row 211 (#1709, #426): a pre-push guard that refuses deletes and non-fast-forward pushes, its installer, and slot-busy.sh that exits 1 on BUSY

scripts/git-hooks/pre-push refuses a delete, a remote tip missing locally and
any update whose remote tip isn't an ancestor of the local tip; a new branch
and a fast-forward pass. scripts/install-git-hooks.sh puts it in the common
.git/hooks, refuses a set core.hooksPath or a different hook without
--force-install, and runs a canary through git hook run. scripts/slot-busy.sh
exits 0 CLEAR, 1 BUSY, 2 error. scripts/test-git-guard.sh (61 cases) and the
mutant runner in agents/darkwing/work/queue-211; TOOLS.md gets the usage.
Not run yet: written during Gander's slot (sage-2801).

agents/darkwing/work/queue-211/BUILD.md

@@ -0,0 +1,142 @@
+# Row 211 (#1709, Vikunja P19 #426): a pre-push hook that refuses force-pushes, and a busy check that fails on BUSY
+
+Darkwing, 2026-10-22. Brief: `docs/plans/2026-10-20_force-push-guard.md` (blob 3a8c2cf13). Moved from Rocko
+to me by sage-2801 (Q220, mos-1301). Base: aca16404c. Branch: `darkwing/row211`. Reviewer: Dewey. Gate
+owner: Sage.
+
+## What ships
+
+- `scripts/git-hooks/pre-push` (new, `/bin/sh`). For each ref line on stdin it refuses:
+  - a delete (local sha all zeros);
+  - a remote tip that isn't in this repository (`git cat-file -e <remote>^{commit}`), with "fetch first,
+    then push again";
+  - a remote tip that isn't an ancestor of the local tip (`git merge-base --is-ancestor`), naming the ref
+    and both shas;
+  - an ancestor check that fails for any other reason (exit above 1);
+  - a short or overlong ref line, or a non-hex object name.
+
+  A new ref (remote sha all zeros) passes, and so does an empty push. It fetches nothing. On any refusal
+  it prints one line per refused ref and a closing line, and exits 1, so git pushes no ref at all.
+- `scripts/install-git-hooks.sh` (new). Installs the hook as `pre-push` in the common `.git/hooks`
+  (`git rev-parse --git-common-dir`), so a `--repo` naming a worktree lands in the shared directory.
+  It refuses (exit 2):
+  - a set `core.hooksPath` at any scope;
+  - a different installed `pre-push`, unless `--force-install` is given;
+  - a symlinked `pre-push`, even with `--force-install`;
+  - any of `GIT_DIR`, `GIT_WORK_TREE`, `GIT_COMMON_DIR`, `GIT_INDEX_FILE`, `GIT_OBJECT_DIRECTORY` or
+    `GIT_ALTERNATE_OBJECT_DIRECTORIES` in the environment;
+  - a directory that isn't a git checkout.
+
+  It never writes the installed file in place. A new hook goes in by `link()` from a temporary copy, so
+  a hook that appeared meanwhile is kept and the install refuses. A replacement goes in by `rename()`.
+  Then it compares the bytes, checks the mode, and runs a canary through `git hook run` (git 2.55 here):
+  an empty push must pass and a delete line must be refused with the guard's message.
+- `scripts/slot-busy.sh` (new). Exit 0 CLEAR, 1 BUSY, 2 its own error, one `BUSY <pid> <rule> cwd=<cwd>
+  <command>` line per match. Rules: `node ... --test`, `*gate*.sh` (after a space, `/`, `_` or `-`), a
+  `mutant` substring, `test-*.sh`, `docker compose` and `docker-compose`. It skips its own pid and its
+  ancestors, empty command lines (kernel threads) and shell-snapshot wrappers.
+- `scripts/test-git-guard.sh` (new), 61 cases, and three Maintenance rows plus a short section in
+  `docs/TOOLS.md`.
+
+## Choices
+
+1. **A separate `scripts/install-git-hooks.sh`, not `queue-commit.sh --install-push-hook`.** The brief
+   left this open. `queue-commit.sh --install-hook` is the queue guard's installer. It is tied to the
+   canonical checkout and to the lead (jason/sage) and runs the queue guard's canary. The push guard has
+   neither tie: it must also be installable in a scratch clone for the tests, and every seat worktree
+   shares it. Putting it in `queue-commit.sh` would widen that script's reach for no gain. The cost is a
+   second installer with a similar shape. The install pattern (copy beside, link or rename, verify,
+   canary) follows `queue-commit.sh`.
+2. **No privilege check in the installer.** The brief makes the canonical install a separate step with
+   Sage after landing, and the tests need to install into scratch clones as the suite's user. So the
+   script installs wherever it is told. The guard is advisory on the client in any case (`--no-verify`).
+3. **The hook source is the working file next to the installer** (`$ROOT/scripts/git-hooks/pre-push`),
+   not a blob from a ref. That lets the mutant runner install a mutated copy from its own tree, and
+   matches how `queue-commit.sh` reads its guard.
+4. **A new `scripts/test-git-guard.sh`, not more cases in `test-land-work.sh`.** The brief allowed either.
+   Land-work's suite is about `land-work.sh`, and the gate runs every `scripts/test-*.sh`, so a new file
+   gets gated with no wiring.
+5. **`--proc-root DIR` is a test seam in `slot-busy.sh`.** On a live host the real `/proc` may hold
+   other seats' runs at any time, so the suite can't demand exit 0 there. Exit 0 and every rule are
+   tested against a fake `/proc` with pids above `pid_max` (4194304), so they can't collide with the
+   ancestor set, which always comes from the real `/proc`. The real `/proc` is tested with a stand-in:
+   a `/bin/sh` script named `node`, started as `node --test standin.test.mjs` in its own cwd. The suite
+   asserts its BUSY line with that cwd, exit 1, that `slot-busy > /dev/null && echo ran` prints nothing,
+   that the suite itself (an ancestor whose command line matches `test-*.sh`) has no BUSY line, and that
+   after the stand-in exits its pid is gone. The brief's "0 without one" is the fake-`/proc` case.
+6. **Ancestors are skipped**, not only its own pid as the brief says. A gate script that runs the busy
+   check matches its own rules (`*gate*.sh`, `test-*.sh`, `mutant`), so checking only `$$` would make
+   every such caller see itself as BUSY.
+7. **The gate rule is broader than `gate.sh`**: `[ /_-]gate[^ /]*\.sh `, so `sage-gate.sh` and
+   `gate-253.sh` match too. A false BUSY costs a wait; a false CLEAR costs a broken run.
+8. **Shell-snapshot wrappers are skipped.** An agent harness runs each command as `bash -c 'source
+   .../shell-snapshots/snapshot-*.sh && eval <command text>'`, so the wrapper's command line holds the
+   text of whatever the agent typed, including a busy check that names `test-*.sh`. The real child
+   processes the wrapper starts are still matched.
+
+## Tests (`scripts/test-git-guard.sh`)
+
+Scratch bare remote, a scratch clone with the guard installed, and a worktree of that clone. Git runs
+with `GIT_CONFIG_NOSYSTEM=1` and an empty `GIT_CONFIG_GLOBAL`, as in `test-land-work.sh`. Nothing touches
+origin, `~/.mosaic`, `~/.mosaic-dev` or `~/.config/mosaic-dev`.
+
+- Install: exit 0, the canary passed, bytes and mode, no temporary file left, and the worktree's git
+  resolves `hooks/pre-push` to the common file.
+- From the worktree: a new branch and a fast-forward pass. A `--force` push of a rewritten branch, a
+  `+refspec` push and a `--force-with-lease` push are refused by the guard's message, naming the ref and
+  both shas, and the remote ref is unchanged. `:ref` and `--delete` are refused and the branch stays.
+- A second clone moves the remote; a `--force` push from the worktree is then refused with "fetch
+  first", the remote stays where the other clone put it, and the guard fetched nothing.
+- The hook fed by hand: a short line, a non-hex name and an unknown local object are refused; an empty
+  push passes.
+- The installer: `core.hooksPath` set refuses with that message and writes no hook (in `.git/hooks` or
+  the hooks path). A different hook refuses and stays byte-identical. `--force-install` replaces it, and
+  a hard link to the old file keeps the old bytes, so the replacement was a rename. A second install of
+  the same bytes exits 0. A symlinked hook refuses even with `--force-install`. A `--repo` naming a
+  worktree lands in the common directory. `GIT_DIR` set, a plain directory and an unknown argument
+  refuse.
+- `slot-busy.sh`, fake `/proc`: CLEAR with exit 0 when nothing matches (a server, a `grep gate`, a
+  snapshot wrapper whose text holds `node --test`, an empty command line). Exit 1 and the BUSY line for
+  `node --test`, with no CLEAR in the output, and `> /dev/null && echo ran` prints nothing. Each rule
+  once more (7 cases). Exit 2 for a missing root, an empty root and an unknown argument.
+- `slot-busy.sh`, real `/proc`: the stand-in cases in choice 5.
+
+## Mutants (`agents/darkwing/work/queue-211/mutants.sh`)
+
+Each mutant copies the four scripts into its own tree, edits one site there (exactly one match), and runs
+that tree's suite under `timeout -k 10 180`. The worktree's files are never edited. The unmutated suite
+runs first and a red base stops the run. The brief's five come first:
+
+| Mutant | Site |
+|---|---|
+| ancestor-dropped | pre-push: `rc=0` in place of the `merge-base --is-ancestor` call |
+| unknown-object-passes | pre-push: the missing-remote-tip branch skips the ref without refusing |
+| delete-passes | pre-push: the delete branch skips the ref without refusing |
+| install-over-hookspath | installer: the `core.hooksPath` refusal is `true \|\| die` |
+| busy-exits-0 | slot-busy: `exit 0` when BUSY |
+| differing-hook-replaced | installer: a different hook is replaced without `--force-install` |
+| replace-in-place | installer: the replacement writes into the installed file (`cat >`) instead of renaming |
+| symlink-followed | installer: no symlink refusal |
+| ancestor-error-passes | pre-push: an ancestor check exiting above 1 doesn't refuse |
+| snapshot-matched | slot-busy: no shell-snapshot skip |
+| ancestors-matched | slot-busy: no ancestor skip |
+| no-node-rule | slot-busy: the `node --test` rule removed |
+| no-compose-rule | slot-busy: the `docker compose` rule removed |
+
+`install-over-hookspath` still exits 2, because its canary then fails (git looks for the hook in the hooks
+path). The suite kills it on the message and on "no hook was written", not on the exit code.
+
+## Runs
+
+Pending: nothing has run yet. Gander held the gate slot (row 253 run 5, from 16:32Z) while this was
+written, so only `bash -n`, `sh -n` and a `DRY=1` mutant pass (every site matched once, no suite run)
+happened. The suite, the mutants and every other `scripts/test-*.sh` run in a CLEAR Sage announces, with
+`TMPDIR` under `/mnt/storage/scratch/tmp`. Results and the measured suite time go here before review.
+
+## Known limits
+
+- `git push --no-verify` skips the hook. Gitea branch protection (ops-01, through Mos) is the guard that
+  holds.
+- The hook doesn't check tags differently from branches: a moved tag is refused like a rewritten branch.
+- `slot-busy.sh` can call a process BUSY that isn't a run, for example an editor with a `test-*.sh` open.
+  It never reads files under a seat's directory, so it can't tell.

agents/darkwing/work/queue-211/mutants.sh

@@ -0,0 +1,74 @@
+#!/usr/bin/env bash
+# Row 211 (#426) mutants. Run from the candidate worktree's root:
+#   TMPDIR=<scratch dir> LOG=<dir> bash agents/darkwing/work/queue-211/mutants.sh > mutants.txt
+# Each mutant copies the four row-211 scripts into its own tree, edits one
+# site there (exactly one match, else NO MATCH), and runs that tree's
+# scripts/test-git-guard.sh under a 180 s timeout. The worktree's files are
+# never edited. "killed" means the suite failed; "survived" means it passed.
+# The unmutated suite runs first, and a red base stops the run (exit 3).
+set -u
+LOG=${LOG:-.}
+FILES="scripts/git-hooks/pre-push scripts/install-git-hooks.sh scripts/slot-busy.sh scripts/test-git-guard.sh"
+WORK=$(mktemp -d "${TMPDIR:-/tmp}/r211-mutants.XXXXXX") || exit 2
+trap 'rm -rf "$WORK"' EXIT
+
+tree() { # $1: name. Prints the copy's root.
+  local t="$WORK/$1" f
+  for f in $FILES; do mkdir -p "$t/$(dirname "$f")" && cp -p -- "$f" "$t/$f" || return 1; done
+  echo "$t"
+}
+suite() { # $1: tree, $2: log
+  timeout -k 10 180 bash "$1/scripts/test-git-guard.sh" > "$2" 2>&1
+}
+edit() { # $1: file, $2: from, $3: to. Exit 3 unless $2 occurs exactly once.
+  python3 -B - "$1" "$2" "$3" <<'PY'
+import sys
+path, old, new = sys.argv[1:4]
+text = open(path).read()
+if text.count(old) != 1:
+    sys.exit(3)
+open(path, "w").write(text.replace(old, new))
+PY
+}
+
+base=$(tree base) || exit 2
+if [ -n "${DRY:-}" ]; then
+  echo "base: not run (DRY)"
+elif suite "$base" "$LOG/mut-base.log"; then
+  echo "base: green ($(tail -1 "$LOG/mut-base.log"))"
+else
+  echo "base: RED, stopping ($(tail -1 "$LOG/mut-base.log"))"
+  exit 3
+fi
+
+run() { # $1: name, $2: file, $3: from, $4: to
+  local name=$1 file=$2 t log
+  t=$(tree "$name") || { echo "$name: COPY FAILED"; return; }
+  if ! edit "$t/$file" "$3" "$4"; then echo "$name: NO MATCH"; return; fi
+  if [ -n "${DRY:-}" ]; then echo "$name: match"; return; fi
+  log="$LOG/mut-$name.log"
+  if suite "$t" "$log"; then
+    echo "$name: survived"
+  else
+    echo "$name: killed (exit $?, $(grep -c '^FAIL' "$log") FAIL; first: $(grep -m1 '^FAIL' "$log" | cut -c6-100))"
+  fi
+}
+H=scripts/git-hooks/pre-push
+I=scripts/install-git-hooks.sh
+S=scripts/slot-busy.sh
+
+# The brief's five.
+run ancestor-dropped $H $'    git merge-base --is-ancestor "$rsha" "$lsha" 2>/dev/null\n    rc=$?' '    rc=0'
+run unknown-object-passes $H $'        refuse "$rref: the remote tip $rsha isn\'t in this repository; fetch first, then push again"\n        continue' '        continue'
+run delete-passes $H $'        refuse "$rref: a delete (remote $rsha); seat branches are never deleted by push"\n        continue' '        continue'
+run install-over-hookspath $I '[ -z "$out" ] || die 2 "refused: core.hooksPath' '[ -z "$out" ] || true || die 2 "refused: core.hooksPath'
+run busy-exits-0 $S 'if [ "$busy" = 1 ]; then exit 1; fi' 'if [ "$busy" = 1 ]; then exit 0; fi'
+# Further sites the suite pins.
+run differing-hook-replaced $I '[ "$FORCE" = 1 ] || die 2 "refused: a different' 'true || die 2 "refused: a different'
+run replace-in-place $I 'mv -f -- "$tmp" "$HOOK" ||' 'cat -- "$tmp" > "$HOOK" && rm -f -- "$tmp" ||'
+run symlink-followed $I 'if [ -L "$HOOK" ]; then die 2' 'if false; then die 2'
+run ancestor-error-passes $H '        *) refuse "$rref: the ancestor check failed' '        *) : "$rref: the ancestor check failed'
+run snapshot-matched $S $'  if [[ "$c" == *"/shell-snapshots/snapshot-"* ]]; then return 1; fi\n' ''
+run ancestors-matched $S '  [ -z "${SKIP[$pid]:-}" ] || continue' '  :'
+run no-node-rule $S 'if [[ "$c" =~ $RE_NODE ]]; then echo "node --test"; return 0; fi' ':'
+run no-compose-rule $S 'if [[ "$c" =~ $RE_COMPOSE ]]; then echo "docker compose"; return 0; fi' ':'

docs/TOOLS.md

@@ -166,8 +166,37 @@ resolved through the CLI.
 | `scripts/test-conductor.sh` | Auto-apply selftests (sandboxed) | 17 cases |
 | `scripts/test-auth.sh` | Auth checkpoint selftests (no Docker) | 13 cases |
 | `scripts/test-land-work.sh` | Land-work selftests (sandboxed, fake Docker) | 84 cases |
+| `scripts/test-git-guard.sh` | Push guard, its installer and `slot-busy.sh` (scratch bare remote, fake `/proc`) | 61 cases |
+| `scripts/install-git-hooks.sh [--repo DIR] [--force-install]` | Install the push guard as `pre-push` in the common `.git/hooks` | Refuses a set `core.hooksPath` or a different installed hook; exit 0 ok · 1 failed · 2 refused · 4 usage |
+| `scripts/slot-busy.sh` | Busy check: one `BUSY` line per gate or test process, or `CLEAR` | Exit 0 CLEAR · 1 BUSY · 2 error; chain on the exit code |
 | `scripts/gitea-api.sh <METHOD> <path> [body]` | Gitea API helper; reads the JSON credential file or a raw per-seat token file | Token never on argv/stdout |
 
+### Push guard and busy check (row 211, #426)
+
+`scripts/git-hooks/pre-push` refuses a push that deletes a branch, or that
+moves a branch to a commit that doesn't descend from the remote tip (a
+`--force`, `--force-with-lease` or `+refspec` push of a rewritten branch). A
+new branch and a fast-forward pass. The hook fetches nothing: when the remote
+tip isn't in the local repository it refuses and says to fetch first.
+`git push --no-verify` skips it, so Gitea's branch protection (ops-01) is the
+guard that holds. `scripts/install-git-hooks.sh` copies it into the common
+`.git/hooks`, which every worktree of the checkout shares, then runs a canary
+through `git hook run`. Installing it in the canonical checkout is a step for
+the lead after the row lands.
+
+Chain the busy check on its exit code, and set `pipefail` in any script that
+pipes it:
+
+```bash
+bash scripts/slot-busy.sh > "$LOG" && gate
+```
+
+Never `bash scripts/slot-busy.sh | tail -1 && gate`: `tail` exits 0, so the
+gate runs while BUSY. It matches `node ... --test`, a `*gate*.sh`, `mutant`, a
+`test-*.sh` and `docker compose` command lines, skips itself, its ancestors
+and agent shell-snapshot wrappers, and reads only `/proc/<pid>/cmdline` and
+the cwd link.
+
 ## Seat launch with registration (`scripts/mosaic`)
 
 ```bash

scripts/git-hooks/pre-push

@@ -0,0 +1,50 @@
+#!/bin/sh
+# Mosaic push guard (row 211, #426). Installed as pre-push in the common
+# .git/hooks by `scripts/install-git-hooks.sh`; never edit the installed copy.
+# For each ref git is about to push, it refuses a delete and any update whose
+# remote tip isn't an ancestor of the local tip: a force-push or a +refspec.
+# A new ref passes. It fetches nothing, so a remote tip that isn't in this
+# repository refuses. Any failure of the check refuses too.
+# `git push --no-verify` skips this hook; Gitea's branch protection is the
+# guard that holds (ops-01). Canon: never force-push.
+refused=0
+refuse() {
+    echo "mosaic push guard: refused: $*" >&2
+    refused=1
+}
+zero() {
+    case "$1" in *[!0]*) return 1 ;; esac
+    return 0
+}
+while read -r lref lsha rref rsha extra || [ -n "${lref:-}" ]; do
+    if [ -z "${rsha:-}" ] || [ -n "${extra:-}" ]; then
+        refuse "unreadable ref line from git: $lref ${lsha:-} ${rref:-} ${rsha:-} ${extra:-}"
+        continue
+    fi
+    case "$lsha$rsha" in
+        *[!0-9a-f]*) refuse "$rref: a ref line with a non-hex object name (local $lsha, remote $rsha)"; continue ;;
+    esac
+    if zero "$lsha"; then
+        refuse "$rref: a delete (remote $rsha); seat branches are never deleted by push"
+        continue
+    fi
+    if zero "$rsha"; then
+        continue
+    fi
+    if ! git cat-file -e "$rsha^{commit}" 2>/dev/null; then
+        refuse "$rref: the remote tip $rsha isn't in this repository; fetch first, then push again"
+        continue
+    fi
+    git merge-base --is-ancestor "$rsha" "$lsha" 2>/dev/null
+    rc=$?
+    case $rc in
+        0) ;;
+        1) refuse "$rref: not a fast-forward; remote $rsha isn't an ancestor of local $lsha (a force-push or a rewritten branch)" ;;
+        *) refuse "$rref: the ancestor check failed (exit $rc) for remote $rsha and local $lsha" ;;
+    esac
+done
+if [ "$refused" != 0 ]; then
+    echo "mosaic push guard: nothing was pushed; a pushed branch only moves forward (canon: never force-push)" >&2
+    exit 1
+fi
+exit 0

scripts/install-git-hooks.sh

@@ -0,0 +1,99 @@
+#!/usr/bin/env bash
+# install-git-hooks.sh: install the push guard (row 211, #426) as pre-push in
+# a repository's common .git/hooks, which every linked worktree shares.
+#
+# Usage:
+#   scripts/install-git-hooks.sh [--repo DIR] [--force-install]
+#
+# DIR defaults to this script's own checkout. The hook's bytes are this
+# checkout's scripts/git-hooks/pre-push. It refuses when core.hooksPath is set
+# (git would not run the hook), and when a different pre-push is installed,
+# unless --force-install is given. It never edits an installed hook in place:
+# a new hook is linked in, a replacement is renamed over the old one. A canary
+# then runs the installed hook through git and needs a delete refused.
+#
+# Exit codes: 0 installed (or already the same bytes); 1 failed; 2 refused;
+# 4 usage. Installing in the canonical checkout is its own step with the lead.
+set -uo pipefail
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd -P)"
+SRC="$ROOT/scripts/git-hooks/pre-push"
+TMPD=""
+
+say() { printf 'install-git-hooks: %s\n' "$*" >&2; }
+die() { local code=$1; shift; say "$*"; exit "$code"; }
+cleanup() { if [ -n "$TMPD" ]; then rm -rf -- "$TMPD"; fi; }
+trap cleanup EXIT
+
+usage() {
+  sed -n '5,6p' "${BASH_SOURCE[0]}" | sed 's/^# \{0,1\}//' >&2
+  exit 4
+}
+
+REPO=$ROOT
+FORCE=0
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --repo) [ $# -ge 2 ] || usage; REPO=$2; shift 2 ;;
+    --force-install) FORCE=1; shift ;;
+    -h|--help) usage ;;
+    *) say "unknown argument: $1"; usage ;;
+  esac
+done
+
+for v in GIT_DIR GIT_WORK_TREE GIT_COMMON_DIR GIT_INDEX_FILE GIT_OBJECT_DIRECTORY GIT_ALTERNATE_OBJECT_DIRECTORIES; do
+  if [ -n "${!v+x}" ]; then die 2 "refused: $v is set; run with the normal git environment"; fi
+done
+[ -f "$SRC" ] && [ ! -L "$SRC" ] || die 2 "refused: $SRC is missing or not a regular file"
+
+g() { git -C "$REPO" "$@"; }
+top=$(g rev-parse --show-toplevel 2>/dev/null) || die 2 "refused: $REPO is not a git checkout"
+common=$(g rev-parse --path-format=absolute --git-common-dir) || die 1 "cannot read $REPO's git common directory"
+HOOKS="$common/hooks"
+HOOK="$HOOKS/pre-push"
+
+out=$(g config --show-scope --get-all core.hooksPath 2>/dev/null)
+[ -z "$out" ] || die 2 "refused: core.hooksPath is set ($(printf '%s' "$out" | tr '\n\t' '; ')), so git would not run the push guard; unset it first"
+
+if [ -L "$HOOK" ]; then die 2 "refused: $HOOK is a symlink; remove it by hand if it is not wanted"; fi
+mkdir -p "$HOOKS" || die 1 "cannot create $HOOKS"
+tmp="$HOOKS/.pre-push.install-git-hooks.$$"
+if [ -e "$HOOK" ]; then
+  [ -f "$HOOK" ] || die 2 "refused: $HOOK is not a regular file"
+  if cmp -s "$SRC" "$HOOK"; then
+    chmod 0755 "$HOOK" || die 1 "chmod failed"
+    note="the same bytes were already there"
+  else
+    [ "$FORCE" = 1 ] || die 2 "refused: a different pre-push hook is installed at $HOOK; compare it with $SRC, and pass --force-install to replace it"
+    cp -- "$SRC" "$tmp" && chmod 0755 "$tmp" || { rm -f -- "$tmp"; die 1 "cannot write $tmp"; }
+    # rename() swaps the whole file; the old hook's bytes are never edited.
+    mv -f -- "$tmp" "$HOOK" || { rm -f -- "$tmp"; die 1 "cannot replace $HOOK"; }
+    note="replaced a different hook (--force-install)"
+  fi
+else
+  cp -- "$SRC" "$tmp" && chmod 0755 "$tmp" || { rm -f -- "$tmp"; die 1 "cannot write $tmp"; }
+  # link() never replaces, so a hook that appeared meanwhile is kept.
+  if ! ln -- "$tmp" "$HOOK" 2>/dev/null; then rm -f -- "$tmp"; die 2 "refused: a pre-push hook appeared at $HOOK while installing"; fi
+  rm -f -- "$tmp"
+  note="new"
+fi
+
+cmp -s "$SRC" "$HOOK" || die 1 "$HOOK differs from $SRC after install"
+[ -x "$HOOK" ] || die 1 "$HOOK is not executable after install"
+
+# The canary: git runs the hook it would run for a push. A delete line needs
+# no objects; the guard must refuse it, and pass an empty push.
+TMPD=$(mktemp -d "${TMPDIR:-/tmp}/install-git-hooks.XXXXXX") || die 1 "mktemp failed"
+z=0000000000000000000000000000000000000000
+printf '(delete) %s refs/heads/install-canary 1%s\n' "$z" "${z#0}" > "$TMPD/delete"
+: > "$TMPD/empty"
+if ! out=$(cd "$top" && git hook run --to-stdin="$TMPD/empty" pre-push -- canary canary 2>&1); then
+  die 2 "refused: the canary's empty push failed, so git is not running the push guard as installed: $out"
+fi
+out=$(cd "$top" && git hook run --to-stdin="$TMPD/delete" pre-push -- canary canary 2>&1); rc=$?
+[ "$rc" -ne 0 ] && [[ "$out" == *"mosaic push guard: refused"* ]] \
+  || die 2 "refused: the canary's delete was not refused by the push guard (exit $rc)"
+
+printf 'ok installed the push guard at %s (%s, sha256 %s, mode 0755); canary passed\n' \
+  "$HOOK" "$note" "$(sha256sum < "$HOOK" | cut -d' ' -f1)"
+exit 0

scripts/slot-busy.sh

@@ -0,0 +1,77 @@
+#!/usr/bin/env bash
+# slot-busy.sh: the shared busy check (row 211, #426). Is a gate or a test run
+# going on this host?
+#
+# Usage:
+#   scripts/slot-busy.sh [--proc-root DIR]
+#
+# It prints one BUSY line per matching process, or one CLEAR line. Exit codes:
+# 0 CLEAR, 1 BUSY, 2 its own error. Chain it on the exit code, never on a
+# filter of its output:
+#   bash scripts/slot-busy.sh > LOG && gate          (set -o pipefail if piped)
+# never `bash scripts/slot-busy.sh | tail -1 && gate`, which runs while BUSY.
+#
+# A process matches when its command line holds `node ... --test`, a
+# `*gate*.sh`, `mutant`, a `test-*.sh` or `docker compose`/`docker-compose`.
+# It skips itself and its ancestors (a gate script may run the check), and an
+# agent harness's shell-snapshot wrapper, whose command line is the agent's
+# command text; that shell's real children are still matched. It reads only
+# /proc/<pid>/cmdline and the cwd link, never a file under a seat's directory.
+# --proc-root names a stand-in for /proc in the selftests.
+set -uo pipefail
+
+PROC=/proc
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --proc-root) [ $# -ge 2 ] || { echo "slot-busy: error: --proc-root needs a directory" >&2; exit 2; }; PROC=$2; shift 2 ;;
+    -h|--help) sed -n '5,6p' "${BASH_SOURCE[0]}" | sed 's/^# \{0,1\}//' >&2; exit 2 ;;
+    *) echo "slot-busy: error: unknown argument: $1" >&2; exit 2 ;;
+  esac
+done
+[ -d "$PROC" ] || { echo "slot-busy: error: $PROC is not a directory" >&2; exit 2; }
+
+# This process and its ancestors, from the real /proc.
+declare -A SKIP=()
+p=$$
+while [ -n "$p" ] && [ "$p" != 0 ]; do
+  SKIP[$p]=1
+  p=$(awk '/^PPid:/{print $2; exit}' "/proc/$p/status" 2>/dev/null) || p=""
+done
+[ "${#SKIP[@]}" -ge 1 ] && [ -n "${SKIP[$$]:-}" ] || { echo "slot-busy: error: cannot read its own process" >&2; exit 2; }
+
+RE_NODE='[ /]node[^ ]* (.* )?--test '
+RE_GATE='[ /_-]gate[^ /]*\.sh '
+RE_TEST='[ /]test-[^ /]*\.sh '
+RE_COMPOSE='[ /]docker( compose|-compose) '
+match() { # $1: the command line, NULs as spaces. Prints the rule that matched.
+  local c=" $1 "
+  if [[ "$c" == *"/shell-snapshots/snapshot-"* ]]; then return 1; fi
+  if [[ "$c" =~ $RE_NODE ]]; then echo "node --test"; return 0; fi
+  if [[ "$c" =~ $RE_GATE ]]; then echo "gate"; return 0; fi
+  if [[ "$c" == *mutant* ]]; then echo "mutant"; return 0; fi
+  if [[ "$c" =~ $RE_TEST ]]; then echo "test script"; return 0; fi
+  if [[ "$c" =~ $RE_COMPOSE ]]; then echo "docker compose"; return 0; fi
+  return 1
+}
+
+seen=0
+busy=0
+for d in "$PROC"/[0-9]*; do
+  [ -d "$d" ] || continue
+  pid=${d##*/}
+  [[ "$pid" =~ ^[0-9]+$ ]] || continue
+  seen=$((seen + 1))
+  [ -z "${SKIP[$pid]:-}" ] || continue
+  # A process can exit between the listing and the read; that's not an error.
+  cmd=$(tr '\0' ' ' < "$d/cmdline" 2>/dev/null) || continue
+  cmd=${cmd% }
+  [ -n "$cmd" ] || continue
+  rule=$(match "$cmd") || continue
+  cwd=$(readlink "$d/cwd" 2>/dev/null) || cwd="?"
+  printf 'BUSY %s %s cwd=%s %s\n' "$pid" "$rule" "$cwd" "${cmd:0:200}"
+  busy=1
+done
+[ "$seen" -ge 1 ] || { echo "slot-busy: error: no processes under $PROC" >&2; exit 2; }
+if [ "$busy" = 1 ]; then exit 1; fi
+echo "CLEAR"
+exit 0

scripts/test-git-guard.sh

@@ -0,0 +1,257 @@
+#!/usr/bin/env bash
+# Sandboxed selftests for the push guard, its installer and the busy check
+# (row 211, #426): scripts/git-hooks/pre-push, scripts/install-git-hooks.sh
+# and scripts/slot-busy.sh.
+#
+# Pushes go from a scratch clone and its worktree to a scratch bare remote,
+# never to origin. Git sees no user or system config. The busy check runs
+# against a fake /proc (pids above pid_max) for the exit codes and rules, and
+# against the real /proc with a sleeping stand-in whose argv is `node --test`.
+set -uo pipefail
+cd "$(dirname "$0")/.."
+REPO="$(pwd)"
+unset GIT_DIR GIT_WORK_TREE GIT_COMMON_DIR GIT_INDEX_FILE GIT_OBJECT_DIRECTORY GIT_ALTERNATE_OBJECT_DIRECTORIES
+
+SANDBOX="$(cd "$(mktemp -d)" && pwd -P)"
+STANDIN=""
+cleanup() {
+    if [ -n "$STANDIN" ]; then kill "$STANDIN" 2>/dev/null; wait "$STANDIN" 2>/dev/null; fi
+    rm -rf "$SANDBOX"
+}
+trap cleanup EXIT
+
+PASS=0
+FAIL=0
+# Status colors: terminal-only, NO_COLOR-respecting; plain when piped.
+if [ -t 1 ] && [ -z "${NO_COLOR:-}" ]; then
+    C_OK=$'\033[0;32m'; C_FAIL=$'\033[0;31m'; C_RESET=$'\033[0m'
+else
+    C_OK=""; C_FAIL=""; C_RESET=""
+fi
+check() {
+    if [ "$2" = "0" ]; then PASS=$((PASS+1)); echo "${C_OK}OK${C_RESET}    $1"; else FAIL=$((FAIL+1)); echo "${C_FAIL}FAIL${C_RESET} $1"; fi
+}
+has() { [[ "$1" == *"$2"* ]]; }
+
+# Git sees no user or system config: no hooks path, no signing, a fixed identity.
+: > "$SANDBOX/gitconfig"
+export GIT_CONFIG_NOSYSTEM=1 GIT_CONFIG_GLOBAL="$SANDBOX/gitconfig"
+export GIT_AUTHOR_NAME=suite GIT_AUTHOR_EMAIL=suite@local GIT_COMMITTER_NAME=suite GIT_COMMITTER_EMAIL=suite@local
+
+HOOK_SRC="$REPO/scripts/git-hooks/pre-push"
+INSTALL="$REPO/scripts/install-git-hooks.sh"
+BUSY="$REPO/scripts/slot-busy.sh"
+
+B="$SANDBOX/remote.git"
+C="$SANDBOX/clone"
+W="$SANDBOX/wt"
+O="$SANDBOX/other"
+commit() { # $1: repo, $2: file and message
+    echo "$2" > "$1/$2" && git -C "$1" add "$2" && git -C "$1" commit -q -m "$2"
+}
+rtip() { git -C "$B" rev-parse -q --verify "refs/heads/$1"; }
+
+# ---- the remote, a clone with the guard, a worktree ----
+git init -q --bare -b main "$B"
+git init -q -b main "$SANDBOX/seed"
+commit "$SANDBOX/seed" base
+git -C "$SANDBOX/seed" push -q "$B" main
+git clone -q "$B" "$C"
+
+echo "== install =="
+out=$(bash "$INSTALL" --repo "$C" 2>&1); rc=$?
+check "the install into a clean clone exits 0" "$rc"
+check "it reports the install and a passed canary" "$(has "$out" "ok installed the push guard" && has "$out" "canary passed"; echo $?)"
+check "the hook is in the common .git/hooks, with the repository's bytes, executable" \
+    "$(cmp -s "$HOOK_SRC" "$C/.git/hooks/pre-push" && [ -x "$C/.git/hooks/pre-push" ]; echo $?)"
+check "no temporary file is left in .git/hooks" \
+    "$(ls -A "$C/.git/hooks" | grep -q '^\.pre-push\.install'; [ $? = 1 ]; echo $?)"
+git -C "$C" worktree add -q -b seat/w "$W" origin/main
+check "the worktree's git runs the common hook" \
+    "$([ "$(git -C "$W" rev-parse --path-format=absolute --git-path hooks/pre-push)" = "$C/.git/hooks/pre-push" ]; echo $?)"
+
+echo "== pushes from the worktree =="
+commit "$W" one
+out=$(git -C "$W" push origin seat/w 2>&1); rc=$?
+check "a new branch passes" "$rc"
+check "the remote branch is at the pushed commit" "$([ "$(rtip seat/w)" = "$(git -C "$W" rev-parse HEAD)" ]; echo $?)"
+
+commit "$W" two
+out=$(git -C "$W" push origin seat/w 2>&1); rc=$?
+check "a fast-forward passes" "$rc"
+check "the remote branch moved forward" "$([ "$(rtip seat/w)" = "$(git -C "$W" rev-parse HEAD)" ]; echo $?)"
+
+before=$(rtip seat/w)
+git -C "$W" reset -q --hard HEAD~1
+commit "$W" two-rewritten
+after=$(git -C "$W" rev-parse HEAD)
+out=$(git -C "$W" push --force origin seat/w 2>&1); rc=$?
+check "push --force of a rewritten branch is refused" "$([ "$rc" -ne 0 ]; echo $?)"
+check "the refusal is the guard's, and names the ref and both shas" \
+    "$(has "$out" "mosaic push guard: refused: refs/heads/seat/w: not a fast-forward" && has "$out" "$before" && has "$out" "$after"; echo $?)"
+check "the remote ref is unchanged after the refused --force" "$([ "$(rtip seat/w)" = "$before" ]; echo $?)"
+
+out=$(git -C "$W" push origin +seat/w:seat/w 2>&1); rc=$?
+check "a +refspec push is refused" "$([ "$rc" -ne 0 ] && has "$out" "not a fast-forward"; echo $?)"
+check "the remote ref is unchanged after the refused +refspec" "$([ "$(rtip seat/w)" = "$before" ]; echo $?)"
+
+out=$(git -C "$W" push --force-with-lease origin seat/w 2>&1); rc=$?
+check "push --force-with-lease is refused too" "$([ "$rc" -ne 0 ] && has "$out" "not a fast-forward"; echo $?)"
+
+out=$(git -C "$W" push origin :seat/w 2>&1); rc=$?
+check "a delete (:ref) is refused" "$([ "$rc" -ne 0 ] && has "$out" "mosaic push guard: refused: refs/heads/seat/w: a delete"; echo $?)"
+out=$(git -C "$W" push --delete origin seat/w 2>&1); rc=$?
+check "a delete (--delete) is refused" "$([ "$rc" -ne 0 ] && has "$out" "a delete"; echo $?)"
+check "the remote branch still exists after the refused deletes" "$([ "$(rtip seat/w)" = "$before" ]; echo $?)"
+
+# Another clone moves the remote; this clone never fetches it.
+git clone -q -b seat/w "$B" "$O"
+commit "$O" from-other
+git -C "$O" push -q origin seat/w
+moved=$(rtip seat/w)
+check "the other clone moved the remote, and this clone lacks that commit" \
+    "$([ "$moved" != "$before" ] && ! git -C "$C" cat-file -e "$moved^{commit}" 2>/dev/null; echo $?)"
+out=$(git -C "$W" push --force origin seat/w 2>&1); rc=$?
+check "a push over a remote tip missing locally is refused, saying fetch first" \
+    "$([ "$rc" -ne 0 ] && has "$out" "isn't in this repository; fetch first"; echo $?)"
+check "the remote ref is unchanged after that refusal" "$([ "$(rtip seat/w)" = "$moved" ]; echo $?)"
+check "the guard fetched nothing" "$(! git -C "$C" cat-file -e "$moved^{commit}" 2>/dev/null; echo $?)"
+
+echo "== the hook, fed by hand =="
+z=0000000000000000000000000000000000000000
+hookrun() { (cd "$C" && printf '%s\n' "$1" | sh "$C/.git/hooks/pre-push" origin "$B" 2>&1); }
+out=$(hookrun "refs/heads/x $(git -C "$C" rev-parse HEAD) refs/heads/x"); rc=$?
+check "a short ref line is refused" "$([ "$rc" -ne 0 ] && has "$out" "unreadable ref line"; echo $?)"
+out=$(hookrun "refs/heads/x HEAD refs/heads/x $z"); rc=$?
+check "a non-hex object name is refused" "$([ "$rc" -ne 0 ] && has "$out" "non-hex"; echo $?)"
+out=$(hookrun "refs/heads/x 1234567890123456789012345678901234567890 refs/heads/x $(git -C "$C" rev-parse HEAD)"); rc=$?
+check "a failed ancestor check (an unknown local object) is refused" \
+    "$([ "$rc" -ne 0 ] && has "$out" "the ancestor check failed"; echo $?)"
+out=$(printf '' | (cd "$C" && sh "$C/.git/hooks/pre-push" origin "$B" 2>&1)); rc=$?
+check "an empty push passes" "$rc"
+
+echo "== installer refusals =="
+fresh() { rm -rf "$1"; git clone -q "$B" "$1"; }
+fresh "$SANDBOX/hp"
+git -C "$SANDBOX/hp" config core.hooksPath .husky
+out=$(bash "$INSTALL" --repo "$SANDBOX/hp" 2>&1); rc=$?
+check "the install refuses with core.hooksPath set (exit 2)" "$([ "$rc" = 2 ]; echo $?)"
+check "it says core.hooksPath is set" "$(has "$out" "core.hooksPath is set"; echo $?)"
+check "no hook was written under the hooksPath refusal" \
+    "$([ ! -e "$SANDBOX/hp/.git/hooks/pre-push" ] && [ ! -e "$SANDBOX/hp/.husky/pre-push" ]; echo $?)"
+
+fresh "$SANDBOX/df"
+printf '#!/bin/sh\nexit 0\n' > "$SANDBOX/df/.git/hooks/pre-push"
+chmod 0755 "$SANDBOX/df/.git/hooks/pre-push"
+ln "$SANDBOX/df/.git/hooks/pre-push" "$SANDBOX/df-old"
+out=$(bash "$INSTALL" --repo "$SANDBOX/df" 2>&1); rc=$?
+check "the install refuses a different installed pre-push (exit 2)" \
+    "$([ "$rc" = 2 ] && has "$out" "a different pre-push hook is installed"; echo $?)"
+check "the different hook is untouched" \
+    "$([ "$(cat "$SANDBOX/df/.git/hooks/pre-push")" = "$(printf '#!/bin/sh\nexit 0')" ]; echo $?)"
+out=$(bash "$INSTALL" --repo "$SANDBOX/df" --force-install 2>&1); rc=$?
+check "--force-install replaces it" \
+    "$([ "$rc" = 0 ] && has "$out" "replaced a different hook" && cmp -s "$HOOK_SRC" "$SANDBOX/df/.git/hooks/pre-push"; echo $?)"
+check "the replacement was a rename: the old file keeps its bytes" \
+    "$([ "$(cat "$SANDBOX/df-old")" = "$(printf '#!/bin/sh\nexit 0')" ]; echo $?)"
+out=$(bash "$INSTALL" --repo "$SANDBOX/df" 2>&1); rc=$?
+check "a second install over the same bytes exits 0" "$([ "$rc" = 0 ] && has "$out" "the same bytes"; echo $?)"
+
+fresh "$SANDBOX/ln"
+ln -s /dev/null "$SANDBOX/ln/.git/hooks/pre-push"
+out=$(bash "$INSTALL" --repo "$SANDBOX/ln" --force-install 2>&1); rc=$?
+check "the install refuses a symlinked pre-push, even with --force-install" "$([ "$rc" = 2 ] && has "$out" "is a symlink"; echo $?)"
+
+fresh "$SANDBOX/wd"
+git -C "$SANDBOX/wd" worktree add -q -b seat/x "$SANDBOX/wd-wt" origin/main
+out=$(bash "$INSTALL" --repo "$SANDBOX/wd-wt" 2>&1); rc=$?
+check "an install named by a worktree path lands in the common .git/hooks" \
+    "$([ "$rc" = 0 ] && cmp -s "$HOOK_SRC" "$SANDBOX/wd/.git/hooks/pre-push"; echo $?)"
+
+out=$(GIT_DIR="$SANDBOX/wd/.git" bash "$INSTALL" --repo "$SANDBOX/wd" 2>&1); rc=$?
+check "the install refuses with GIT_DIR set" "$([ "$rc" = 2 ] && has "$out" "GIT_DIR is set"; echo $?)"
+mkdir -p "$SANDBOX/plain"
+out=$(bash "$INSTALL" --repo "$SANDBOX/plain" 2>&1); rc=$?
+check "the install refuses a directory that isn't a checkout" "$([ "$rc" = 2 ]; echo $?)"
+out=$(bash "$INSTALL" --nope 2>&1); rc=$?
+check "an unknown argument exits 4" "$([ "$rc" = 4 ]; echo $?)"
+
+echo "== slot-busy, fake /proc =="
+P="$SANDBOX/proc"
+fakeproc() { # $1: pid, $2: cwd, rest: argv
+    local pid=$1 cwd=$2; shift 2
+    mkdir -p "$P/$pid"
+    printf '%s\0' "$@" > "$P/$pid/cmdline"
+    ln -sfn "$cwd" "$P/$pid/cwd"
+}
+busyrun() { bash "$BUSY" --proc-root "$P" 2>&1; }
+fakeproc 4194401 /srv node server.mjs
+fakeproc 4194402 /srv grep -r gate .
+fakeproc 4194403 /srv /bin/bash -c "source /home/u/.claude/shell-snapshots/snapshot-bash-1.sh && eval 'node --test x.test.mjs'"
+mkdir -p "$P/4194404"; : > "$P/4194404/cmdline"
+out=$(busyrun); rc=$?
+check "no matching process: exit 0 and CLEAR" "$([ "$rc" = 0 ] && [ "$out" = CLEAR ]; echo $?)"
+check "an agent's shell-snapshot wrapper isn't a match" "$(! has "$out" "4194403"; echo $?)"
+
+fakeproc 4194410 /seat/a node --test packages/x/tests/a.test.mjs
+out=$(busyrun); rc=$?
+check "a node --test process: exit 1" "$([ "$rc" = 1 ]; echo $?)"
+check "its BUSY line names pid, rule and cwd" "$(has "$out" "BUSY 4194410 node --test cwd=/seat/a node --test"; echo $?)"
+check "BUSY output never says CLEAR" "$(! has "$out" "CLEAR"; echo $?)"
+ran=$(bash "$BUSY" --proc-root "$P" >/dev/null && echo ran)
+check "\`slot-busy > LOG && step\` doesn't run the step while BUSY" "$([ -z "$ran" ]; echo $?)"
+rm -rf "${P:?}/4194410"
+
+rule() { # $1: expected rule, rest: argv
+    local want=$1; shift
+    rm -rf "${P:?}/4194420"; fakeproc 4194420 /seat/b "$@"
+    out=$(busyrun); rc=$?
+    check "busy on: $* ($want)" "$([ "$rc" = 1 ] && has "$out" "BUSY 4194420 $want "; echo $?)"
+}
+rule "node --test" /usr/bin/node --experimental-strip-types --test a.test.ts
+rule "gate" bash /mnt/scratch/sage-gate.sh 253
+rule "gate" bash ./gate.sh
+rule "mutant" bash agents/x/work/queue-1/mutants.sh M3
+rule "test script" bash scripts/test-land-work.sh
+rule "docker compose" docker compose -p x up -d
+rule "docker compose" /usr/bin/docker-compose up
+rm -rf "${P:?}/4194420"
+
+out=$(bash "$BUSY" --proc-root "$SANDBOX/no-such-proc" 2>&1); rc=$?
+check "a missing proc root: exit 2" "$([ "$rc" = 2 ]; echo $?)"
+mkdir -p "$SANDBOX/empty-proc"
+out=$(bash "$BUSY" --proc-root "$SANDBOX/empty-proc" 2>&1); rc=$?
+check "a proc root with no processes: exit 2" "$([ "$rc" = 2 ]; echo $?)"
+out=$(bash "$BUSY" --nope 2>&1); rc=$?
+check "an unknown argument: exit 2" "$([ "$rc" = 2 ]; echo $?)"
+
+echo "== slot-busy, real /proc =="
+mkdir -p "$SANDBOX/bin" "$SANDBOX/standin"
+cat > "$SANDBOX/bin/node" <<'EOF'
+#!/bin/sh
+trap 'exit 0' TERM
+while :; do sleep 0.2; done
+EOF
+chmod +x "$SANDBOX/bin/node"
+(cd "$SANDBOX/standin" && exec "$SANDBOX/bin/node" --test standin.test.mjs) &
+STANDIN=$!
+for _ in $(seq 50); do
+    has "$(tr '\0' ' ' < "/proc/$STANDIN/cmdline" 2>/dev/null)" "--test standin" && break
+    sleep 0.1
+done
+out=$(bash "$BUSY" 2>&1); rc=$?
+check "with a stand-in node --test running: exit 1" "$([ "$rc" = 1 ]; echo $?)"
+check "the stand-in has a BUSY line with its cwd" \
+    "$(printf '%s\n' "$out" | grep -q "^BUSY $STANDIN node --test cwd=$SANDBOX/standin "; echo $?)"
+check "this suite, an ancestor, has no BUSY line" "$(! printf '%s\n' "$out" | grep -q "^BUSY $$ "; echo $?)"
+ran=$(bash "$BUSY" >/dev/null && echo ran)
+check "\`slot-busy > LOG && step\` doesn't run the step with the stand-in up" "$([ -z "$ran" ]; echo $?)"
+kill "$STANDIN" 2>/dev/null; wait "$STANDIN" 2>/dev/null
+gone=$STANDIN; STANDIN=""
+out=$(bash "$BUSY" 2>&1); rc=$?
+check "after the stand-in exits, it has no BUSY line" "$(! printf '%s\n' "$out" | grep -q "^BUSY $gone "; echo $?)"
+check "the real-/proc run exits 0 or 1, never 2" "$([ "$rc" = 0 ] || [ "$rc" = 1 ]; echo $?)"
+
+echo "---"
+echo "git-guard selftests: $PASS passed, $FAIL failed"
+[ "$FAIL" -eq 0 ]