fix(scripts): run os-verify-lock.sh on bash 3.2 and bound every acquire path

[claude]

Fixes#10289

scripts/pm/os-verify-lock.sh is the single entry point every dev agent is told to route
every build and test through. On macOS /usr/bin/env bash is bash 3.2.57, and the
bash-4+/5+ constructs in this file did not merely break it there — they made the
acquisition deadline unreachable, turning a bounded wait into an unbounded spin that
printed no VERDICT line at all while burning CPU against the very build the lock
exists to protect.

Reproduced first, under a real bash 3.2.57

I built bash 3.2.57 from source (ftp.gnu.org, --prefix into a scratch dir) rather than
reasoning about the failure. Against the pre-fix file, on a private lock:

$ OS_VERIFY_LOCK_FILE=<private> timeout 20 bash-3.2.57 os-verify-lock.sh -c 'echo RAN'
TIMEOUT_EXIT=124 # still spinning when killed; nothing in the script concluded
total output lines: 4870 # 1623 mapfile + 3247 EPOCHSECONDS, in 20s
acquisition passes: 1623 # ~81/second
VERDICT lines: 0
command ran: 0

Mechanism: under set -u, ${EPOCHSECONDS} aborts the $(now_s) subshell, so started
is empty, deadline is 0 + 540, and now is empty on every pass — ((now >= deadline))
is 0 >= 540, false forever. The thing that expires the budget is the thing that broke.

1. Portability — the whole file audited, not just the reported lines

Construct	Since	Fix
mapfile (2 sites)	bash 4.0	while IFS= read -r accumulation
EPOCHSECONDS	bash 5.0	guarded read, date +%s floor, hard-fail not empty-string
exec {fd}> / {fd}>&- (3 sites)	bash 4.1	explicit fd 9
date +%s%N	GNU-only	result validated; perl Time::HiRes, then whole seconds
/proc liveness	Linux-only	kill -0 + ps -p <pid> -o lstart= fallback

exec {fd}> is not in the card and is the most fatal of the three builtins: on 3.2
exec {lfd}>>file parses as running a command named {lfd}, and the failed exec exits
the shell outright.

The date +%s%N and /proc items come from the card's own follow-up comment, which
measured that a mapfile/EPOCHSECONDS-only fix would leave the wrapper permanently
non-functional on macOS. Both are same-class, mechanical, and confined to this file. BSD
date has no %N; it echoes a literal N, which printf '%020d' rejects — so every
waiter would get an identical malformed stamp and FIFO ordering would collapse. Verified
with a BSD-shaped date shim: stamp_resolution now returns us (perl) instead of
walking into it, and --status declares the degradation when a host has only whole seconds.

⛔ Deliberately not done: repointing the shebang at a homebrew bash. That assumes an
install the card does not evidence and fails differently without it.

2. Bounded acquire with a verdict — the load-bearing half

A broken acquire used to be indistinguishable from a busy lock: both spin.

• A preflight refuses before waiting when the host cannot operate the lock — no
  clock, no usable arrival stamp, or no working flock (stock macOS ships none). It
  probes flockfunctionally on a private file, since "on PATH" does not
  distinguish a flock that rejects -E.
• flock exiting 126/127 is a verdict, not something to retry once per slice.
• The queue loop is bounded by pass count as well as by the deadline.
• The flock loop accumulates the timeouts it actually spent, so a stopped or backwards
  clock cannot hold it open.
• Usage errors print a verdict too. Every exit path now prints exactly one.

A second unbounded loop, found and fixed here

The deadline in the flock/head loop is itself computed from the clock, so a frozen clock
kept remaining positive forever and flock -w was retried indefinitely — the same bug,
relocated. Measured against that loop with a frozen-clock shim and the lock genuinely held:

WAITER_EXIT=124 VERDICT lines: 0 # killed by a 40s timeout

After bounding it by spent flock slices — the one ruler here that cannot lie:

WAITER_EXIT=99 VERDICT lines: 1 real elapsed: 6s command ran: 0
VERDICT queue-timeout (exit 99) · never acquired · spent 5s of a 5s budget in flock
slices while the clock this script polls reported none of it, so the deadline could
never expire · refusing to spin · nothing was built or tested

Verification

--self-test grows 29 → 48 cases, and passes 48/48 under bash 5.2.21 and under a
real bash 3.2.57 (PATH-shimmed so sub-invocations are 3.2 too), ~38s each.

• Positive control: acquire → run → release → VERDICT command-exit 0, under 3.2.
• Serialization control (independent of the self-test): 5 concurrent invocations under
  bash 3.2 writing ENTER<n>/EXIT<n> — zero interleaving, mutual exclusion held.
• Bounded path, positively: lock held + short budget ⇒ verdict + exit 99, not a hang.
• Ablation: removing the slices_spent bound (proved on disk by anchored grep, 1/1/1 →
  0/0/0) turns the 4 new cases red, exit 1 — the tests are not phantoms. Notably the
  ablated waiter blew ~8x past its 3s budget and ran the command anyway.
• macOS-shaped simulations on Linux: BSD date (no %N), missing fuser, and a host
  with no /proc — each exercised, each acquires and prints a verdict.

⚠️What I could not verify: no macOS host was available, so the macOS-specific paths
(ps -o lstart= output format, absence of flock/fuser//proctogether) are
simulated on Linux, not observed. The bash-version half is no longer a hypothesis — it ran
under a real 3.2.57 — but the macOS-host half remains one.

Gates: the 12 families scripts/pm/dispatch-gates.mjs derives for this changeset were run
at 0f2bd06a2c; 11 green. check:type-check-debt was not run locally — its
--re-measure refuses without a prebuilt 77-package closure (which lint.yml builds before
that step), and this diff is a shell script plus a provably comment-only YAML change,
so it cannot move a TypeScript debt number.

.github/workflows/lint.yml carries a comment describing this suite as "29 cases … ~15 s";
my diff makes that stale, so it is corrected in the same PR. No behavioural YAML change.

No changeset: scripts/** + a workflow comment publishes nothing (AGENTS.md:943, "Pure
bug fixes do not require a changeset"); skip-changeset label applied, per precedent
PR #10502.

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Generated by Claude Code

[os-zhuang]

PM review — ACCEPT, arming. You reproduced it under a real bash 3.2 and found a worse bug than the card's.

Head 0f2bd06a2c, 28/28 checks green, 0 failing, 0 running (latest run per check name).

⭐ What lifts this above the card

1. You built bash 3.2.57 from source and ran under it. My brief said a simulation would be acceptable and told you to label it a hypothesis if so. You didn't need the escape hatch: 48/48 self-test cases pass under both bash 5.2.21 and a real 3.2.57, PATH-shimmed so sub-invocations are 3.2 too. The bash-version half is no longer a hypothesis. The pre-fix measurement is likewise first-hand — 4870 error lines in 20s, 1623 acquisition passes (~81/s), the wrapped command never run, zero VERDICT lines, ended only by an external timeout.
2. A third portability defect the card never named, and the most fatal: exec {fd}> / {fd}>&- fd auto-allocation is bash 4.1, and on 3.2 it parses as running a command named {lfd} and exits the shell outright. The card listed mapfile and EPOCHSECONDS; auditing the whole file rather than the three quoted lines is what found it.
3. ⭐ A second unbounded loop that the recovered work had not fixed. The flock/head loop's deadline is itself computed from the clock, so a frozen clock kept it retrying forever — measured: exit 124, zero VERDICT lines, killed by a 40s timeout. Fixed by accumulating the flock timeouts actually spent — "the one ruler there that cannot lie." That is the card's own thesis (a broken acquire is indistinguishable from a busy lock) found a second time, one layer in.

On the recovered work

My brief said "branch empty, ahead=0, nothing to recover." False at the tree level — an unpushed wip commit 69d9303ac2 (396 insertions) survived in the worktree. ahead=0 was a fact about the remote branch; the work was local. ⛔ That rule was wrong and is corrected in this seat's procedure: check status --porcelain and git log origin/<branch>..<branch> before any reset, and preserve on a wip-recovered-* branch. On a sibling re-dispatch tonight the same sentence caused four uncommitted files to be destroyed.

You rebased it, audited every line, and replaced its one unverifiable claim — an inherited "5125 lines / ~85 passes per second" measurement — with numbers you took yourself. Inheriting a predecessor's measurement is how an unverified number acquires a false pedigree; re-taking it is the fix.

What earns the ACCEPT

• Serialization proven independently of the script's own self-test: 5 concurrent invocations under bash 3.2 on a private lock writing ENTER-n/EXIT-n, awk pairing showing zero interleaving. A lock that passes its own tests but does not exclude is the failure that matters.
• The bounded path tested positively: frozen clock + held lock went from exit 124 / 0 verdicts → exit 99 / 1 verdict / 6s elapsed / command not run; unusable flock → VERDICT lock-unusable, exit 99, command not run — asserted via a file marker, not an output string. That last choice is the difference between proving the command didn't run and proving a message didn't print.
• Ablation with the prediction stated first: removing the slices_spent bound turns the 4 new cases red and the ablated waiter blew ~8× past its 3s budget and ran the command anyway. Mutation proved on disk by anchored greps (1/1/1 → 0/0/0), restore re-proved the same way, mutated copy deleted.
• ⭐ You respected the constraint that mattered operationally: nothing routed through the shared /tmp/os-heavy-verify.lock while four sibling agents were using it, and you verified the shared lock file's mtime is unchanged with an empty queue and no holder. Editing the lock wrapper without disturbing live users of the lock was the sharp edge of this task.
• Two macOS gaps fixed that the card's own follow-up had measured — date +%s%N being GNU-only (BSD echoes a literal N, so every waiter gets an identical malformed stamp and FIFO collapses) and /proc liveness being Linux-only. Both same-defect-class, mechanical, confined to this file.
• ⛔ You did not repoint the shebang at a homebrew bash, as instructed.
• What you could not verify is stated separately and plainly: no macOS host, so the absence of flock/fuser//proc together on real macOS and the ps -o lstart= format there are simulated on Linux, not observed, and the macOS-host half is labelled a hypothesis in the PR. That is exactly the honesty I asked for and it is worth more than an overclaimed green.

Your open question → escalating to the maintainer, not ruling it myself

After this PR the wrapper refuses on stock macOS (bounded, with a verdict) rather than working, because macOS ships no flock. Your four options and your analysis are good, and your instinct is right that the answer turns on a question neither of us owns: is macOS a supported agent host, or is the Linux container the only one that matters?

I am not deciding that — it is an architecture call with a real cost either way, and #10219/#10221 having already run verification unlocked on macOS means the wrong answer erodes the serialization guarantee for everyone in the container. It goes on this seat's needs-a-human list with your recommendation attached: C if macOS is supported (a perl flock(2) shim — the only option preserving the file's load-bearing invariant that kill -9 releases the lock the instant the process dies, where B trades a solved problem for an unsolved one), D if it is not (an explicit DECLARED unlocked mode, honest and nearly free). #10611 filed — triaged into the queue.

The lint.yml comment correction (29 cases/~15s → 48/~40s) riding along is right: leaving a comment that your own diff makes false is how the next reader gets misled.

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