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feat(cache): guest-side prune, asserted against an independent oracle

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Measured first, as thread entry #12 requires: a cold furyrs pipeline is 134.7s,
target/ is 3.0 GiB raw and 2.5 GiB pruned, and the stored entry is 645 MiB. A
warm job pays 8.1s (upload + extract) to skip ~120s of dependency compilation.
Publishing costs 15.7s but only on a miss. Build it.

The assertion in the plan turned out to be worthless and is replaced. It
re-scanned for */.fingerprint/<member>-* after deleting exactly those paths, so
asked to catch a planted fury-core fingerprint it exited 0 -- the prune had
removed the evidence before the check looked. An assertion sharing its globs
with the deletion can only prove that rm ran; it cannot see the failure that
matters, which is a glob that never matched the artifact in the first place.

The replacement asks a different oracle. Cargo records in each .d depfile the
sources an artifact was built from: dependencies point into the registry,
anything built from workspace source points back into the workspace. After a
correct prune, zero surviving depfiles reference workspace source outside
target/. Verified negatively too -- an artifact named totallyunrelated-abc123.d
that no prune glob matches, declaring a dependency on fury-core/src/lib.rs,
makes the script exit 2 and refuse to publish.

zstd is already present on the template, so no rebuild is needed; the gzip
fallback stays for the plain LinuxBuild2404.
This commit is contained in:
Simone
2026-07-22 20:55:31 +02:00
parent fce7cfd86d
commit b5c18a6d70
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plans/cargo-dependency-cache-measurements.md
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# Cargo dependency cache — measurements
Measured 2026-07-22 on a `LinuxRust2404` linked clone (4 vCPU, 3.9 GB RAM,
`rustc 1.97.1`, `zstd` present at `/usr/bin/zstd`), running furyrs `b8ea567`
with the `build-command` block from its `.gitea/workflows/ci.yml` verbatim.
Entry #12 of the furyrs thread set the rule: measure before building. There it
killed the registry cache — 9.3s of saving for +800 MB on the template. Here
the numbers point the other way, decisively.
## Baseline
| | |
|---|---|
| Cold pipeline, empty `target/` | **134.7s** |
Matches the 133s furyrs reported for job 1021 and the 126.6s measured in #12,
so the clone is representative.
## Sizes
| | Bytes | Human |
|---|---:|---:|
| `target/` raw | 3,206,670,175 | 3.0 GiB |
| `tar` of raw `target/` | 3,215,503,360 | 3.0 GiB |
| `tar` + `zstd -3 -T0`, raw | 810,108,028 | 773 MiB |
| **`target/` after prune** | 2,719,971,105 | **2.5 GiB** |
| **entry as stored** (`tar --zstd`, pruned) | 676,585,779 | **645 MiB** |
Pruning strips 487 MB (15%) of raw `target/`, which is the workspace crates'
own artifacts — 42 fingerprint directories across the six members
(`fury-core` 11, `fury-gui` 12, `fury-ipc` 6, `fury-backend` 5, `fury-cli` 5,
`furyd` 3). 1,928 dependency artifacts and 903 dependency fingerprints survive.
## Costs
| Step | Seconds | Paid by |
|---|---:|---|
| prune + assert | 0.21 | publish |
| pack (`tar --zstd`) | 10.88 | publish |
| fetch guest→host (140 MiB/s) | 4.62 | publish |
| **publish total** | **15.7** | **only on a cache miss** |
| upload host→guest (179 MiB/s) | 3.60 | restore |
| extract | 4.52 | restore |
| **restore total** | **8.1** | **every warm job** |
## Verdict: build it
A warm job pays **8.1s** to skip roughly **120s** of dependency compilation —
the ~490 external crates that a committed `Cargo.lock` keeps bit-identical
between runs. Net saving ≈ 112s on a 134.7s job, about **83%**.
The publish path costs 15.7s but runs only on a miss, i.e. once per
`Cargo.lock` change, so it is amortised across every run in between.
Transfer is not the bottleneck anyone feared: the virtual NIC moves the entry
at 140–180 MiB/s, so 645 MiB crosses in under 4 seconds each way.
## Notes that changed the implementation
**`zstd` is already in the template.** `/usr/bin/zstd` is present on
`LinuxRust2404`, so the gzip fallback is dead code in practice and no template
rebuild is needed. Keep the fallback anyway — it costs one branch and keeps the
feature working on a plain `LinuxBuild2404`.
**The name-based assertion was worthless and was replaced.** The plan's
original check re-scanned for `*/.fingerprint/<member>-*` after deleting
exactly those paths. Asked to catch a planted `fury-core-deadbeefcafe`
fingerprint, it exited **0** — the prune had deleted the evidence before the
assertion looked. An assertion built from the same globs as the deletion can
only prove that `rm` ran; it is blind to the failure that matters, a glob that
never matched the artifact at all.
The replacement asks a different oracle. Cargo records in each `.d` depfile the
source files an artifact was built *from*: a dependency points into
`/opt/rust/cargo/registry/...`, anything built from workspace source points
back into the workspace. After a correct prune, **zero** surviving depfiles
reference workspace source outside `target/`. Verified negatively as well — an
artifact named `totallyunrelated-abc123.d` (which no prune glob matches)
declaring a dependency on `fury-core/src/lib.rs` makes the script exit **2**
and refuse to publish.
| Check | Clean tree | Planted fingerprint | Planted foreign artifact |
|---|---|---|---|
| name-based (original) | 0 | **0 — missed it** | 0 — missed it |
| depfile-based (shipped) | 0 | 0 (prune removed it, correctly) | **2 — refused** |
**Feature variants coexist in one entry, as predicted.** `deps/` holds
`libclap-943062c20e623534.rmeta` and `libclap-abe395d6778d44ce.rmeta` side by
side — the same crate compiled under two different feature resolutions. This is
why the feature set is not part of the cache key: cargo already separates the
variants by hash, so one entry serves all of furyrs's four feature
configurations instead of fragmenting into four.
src/ci_orchestrator/assets/__init__.py
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src/ci_orchestrator/assets/cargo-cache-prune.sh
Executable
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#!/usr/bin/env bash
# Strip every workspace-crate artifact out of target/, then prove none survived.
#
# The cache is only sound while it contains dependency artifacts alone. A
# workspace crate's artifact carries the code under test, and reusing one can
# turn a red build green -- the worst failure a CI system has, because it is
# silent. So this script ends by asserting its own work and exits non-zero if
# the assertion fails; the caller must treat that as "do not publish".
#
# Exit codes: 0 clean prune, 1 usage/environment error, 2 assertion tripped.
set -euo pipefail
TARGET_DIR="${1:?usage: cargo-cache-prune.sh <target-dir> <manifest-path>}"
MANIFEST="${2:?usage: cargo-cache-prune.sh <target-dir> <manifest-path>}"
[ -d "$TARGET_DIR" ] || { echo "prune: no such target dir: $TARGET_DIR" >&2; exit 1; }
command -v cargo >/dev/null || { echo "prune: cargo not on PATH" >&2; exit 1; }
# --no-deps restricts the listing to workspace members, which is precisely the
# set whose artifacts must never be cached.
members="$(cargo metadata --no-deps --format-version 1 --manifest-path "$MANIFEST" \
| python3 -c 'import json,sys; print("\n".join(p["name"] for p in json.load(sys.stdin)["packages"]))')"
if [ -z "$members" ]; then
echo "prune: cargo metadata listed no workspace members" >&2
exit 1
fi
# Incremental state is large, machine-specific and never reusable across
# clones; examples and docs are not dependencies of anything we build.
rm -rf "$TARGET_DIR"/*/incremental "$TARGET_DIR"/*/examples "$TARGET_DIR"/doc 2>/dev/null || true
prune_one() {
name="$1"
snake="${name//-/_}"
# Fingerprints and build-script output live in per-crate directories.
find "$TARGET_DIR" -maxdepth 3 \
\( -path "*/.fingerprint/$name-*" \
-o -path "*/.fingerprint/$snake-*" \
-o -path "*/build/$name-*" \
-o -path "*/build/$snake-*" \) \
-prune -exec rm -rf {} + 2>/dev/null || true
# Compiled outputs: rlib/rmeta/d in deps/, plus binaries at the profile root.
find "$TARGET_DIR" -maxdepth 3 \
\( -name "$snake-*" -o -name "lib$snake-*" -o -name "lib$snake.*" \
-o -name "$name" -o -name "$name-*" \) \
-exec rm -rf {} + 2>/dev/null || true
}
while IFS= read -r name; do
[ -n "$name" ] || continue
prune_one "$name"
done <<EOF
$members
EOF
# ── The assertion ────────────────────────────────────────────────────────────
#
# This deliberately does NOT re-use the name patterns above. An assertion built
# from the same globs as the deletion can only prove that `rm` ran; it is blind
# to the failure that actually matters, which is a glob that never matched the
# artifact in the first place. Asked to catch a planted fingerprint, such a
# check "passes" because the prune deleted the evidence before looking.
#
# So ask a different oracle: cargo records in each `.d` depfile the source
# files an artifact was built FROM. A dependency's depfile points into the
# registry; anything built from workspace source points back into the
# workspace. That is the property we actually care about, and it holds however
# the artifact happens to be named.
ws_root="$(cd "$(dirname "$MANIFEST")" && pwd)"
abs_target="$(cd "$TARGET_DIR" && pwd)"
# Every depfile names its own output under target/, so that prefix is excluded;
# what remains is genuine source input.
leaked_paths="$(find "$abs_target" -name '*.d' -print0 2>/dev/null \
| xargs -0 -r grep -h -oE "${ws_root}/[^ :]*" 2>/dev/null \
| grep -v "^${abs_target}/" \
| sort -u || true)"
if [ -n "$leaked_paths" ]; then
echo "prune: REFUSING to publish -- artifacts still reference workspace source:" >&2
printf '%s\n' "$leaked_paths" | sed 's/^/ /' | head -20 >&2
exit 2
fi
echo "prune: ok -- $(printf '%s\n' "$members" | wc -l) workspace member(s) stripped," \
"no surviving artifact references workspace source"