Rework follow-up on the per-job TERMINAL_CWD readers-writer lock.
The lock was acquired BEFORE the try: whose finally: is the only release
site, with the env-override statements (os.environ[TERMINAL_CWD] = workdir;
logger.info) sitting in the unprotected window between acquire and try. Any
exception there — a raising log handler, an os.environ error, a thread
interrupt — propagated out of run_job WITHOUT running the finally, leaking
the lock. A leaked writer permanently deadlocks the whole scheduler (every
future cron job blocks on acquire_*); a leaked reader blocks all writers.
- Snapshot _prior_terminal_cwd before the acquire (so the finally can always
restore env even if the body raises before the override).
- Open the try: immediately after acquire and move the env-override lines
inside it, so the existing finally always releases the lock.
- Add a mutation-verified regression test: a workdir job whose in-window
logger.info raises must still release the writer lock (a subsequent
acquire_write must not block).
A cron job with a per-job `workdir` overrides the process-global
`os.environ["TERMINAL_CWD"]` for the entire duration of its agent run and
restores it afterwards. The scheduler dispatches workdir jobs on a
single-thread sequential pool and workdir-less jobs on a separate parallel
pool, and the in-code comments claimed this made the override safe.
That only prevents two workdir jobs from overlapping each other. The two
pools run concurrently in the same process and share `os.environ`, so while
a workdir job has `TERMINAL_CWD` pointed at its project directory, any
workdir-less job firing in the same window reads that same global through the
terminal, file, and code-exec tools and runs its commands in the wrong
directory. The corruption window spans the whole workdir-job run, and a file
write or delete can land in another job's tree.
This serializes the override with a writer-preferring readers-writer lock.
Workdir jobs acquire it as writers (exclusive for their whole run); workdir-
less jobs acquire it as readers, so they still run in parallel with each
other but never alongside a workdir job's override. The guarantee is based on
run overlap rather than tick boundaries, so it also holds when a workdir job
spans ticks.
## What does this PR do?
Fixes a directory-isolation bug in the cron scheduler: a workdir cron job's
process-global `TERMINAL_CWD` override could be observed by a concurrently
running workdir-less cron job, causing that job's shell/file/code-exec
commands to execute in the wrong directory.
## Related Issue
N/A
## Type of Change
- [x] 🐛 Bug fix (non-breaking change that fixes an issue)
- [ ] ✨ New feature (non-breaking change that adds functionality)
- [ ] 🔒 Security fix
- [ ] 📝 Documentation update
- [ ] ✅ Tests (adding or improving test coverage)
- [ ] ♻️ Refactor (no behavior change)
- [ ] 🎯 New skill (bundled or hub)
## Changes Made
- `cron/scheduler.py`: add `_ReadWriteLock` (writer-preferring) and the
module-global `_terminal_cwd_lock`.
- `cron/scheduler.py`: in `run_job`, acquire the lock as a writer for workdir
jobs and as a reader for workdir-less jobs, spanning the `TERMINAL_CWD`
override and its restore in the `finally` block.
- `cron/scheduler.py`: correct the stale comments in `run_job` and `tick` that
claimed the sequential pool alone made the override safe.
- `tests/cron/test_terminal_cwd_lock.py`: new tests for reader concurrency,
writer exclusion, and the no-cross-observation regression.
## How to Test
1. `python -m pytest tests/cron/test_terminal_cwd_lock.py -q` — the regression
test `test_reader_never_observes_writer_override` fails without the lock and
passes with it.
2. `python -m pytest tests/cron/test_cron_workdir.py tests/cron/test_parallel_pool.py -q`
— confirms the existing `TERMINAL_CWD` set/restore and pool behaviour are
unchanged.
## Checklist
### Code
- [x] I've read the Contributing Guide
- [x] My commit messages follow Conventional Commits (`fix(scope):`, etc.)
- [x] I searched for existing PRs to make sure this isn't a duplicate
- [x] My PR contains only changes related to this fix
- [x] I've run the affected `tests/cron/` suites and all tests pass
- [x] I've added tests for my changes (required for bug fixes)
- [x] I've tested on my platform: macOS 15 (Darwin 25.5)
### Documentation & Housekeeping
- [x] I've updated relevant documentation (docstrings/comments) — or N/A
- [x] I've updated `cli-config.yaml.example` if I added/changed config keys — N/A
- [x] I've updated `CONTRIBUTING.md` or `AGENTS.md` if I changed architecture — N/A
- [x] I've considered cross-platform impact (Windows, macOS) — uses stdlib `threading` only
- [x] I've updated tool descriptions/schemas if I changed tool behavior — N/A
