both under
the arbiter's enumeration is tracked as `daniel/gitdan#30`.

The two lists are meant to be the same length — the five names above, and the
prefix constants `ci-cache-reclaim.sh` *declares*. Not the subset it enumerates
as reclaim candidates: that one is smaller, because `.reading-` is read and
never swept. A mismatch means one side gained a shape without telling the
other, which is the drift the rule exists to catch and the cheapest thing to
check.

**The staleness constants are part of the same contract, and that half has a
direction.** `CACHE_READ_STALE_SECONDS` (`cache-lib.sh`) and
`STALE_LOCK_SECONDS` (`prune-cache.sh`) are mirrored there, and the arbiter's
copies must be **greater than or equal to** these. Raising one here for longer
jobs, without raising its mirror first, makes the arbiter treat a marker whose
owner still considers it live as stale and delete a tree under an in-flight
clone — and its own minimum-age guard does not back-stop that, since a clone
holding a three-hour-old marker has a roughly three-hour-old staging tree.
Lowering either here needs no coordination: the arbiter then only defers a
reclamation this side would already have permitted, which costs disk rather
than correctness.

### Names this repo doesn't reclaim, but the arbiter depends on

The five names above are leftovers — dead trees gitdan's arbiter finds and
deletes. Four other dot-prefixed names change that arbiter's *behaviour*
without ever being a leftover: it reads them to make a correctness decision
and never reclaims them. They are a real cross-repo dependency too, on the
name's spelling rather than on the tree's lifetime — gitdan's
`DEPENDED-UPON NAMES CONTRACT` block (in `scripts/ci-cache-reclaim.sh`) is the
canonical description; this is the producer's half for the two we own.

| Name | Produced here by | Consequence of an unannounced rename |
|---|---|---|
| `.ci-lock-<id>` | `scripts/cache-lock.sh` (acquire/release) and `cache-lib.sh`'s `write_cache_lock()` | The arbiter's lock check silently stops matching — a live job's staging tree loses its liveness guard and becomes an ordinary reclaim candidate while still in use |
| `.cache-last-used` | `cargo-cache/action.yml`, stamped every run | The arbiter falls back to directory mtime — silently reordering its eviction order, and possibly failing to recognise the directory as a cache dir at all |

Two more names are in gitdan's list — `.gitea-last-used` and `.ci-keep` — and
neither is produced by anything in this repo, so there is no producer-side
half to write here. `.gitea-last-used` is a naming convention individual
repos used before adopting this shared action; nothing here writes it, and
gitdan's script reads it only for compatibility with directories created
under that older scheme. `.ci-keep` is a per-repo, hand-placed opt-out a
consuming repo's own workflow drops directly into a cache directory it wants
exempted from eviction — never something this action or its scripts write.

**Adding a fifth depended-upon name counts as much as renaming one of the two
above.** If a future change here makes gitdan's arbiter start depending on the
spelling of some new dot-prefixed name — a name it reads for a decision but
never reclaims — that is exactly this category, and it needs the matching
`DEPEND_*` entry on gitdan's side before it ships, not after.

---

## Inputs

### `cargo-cache`

| input | default | meaning |
|---|---|---|
| `cache-root` | `/cache` | mount point of the persistent volume inside the job container |
| `protected-branches` | `dev main` | refs that publish snapshots and are never evicted |
| `min-free-percent` | `10` | prune when free space drops below this |
| `restore-mtimes` | `true` | restore tracked-file mtimes from git history |
| `prune` | `true` | run the eviction pass |
| `liveness-prune` | `true` | within eviction, remove caches for branches gone from origin |
| `own-ref` | *(auto)* | override; defaults to `github.head_ref`, else `github.ref_name` |
| `base-ref` | *(auto)* | override; defaults to `github.base_ref` (empty on push) |
| `seed-fallback-dir` | *(empty)* | absolute path to seed from when no snapshot exists — for migrating off an existing flat cache |
| `watermark-file` | `.ci-watermark-<job>-sha` | must differ per job when two jobs share one cache key |
| `lock-id` | `<job>-<run_id>` | identifies this job's cache lock |
| `stale-lock-seconds` | `7200` | age past which another job's lock is treated as abandoned |

Outputs: `target-dir`, `cache-key`, `seeded-from` (`own` \| `base-snapshot` \|
`own-snapshot` \| `fallback-dir` \| `concurrent-peer` \| `cold`).

Exports to the job environment: `CARGO_TARGET_DIR`, `CARGO_CACHE_ROOT`,
`CARGO_CACHE_KEY`, `CARGO_CACHE_LOCK_ID`, `CARGO_CACHE_SCRIPTS`,
`CI_WATERMARK_FILE`.

### `cargo-cache-publish`

| input | default | meaning |
|---|---|---|
| `cache-root` | `/cache` | must match the consume action |
| `protected-branches` | `dev main` | refs that publish snapshots |
| `mode` | `publish` | `publish`, or `release-lock` for the `if: always()` step |
| `own-ref` | *(auto)* | override; defaults to `github.head_ref`, else `github.ref_name` |
| `publish-on-events` | `push` | events on which a protected ref actually publishes |
| `read-grace-seconds` | `300` | how long the swap waits for in-flight clones of the generation it replaces before reclaiming it; on timeout the reclamation is deferred, never forced |
| `reader-stale-seconds` | `7200` | age past which a consumer's read marker is treated as abandoned by a killed job |
| `record-watermark` | `true` | record HEAD as this cache's watermark (PR runs too) |

`publish-on-events` defaults to `push` on purpose: a `pull_request` run from
`dev` into `main` has `own-ref` `dev` and would otherwise publish a snapshot
of a merge-preview build, which is not what `dev` is.

---

## Multiple jobs in one workflow

Jobs sharing a cache key (a `ci` job and a `wasm` job on the same branch, say)
each need their **own** watermark file. A shared one breaks the moment two
jobs run in sequence within one trigger: job A advances the watermark to HEAD,
and job B then reads that just-advanced value, computes an empty diff, and
loses the merge protection entirely. The default (`.ci-watermark-<job>-sha`)
already gives each job its own; only override `watermark-file` if you also
override `lock-id`, and then keep both distinct per job.

---

## Constraints of this runner

- **The repository must be public.** act_runner fetches actions by anonymous
  git clone and has no credentialed-fetch option, so a private action
  repository simply fails to resolve. Nothing secret goes in here.
- **`uses:` needs the absolute URL.** A bare `owner/repo` resolves against
  github.com, because Gitea's `DEFAULT_ACTIONS_URL` is unset — and it has to
  stay unset, or `actions/checkout`, `dtolnay/rust-toolchain` and
  `taiki-e/install-action` stop resolving.
- **The runner pre-fetches every referenced action before running any step**,
  so a bad action reference fails the job at step 0 rather than where it is
  used.
- **`container.volumes` is job-level** and cannot be set from inside a
  composite action. The consuming workflow declares it; see the quick start.
- **The cache volume is ext4** — no reflink support, which is precisely why
  hardlinks are the mechanism that makes cloning cheap.

---

## Versioning

Pin `@v1`. It is a moving major tag: fixes and backward-compatible inputs move
it forward, and anything that would break an existing consumer gets `v2`
instead. Pin a commit SHA if you want a frozen version.

The alternative is immutable release tags — `v1.0.1`, `v1.0.2`, … — with every
consumer edited to point at the new one per fix. That is the safer model in
general and the wrong one here. What it buys is the ability to hold one
consumer back on a known-good version; what it costs is a PR in every consumer
repo per fix, and its real failure mode with two consumers and one operator is
that the second one is simply never updated and quietly runs a version nobody
is testing. The moving pointer makes a release one action with one blast
radius, which is the thing worth being deliberate about. Anyone who wants the
immutable behaviour already has it, by pinning a SHA.

**What `@v1` promises** is that whatever it points at works with the inputs
documented above, spelled as they are documented. A change that renames or
removes an input, changes a default in a way that changes behaviour, or
requires something new of the consuming workflow — another `container.volumes`
entry, another permission — is a `v2`, not a `v1` move. Everything else moves
`v1`: correctness fixes, new optional inputs, and anything internal to
`scripts/`.

**Moving the tag is a release step, and it is the operator's.** Merging to
`main` ships nothing to anybody. `v1` is a lightweight tag and does not follow
a branch, so until it is re-pointed every consumer keeps fetching the commit it
already named, whatever `main` now says. The gap is deliberate: re-pointing
`v1` changes what another repository's CI executes on its next run, so it is a
decision taken once, knowingly, after the merge — never something a merge does
by itself.

```bash
git fetch origin
git tag -f v1 origin/main
git push -f origin v1
git ls-remote --tags origin v1     # must equal git rev-parse origin/main
```

**Downstream** are emowheel, which pins `cargo-cache@v1` and
`cargo-cache-publish@v1` across its CI workflow, and zemyna, migrating to the
same pin. Both pick a move up on their next run with no change on their side,
which is the whole point of the moving pointer and also the reason the move is
not automatic.

---

## Development

```bash
shellcheck -x --source-path=scripts scripts/*.sh
bash scripts/selftest.sh          # everything (needs cargo)
bash scripts/selftest.sh --fast   # fixture-only suites, no compiler
```

Both run in CI — `.gitea/workflows/ci.yaml`, one job, on pushes to `main` and
on PRs that were non-draft when the run was created. It installs shellcheck
and a stable Rust toolchain and references no credentials; the scratch
workspaces the compiler-backed suites build use path dependencies only, so
nothing reaches crates.io. It runs the full suite rather than `--fast`,
because the two compiler-backed suites are the ones that check this scheme
against real Cargo instead of against a fixture. Draft (`WIP:`-titled) PRs
skip it, and un-drafting does **not** un-skip them — the guard is evaluated
when a run is created and un-drafting creates none, so push an empty commit
after un-WIP'ing.

This repo is consumed by three other repos' CI at `@v1`, a moving tag, so a
change here reaches all of them at once. That is what the gate is for.

| suite | covers |
|---|---|
| `hardlink-clone-selftest.sh` | that a build in a clone cannot mutate its source — with a control proving a raw `cp -al` does. Needs a real compiler. |
| `seed-target-dir-selftest.sh` | seed-source preference, lock-file stripping, two jobs racing on one cache key, **and one scenario per check a hardlink clone is validated against**: a source rotated wholesale, a subtree silently lost from the walk, a copy that reports failure over a tree both other checks read as whole, and a source identity that resolved at neither end — plus a staging tree that could not be privately owned being discarded rather than published, and the publisher's log showing it waited on the consumer's own reader-lock marker before reclaiming a rotated snapshot |
| `publish-snapshot-selftest.sh` | the atomic swap, that a live consumer survives a republish, and the publisher's side of the rotation race: deferred reclamation under a live reader, and its sweep once the reader is gone |
| `prune-cache-selftest.sh` | liveness, protection, locking, eviction order, self-clear, **and that a cache a job claims *inside* the check-to-unlink window survives it** — against a real scratch `origin` |
| `restore-mtimes-selftest.sh` | the merge hazard and the watermark that closes it, including the two-jobs-one-namespace case. Needs a real compiler. |

Every suite runs the actual script, not a reimplementation of its logic, and
every fix scenario is paired with a control that reproduces the bug — a
scenario that passes either way proves nothing.

The concurrency scenarios take one of three shapes, and none of them races
for the interleaving its assertion depends on.

**A genuine race whose asserted invariant holds under any interleaving.**
`seed-target-dir-selftest.sh` scenario 7 starts two real seeds on one cache
key and asserts only what must be true whichever of them wins the rename.

**A `PATH` stub on a command the code under test calls at a known point**,
which places the interference inside the window rather than hoping it lands
there. `prune-cache-selftest.sh` scenario 12 stubs `du`, so the pass's own
measurement publishes a reader marker strictly between its check and its
unlink. `seed-target-dir-selftest.sh` uses the shape five times over, on two
different commands. Scenarios 8a, 8b and 8c stub `cp`, so the consumer's own
clone is what rotates the snapshot underneath it, loses a subtree of its own
source, or reports a failure over a tree that is in fact whole — each strictly
inside the window that clone's checks cover; scenario 10 stubs `cp` one level
down instead, refusing the per-file copies that unshare the mutable paths.
Scenario 8d stubs `stat`, because the check it pins fires on an identity that
could not be READ rather than on one that changed, and the only way to make
that the sole witness is to fail the identity reads while the copy between
them succeeds. 8a also starts a second real process — the actual
`publish-snapshot.sh` — but the stub is what fixes where its swap lands; the
concurrency is incidental to the determinism. Scenario 11 reuses 8a's exact
stub and the same forced rotation, but reads a different witness: not the
consumer's own checks, but a line in the *publisher's* log reporting that it
waited on the reader marker this consumer's clone wrote — `reader_lock_acquire`
is exercised by 8a already, but nothing asserts it actually fired until this
scenario reads that line back. Every stub asserts that it fired, because a
scenario whose interference silently did not happen passes for the wrong
reason.

**A synthetic stand-in for the other side, where that artefact *is* the
contract.** `publish-snapshot-selftest.sh` scenarios 6 to 8 hold a
`.reading-*` marker instead of running a slow consumer: the marker is the
whole agreement between reader and publisher, so holding one is being a
reader, and racing a real one would make the suite's runtime the thing under
test.

Gating the interfering step on *observed progress* of the step it interferes
with was an earlier answer here, and it is not one: seeing that a walk has
started says nothing about where it will be when the interference lands, so
the assertion downstream held only some of the time (issue #3). No scenario
does it any more.

Where more than one guard could catch a fault, a scenario should assert
*which* one did — otherwise deleting the guard under test leaves the suite
green because a sibling fires in its place. Scenarios 8a to 8d and 10 of the
seed suite do, and each is reddened by exactly one mutation of the clone's
checks. Scenario 9 does not, and a mutation still survives it: with its source
unreadable, `cp -al` leaves the staging directory at mode `000`, so deleting
the exit-status check does not change the outcome — the unshare pass aborts
the clone instead, and the assertion is satisfied down a path it was not
written for.

That is the standing hazard here, and it is not hypothetical. Two guards that
can each catch the same fault mask each other, so **neither** is individually
necessary and no fixture built around that fault can pin either one — which is
how both `[ "$cp_rc" -eq 0 ]` and `[ "$i_before" != missing ]` sat unpinned
(issue #5) while looking well covered. Isolating a check means constructing
the state only it can see, not the state that trips several at once.

Scenario 11 applies the same discipline to a witness outside the clone
entirely: not which of several checks inside `hardlink_clone_into` caught a
fault, but whether `reader_lock_acquire`'s marker was observed by anything
outside it at all. The consumer's own log and exit status are silent either
way — a run with the marker deleted still succeeds — so what is asserted is
one line in the *publisher's* log reporting that it waited. Deleting
`reader_lock_acquire` (issue #10) leaves that line unwritten without failing
anything else in the suite.

The action YAML holds no logic beyond wiring; everything testable lives in
`scripts/`. A composite action needs `shell: bash` on every `run:` step, and
the actions reach their shared scripts through
`${{ github.action_path }}/../scripts`, which works because the runner clones
the whole repository when it fetches an action.