gitdan-actions/scripts/hardlink-clone-selftest.sh

#!/usr/bin/env bash
# Regression test for the single assumption this whole caching scheme rests
# on: that a build running inside a hardlink clone cannot mutate the directory
# it was cloned from.
#
# That assumption is FALSE for a plain `cp -al`. Measured, and asserted below
# as an explicit control: build in a raw `cp -al` clone and the source's
# `.fingerprint/<unit>/dep-*` (under CARGO_UNSTABLE_CHECKSUM_FRESHNESS),
# `build/<pkg>/output`, `build/<pkg>/out/**` and `deps/*.d` all change,
# because Cargo and build scripts write those with a plain truncating write
# rather than the write-then-rename Cargo uses for real artifacts.
#
# The consequence is not a slow build, it is a wrong one: 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 sources; once the PR merges,
# the base's next run finds the checksums match its (now merged) sources,
# reports `Fresh`, and links a binary built from the pre-merge code.
#
# cache-lib.sh's unshare_mutable_paths() is what closes that, and this test is
# what proves it stays closed. The control matters as much as the fix: a
# scenario that passes for both would prove nothing.
#
# Needs a working cargo on PATH. Everything happens under a mktemp -d scratch
# tree. Run by hand: bash scripts/hardlink-clone-selftest.sh
set -euo pipefail
script_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
. "$script_dir/cache-lib.sh"

command -v cargo >/dev/null || { echo "SKIP: no cargo on PATH"; exit 0; }

scratch=$(mktemp -d)
trap 'rm -rf "$scratch"' EXIT
pass_count=0

fail() { echo "ASSERTION FAILED: $*" >&2; exit 1; }
ok() { pass_count=$((pass_count + 1)); echo "PASS: $*"; }

# Content hash of every file in a tree, keyed by relative path.
snapshot_tree() { (cd "$1" && find . -type f -print0 | sort -z | xargs -0 -r sha1sum) 2>/dev/null; }

# `diff` exits 1 when the trees differ, which is the expected case here and
# must not trip `pipefail` — the difference IS the result.
mutated_paths() {
  { diff <(printf '%s' "$1") <(printf '%s' "$2") || true; } 2>/dev/null \
    | awk '/^[<>]/ { print $3 }' | sort -u
}

# A crate with a build script, because build-script OUT_DIR writes are one of
# the two mutation families and are invisible without one.
mkcrate() {
  local dir="$1"
  mkdir -p "$dir/src"
  cat > "$dir/Cargo.toml" <<'TOML'
[package]
name = "probe"
version = "0.1.0"
edition = "2021"
[workspace]
TOML
  cat > "$dir/build.rs" <<'RS'
use std::{env, fs, path::PathBuf};
fn main() {
    println!("cargo::rerun-if-changed=src/lib.rs");
    let out = PathBuf::from(env::var("OUT_DIR").unwrap());
    let src = fs::read_to_string("src/lib.rs").unwrap();
    fs::write(out.join("gen.txt"), format!("generated from {} bytes", src.len())).unwrap();
}
RS
}

crate_dir="$scratch/probe"
mkcrate "$crate_dir"
cd "$crate_dir"

export CARGO_INCREMENTAL=0

# Every assertion below reads cargo's own words out of a build log
# (`Compiling libdep`, `Fresh probe`). A CI image that forces colour splices an
# ANSI reset between the status word and the crate name, at which point every
# one of those greps silently stops matching and the suite reports the
# opposite of what happened — observed on gitdan-ci's runner image, where
# scenario 2 failed while the log it printed plainly showed `Compiling libdep`.
# Pin the format the assertions are written against.
export CARGO_TERM_COLOR=never
# Checksum freshness is where the worst failure lives (the dep-* file carries
# per-source checksums and is rewritten in place). Only available on nightly;
# without it the test still covers the build/ and *.d families.
CONTENT_A='pub fn f() -> u32 { 1 }'
CONTENT_B='pub fn f() -> u32 { 22222 } pub fn g() -> u32 { 7 }'

# Probe the BEHAVIOUR, not the channel and not the flag. Two weaker probes
# were tried against gitdan-ci's runner and each let the suite assert a
# property the toolchain did not have:
#
#   `cargo +nightly -V`                     — answers "did a proxy called with
#                                             +nightly exit 0". `-V`
#                                             short-circuits before `-Z` is
#                                             even parsed.
#   `cargo +nightly -Z checksum-freshness   — answers "is this flag still
#    locate-project`                          accepted". 1.100.0-nightly
#                                             (2026-08-25) accepts it and does
#                                             not resolve freshness by content
#                                             anyway.
#
# The scenario at the end of this file depends on one thing and it is neither
# of those: that changed content with an OLDER mtime rebuilds. Under mtime
# freshness the correct answer is Fresh, so under mtime freshness that
# scenario asserts a bug. So the probe simply performs that experiment, on its
# own crate and its own target dir, with no clone anywhere near it — which is
# also what makes it a control rather than a restatement of the scenario: the
# probe establishes that the toolchain rebuilds on content, the scenario
# establishes that a hardlink clone did not take that away.
CHECKSUM_MODE="off"
CARGO_BIN=(cargo)
checksum_freshness_active() {
  local d="$scratch/freshness-probe" t="$scratch/freshness-probe-target"
  mkcrate "$d"
  (
    cd "$d" || exit 1
    printf '%s\n' "$CONTENT_A" > src/lib.rs
    CARGO_TARGET_DIR="$t" cargo +nightly build -q > /dev/null 2>&1 || exit 1
    printf '%s\n' "$CONTENT_B" > src/lib.rs
    touch -d '@1000000000' src/lib.rs
    CARGO_TARGET_DIR="$t" cargo +nightly build -v > "$scratch/freshness-probe.log" 2>&1 || exit 1
    ! grep -qE '^\s+Fresh probe' "$scratch/freshness-probe.log"
  )
}
if cargo +nightly -Z checksum-freshness locate-project > /dev/null 2>&1; then
  export CARGO_UNSTABLE_CHECKSUM_FRESHNESS=true
  if checksum_freshness_active; then
    CARGO_BIN=(cargo +nightly)
    CHECKSUM_MODE="on"
  else
    unset CARGO_UNSTABLE_CHECKSUM_FRESHNESS
    echo "note: this nightly accepts -Z checksum-freshness but still resolves freshness by mtime"
  fi
fi
cd "$crate_dir"
echo "=== checksum-freshness mode: ${CHECKSUM_MODE} ==="

build_base() {
  local dir="$1"
  printf '%s\n' "$CONTENT_A" > src/lib.rs
  CARGO_TARGET_DIR="$dir" "${CARGO_BIN[@]}" build -q
}

echo
echo "=== control: a raw \`cp -al\` clone DOES mutate its source ==="
base_ctl="$scratch/base-ctl"; clone_ctl="$scratch/clone-ctl"
build_base "$base_ctl"
before=$(snapshot_tree "$base_ctl")
cp -al "$base_ctl" "$clone_ctl"
strip_cargo_locks "$clone_ctl"            # locks alone are not the hazard under test
printf '%s\n' "$CONTENT_B" > src/lib.rs
CARGO_TARGET_DIR="$clone_ctl" "${CARGO_BIN[@]}" build -q
after=$(snapshot_tree "$base_ctl")
ctl_mutated=$(mutated_paths "$before" "$after")
if [ -z "$ctl_mutated" ]; then
  fail "control produced no mutation — the test can no longer distinguish fixed from broken"
fi
ok "raw cp -al clone mutates the source ($(printf '%s\n' "$ctl_mutated" | wc -l) paths)"
printf '%s\n' "$ctl_mutated" | sed 's/^/       /'

# Reported, not asserted, and the distinction is the point. The control's job
# is to prove the hazard exists at all, which the non-empty set above already
# does; this line records WHICH families a given Cargo exhibits.
#
# `.fingerprint/*/dep-*` is the worst of them — it carries the per-source
# checksums, so mutating it through a shared inode turns a hardlink clone into
# silent stale-artifact reuse rather than a slow build. It was measured on
# cargo 1.9x nightly (see unshare_mutable_paths in cache-lib.sh) and is NOT
# reproduced on 1.100.0-nightly (2026-08-25), where the control mutates only
# the build/ and *.d families. Failing on its absence would mean this suite
# goes red whenever upstream stops doing something we never wanted it to do —
# and it would go red in the CONTROL, where a failure reads as "the hazard is
# gone" rather than "upstream changed". Nothing is lost by reporting it: the
# fix scenario below asserts the source is byte-identical after a full rebuild
# in the clone, which covers every family this Cargo has, named or not.
if [ "$CHECKSUM_MODE" = "on" ]; then
  if printf '%s' "$ctl_mutated" | grep -q '\.fingerprint/.*/dep-'; then
    echo "  note: this cargo DOES rewrite .fingerprint/*/dep-* in place under checksum freshness"
  else
    echo "  note: this cargo does NOT rewrite .fingerprint/*/dep-* in place; only the build/ and *.d families appear above"
  fi
fi

echo
echo "=== fix: hardlink_clone_into() leaves the source byte-identical ==="
base_fix="$scratch/base-fix"; clone_fix="$scratch/clone-fix"
build_base "$base_fix"
before=$(snapshot_tree "$base_fix")
hardlink_clone_into "$base_fix" "$clone_fix" "selftest" || fail "hardlink_clone_into reported the destination already existed"

# The clone's contract, asserted before anything builds in it: artifacts
# share inodes (that is what makes the clone near-free), and every file Cargo
# rewrites in place does not (that is what makes it sound). Checking after a
# rebuild would prove nothing — the rebuild replaces those files anyway.
shared=0; unshared=0
while IFS= read -r f; do
  rel="${f#"$base_fix"/}"
  [ -e "$clone_fix/$rel" ] || continue
  if [ "$(stat -c '%i' "$f")" = "$(stat -c '%i' "$clone_fix/$rel")" ]; then
    case "$rel" in
      */.fingerprint/*|*/build/*|*.d|.rustc_info.json)
        fail "mutable path still shares an inode with the source: $rel" ;;
    esac
    shared=$((shared + 1))
  else
    unshared=$((unshared + 1))
  fi
done < <(find "$base_fix" -type f)
[ "$shared" -gt 0 ] || fail "nothing is shared — the clone degenerated into a full copy"
[ "$unshared" -gt 0 ] || fail "nothing was unshared — unshare_mutable_paths did not run"
ok "fresh clone shares ${shared} artifact files and privately owns ${unshared} mutable ones"

printf '%s\n' "$CONTENT_B" > src/lib.rs
CARGO_TARGET_DIR="$clone_fix" "${CARGO_BIN[@]}" build -q
after=$(snapshot_tree "$base_fix")
fix_mutated=$(mutated_paths "$before" "$after")
if [ -n "$fix_mutated" ]; then
  echo "  still mutated:" >&2
  printf '%s\n' "$fix_mutated" | sed 's/^/       /' >&2
  fail "a build in the clone mutated the source through a shared inode"
fi
ok "no file in the source changed after a full rebuild in the clone"

echo
if [ "$CHECKSUM_MODE" = "on" ]; then
  echo "=== the whole point: the source's next build is still correct ==="
  # The source's cache holds artifacts built from CONTENT_A. Advance the
  # source to CONTENT_B (as a merge would) and rebuild in it. If the clone had
  # corrupted its dep-info, Cargo would report Fresh and keep the stale rlib.
  #
  # CHECKSUM-FRESHNESS ONLY, and the backdated mtime is why. Under checksum
  # freshness the dep-info file's per-source checksums decide, so a 2001
  # timestamp on changed content must still rebuild — the assertion below.
  # Under Cargo's ordinary MTIME freshness the same timestamp means the source
  # is older than the artifact, and reporting Fresh is the correct answer;
  # asserting otherwise asserts a bug. This scenario was written against a
  # machine with a nightly installed and, run without one, failed on that
  # correct answer.
  printf '%s\n' "$CONTENT_B" > src/lib.rs
  touch -d '@1000000000' src/lib.rs
  log="$scratch/rebuild.log"
  CARGO_TARGET_DIR="$base_fix" "${CARGO_BIN[@]}" build -v > "$log" 2>&1 || { cat "$log"; fail "rebuild in the source failed"; }
  if grep -qE '^\s+Fresh probe' "$log"; then
    fail "source declared its own crate Fresh against sources it has never built — stale-artifact reuse"
  fi
  ok "source correctly rebuilt its crate after advancing to the clone's content"
else
  echo "=== skipped: the source's-next-build scenario needs checksum freshness ==="
  echo "  (a nightly cargo accepting -Z checksum-freshness; see the probe above)"
fi

echo
echo "hardlink-clone-selftest: ${pass_count} assertions passed"