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perf(ci): the warm job compiled 145s to write nothing on every merge but one - #628
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CLOUD-840 Every PR compiles the Rust tree cold: the caches exist, are byte-identical across PRs, and no PR can read another's — so `windows` writes 182MB per run that nothing ever restores
Why CLOUD-813 measured Measured 2026-08-21 from the Actions cache API, not inferred from step timings:
This reverses a recorded decision, and that decision named the conditions. CLOUD-176 considered base-branch warming and recommended against it: about 178s of job-time per merge to save about 60s on one pull request's first run, "roughly 3× the job-time it saves." That costing lists four legs — It also wrote down when to revisit, and both conditions now hold:
Recording the reversal rather than quietly re-filing, because CLOUD-176's recommendation is written down and someone will find it. Refinement — Ready (warm the base branch, and get under the cap so the warm entry survives)
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Included review availability: Your plan provides up to 10 included reviews per hour; 5 remain after this review. 📝 WalkthroughWalkthroughThe release workflow now skips Rust cache-warm compilation on exact cache hits. CI parity validation checks the required guard and cache-step ID. Bats tests cover invalid guards, missing IDs, and valid conditions. ChangesCache-warming guard enforcement
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The workflow change is mergeable with owner awareness, but the new validation can accept cache conditions that do not actually restrict compilation to cache misses, allowing the wasted build behavior to return silently; tightening this predicate should follow up. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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…but one Read off the cache API on 2026-08-21: the repository held exactly two `v0-rust-windows-...` entries and both carried the SAME key, `v0-rust-windows-Windows_NT-x64-d44ea756-97231082`, created 15:19 on `refs/heads/main` and 15:24 on `refs/pull/572/merge`. Five merges landed between 15:19 and 18:28, several touching `crates/**`, and the key never moved. It follows the toolchain and the dependency set, not workspace source, and `rust-cache` skips saving when the key already exists. So every cycle after the first restored 182MB, recompiled `batten` for ~145s, wrote nothing, and billed ~6.4 minutes at the 2x Windows multiplier. The recompile was doubly wasted: `cache-workspace-crates: false` means the workspace crate's artifacts were never going into the cache this job exists to fill. Guarding the compile on the restore step's `cache-hit` pays the full price exactly when the key moves -- a dependency bump or a toolchain change, which is what makes a pull request miss -- and a runner boot otherwise. The benefit side is unchanged and now measured: the one cache miss (run 32496183419, restore 6s) spent 681s in `cargo nextest run --workspace` against a 419s mean over twelve hits, so an avoided miss is worth ~262s of wall clock on the longest job in the matrix. Ships with its mechanism. `ci-local-parity` property 17 refuses a `--no-run` compile that carries no `cache-hit` guard, and refuses a guard naming a step id no step declares -- the second because that is the direction the rot is silent in. If the action stops emitting `cache-hit` the expression is empty, the guard holds, and the compile runs: wasteful but visible in the bill. If the `id:` is renamed while the `if:` keeps naming it, the expression is ALSO empty and the job quietly goes back to compiling every time. Same symptom, no signal. Scoped to workflows that restore a cache, so a `--no-run` build with nothing to warm is not asked to guard against an output that cannot exist. Three bats cases: both refusals and the passing shape, so the property is shown to discriminate rather than to refuse everything with `--no-run` in it. Refs: CLOUD-840
…survived `mutant` caught this, which is the point of it: 118 of 119 declared mutations were caught and the one that survived was mine. The gate refuses through `if ! grep -qE ... ; then problem ...`. Replacing the grep with `false` makes `! false` true, so the refusal fires on every step instead of none. The bats case asserts exit 1, the mutated gate still exits 1, the case still passes -- a test green on broken code, which is the exact shape CLOUD-418 built this harness to refuse. `true` is the direction that disables the refusal: `! true` is false, no problem is reported, the gate exits 0, and the case asserting exit 1 goes red. The sibling declaration already had it right, which is why only one of the two survived. Refs: CLOUD-840
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Inline comments:
In `@mise-tasks/ci-local-parity`:
- Around line 954-965: Update the cache-hit validation around the existing
--no-run guard in mise-tasks/ci-local-parity lines 954-965 to reject true-only
predicates and if: lines where cache-hit appears only in comments; require a
steps.<id>.outputs.cache-hit != 'true' predicate before validating the
referenced step ID. Add refusal cases covering both invalid forms in
tests/ci-local-parity.bats lines 494-519.
Apply the same fix in @.github/workflows/release-plz.yml at line 175.
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What this changes
The base-branch cache warm job (
release-plz.yml, CLOUD-840) compiled theworkspace on a Windows runner on every push to
main. This guards that compileon the restore step's
cache-hit, so it pays the full price only when there issomething to warm.
Why — read off the cache API, not reasoned
On 2026-08-21 the repository held exactly two
v0-rust-windows-…entries,and both carried the same key
v0-rust-windows-Windows_NT-x64-d44ea756-97231082— created 15:19 onrefs/heads/mainand 15:24 onrefs/pull/572/merge.Five merges landed between 15:19 and 18:28, several of them touching
crates/**, and the key never moved. It follows the toolchain and thedependency set, not workspace source, and
rust-cacheskips saving when the keyalready exists.
So every cycle after the first restored 182MB, recompiled
battenfor ~145s,wrote nothing, and billed ~6.4 minutes at the 2× Windows multiplier. The
recompile was doubly wasted:
cache-workspace-crates: falsemeans the workspacecrate's artifacts were never going into the cache this job exists to fill.
The benefit side is unchanged and now measured rather than assumed: the one
cache miss (run 32496183419, restore 6s) spent 681s in
cargo nextest run --workspace, against a 419s mean over twelve cache hits.An avoided miss is worth ~262s of wall clock on the longest job in the matrix.
The mechanism it ships with
ci-local-parityproperty 17 refuses two things:--no-runcompile carrying nocache-hitguard;The second matters because it is the direction the rot is silent in. If the
action stops emitting
cache-hit, the expression is empty, the guard holds, andthe compile runs — wasteful but visible in the bill. If the
id:is renamedwhile the
if:keeps naming it, the expression is also empty and the jobquietly goes back to compiling every time. Same symptom, no signal.
Scoped to workflows that restore a cache, so a
--no-runbuild with nothing towarm is not asked to guard against an output that cannot exist.
Tests
tests/ci-local-parity.bats— three new cases: both refusals and the passingshape, so the property is shown to discriminate rather than to refuse
everything with
--no-runin it. All 87 cases pass.mise run mutant— 119 declared mutations, every one caught. The secondcommit fixes a mutation of mine that survived: the gate refuses through
if ! grep …, so replacing the grep withfalsemade the refusal fire onevery step rather than none, and the case asserting exit 1 still passed.
trueis the direction that disables the refusal.
Refs CLOUD-840
DO-NOT-CLOSE: CLOUD-840 also owns extending the warm set beyond
windowsandthe keep-or-revert decision against the next warm cycle, neither of which this
touches.
Summary by CodeRabbit
CI Improvements
Bug Fixes
Tests