idle-draft: process-group child termination, unique tmp paths, stale sweep

Lock-contention test added; children die with the dispatcher (SIGTERM/
SIGINT handlers + atexit); tmp files are per-invocation (.tmp.<pid>) so
a SIGKILL-orphaned child can never collide with a new instance.

Claude-Session: https://claude.ai/code/session_01YQDoWNM7XPPii28khFWoMc

scripts/idle-draft

@@ -7,15 +7,19 @@ Design contract: ~/dev/claude/writing/oreillyconsulting/IDLE-DRAFT-PLAN.md
 
 from __future__ import annotations
 
+import atexit
 import concurrent.futures
 import fcntl
 import hashlib
 import json
 import os
 import re
+import signal
 import subprocess
 import sys
 import tempfile
+import threading
+import time
 from datetime import datetime, timezone
 from pathlib import Path
 from string import Template
@@ -828,7 +832,7 @@ def dispatch_preview(repo: Path, config: dict, candidate: dict, max_turns: int)
     allowed_tools = resolve_allowed_tools(config, work_type)
     argv = build_claude_argv(model_id, max_turns, add_dirs, allowed_tools)
     canonical = canonical_output_path(repo, candidate)
-    tmp_path = canonical.with_name(canonical.name + ".tmp")
+    tmp_path = tmp_output_path(canonical)
     parsed_env = parse_provider_env(profile_dir / "provider.env")
     return {
         "dossier": dossier,
@@ -846,6 +850,133 @@ def dispatch_preview(repo: Path, config: dict, candidate: dict, max_turns: int)
     }
 
 
+# === Child process tracking, unique tmp paths, stale-tmp sweep ===
+#
+# Concurrency-protection hardening (see specs/idle-draft.spec.md "Concurrency
+# protection"): the flock (§1) already keeps two dispatcher instances from
+# running concurrently in the common case. The gap it does *not* close is a
+# dispatcher that dies without releasing its children (SIGTERM/SIGKILL,
+# machine reboot, OOM kill): the flock is released when the fd closes, but
+# `claude` grandchildren spawned via subprocess survive as orphans and can
+# still be mid-write to a tmp file when a fresh cron-fired instance starts.
+# Two independent mechanisms close that gap:
+#   (a) children run in their own process group (`start_new_session=True`);
+#       a caught signal (SIGTERM/SIGINT) or normal interpreter exit kills
+#       every live child's group via os.killpg. This cannot catch SIGKILL of
+#       the dispatcher itself -- hence (b).
+#   (b) every task's tmp output path is unique per dispatcher invocation
+#       (`<canonical>.tmp.<dispatcher-pid>`), so even an orphan that outlives
+#       its dispatcher can never collide with the tmp path a *new* dispatcher
+#       instance uses for the same item. At startup a new instance sweeps and
+#       deletes tmp files older than the task timeout -- they are inert
+#       litter (never promoted by anyone) but worth reclaiming.
+
+_live_children_lock = threading.Lock()
+_live_children: set[int] = set()
+
+
+def register_child(pid: int) -> None:
+    with _live_children_lock:
+        _live_children.add(pid)
+
+
+def unregister_child(pid: int) -> None:
+    with _live_children_lock:
+        _live_children.discard(pid)
+
+
+def live_children() -> set[int]:
+    """Snapshot of currently-tracked child pids. Test/inspection hook."""
+    with _live_children_lock:
+        return set(_live_children)
+
+
+def kill_all_children() -> None:
+    """Kill the process group of every tracked live child. Safe to call
+    repeatedly / with a stale or empty registry -- a dead or nonexistent pid
+    just raises ProcessLookupError, which is swallowed. Each child was
+    started with start_new_session=True, so its pid is also its pgid."""
+    with _live_children_lock:
+        pids = list(_live_children)
+        _live_children.clear()
+    for pid in pids:
+        try:
+            os.killpg(pid, signal.SIGKILL)
+        except (ProcessLookupError, PermissionError, OSError):
+            pass
+
+
+def install_signal_handlers() -> None:
+    """Install SIGTERM/SIGINT handlers so a killed dispatcher takes its
+    in-flight `claude` children down with it rather than orphaning them.
+    Only meaningful in the main thread of a real (non-dryrun) dispatch run,
+    where children actually get spawned."""
+
+    def _handler(signum, frame):
+        kill_all_children()
+        sys.exit(1)
+
+    signal.signal(signal.SIGTERM, _handler)
+    signal.signal(signal.SIGINT, _handler)
+
+
+# Belt-and-braces: even a normal unhandled-exception exit (no signal
+# involved) should not leave children behind. atexit fires on interpreter
+# shutdown from any cause except os._exit()/SIGKILL.
+atexit.register(kill_all_children)
+
+
+TMP_SUFFIX_RE = re.compile(r"\.tmp\.\d+$")
+
+
+def tmp_output_path(canonical: Path, pid: int | None = None) -> Path:
+    """Per-invocation-unique tmp path for a task's output. Defaults to this
+    process's pid (shared by all worker threads of one dispatcher run, so
+    every task in one invocation uses the same suffix); a pid can be passed
+    explicitly for testing or preview purposes."""
+    pid = pid if pid is not None else os.getpid()
+    return canonical.with_name(canonical.name + f".tmp.{pid}")
+
+
+def find_tmp_files(repo: Path, dossiers: list) -> list:
+    """Every leftover `*.tmp.<pid>` file directly under any configured
+    dossier directory (where canonical outputs and their tmp siblings live)."""
+    found = []
+    for dossier in dossiers:
+        d = repo / dossier
+        if not d.is_dir():
+            continue
+        for f in d.iterdir():
+            if f.is_file() and TMP_SUFFIX_RE.search(f.name):
+                found.append(f)
+    return found
+
+
+def sweep_stale_tmp_files(repo: Path, dossiers: list, task_timeout: int, now: float | None = None) -> list:
+    """Delete tmp files whose mtime is older than task_timeout seconds --
+    orphaned litter from a dispatcher that died mid-task (GAP 2b). Tmp files
+    younger than the timeout are left alone: they could belong to a live
+    orphan still finishing up, or to another concurrent dispatch cycle's
+    in-flight task. Since a tmp file is only ever promoted by the exact
+    run_task() call that created it, an untouched one is inert at worst --
+    never silently adopted by anyone else. Returns the paths actually
+    deleted, for logging by the caller."""
+    now = now if now is not None else time.time()
+    deleted = []
+    for f in find_tmp_files(repo, dossiers):
+        try:
+            mtime = f.stat().st_mtime
+        except OSError:
+            continue
+        if now - mtime > task_timeout:
+            try:
+                f.unlink()
+                deleted.append(f)
+            except OSError:
+                continue
+    return deleted
+
+
 # === Real task execution ===
 
 
@@ -856,7 +987,7 @@ def run_task(repo: Path, config: dict, candidate: dict, max_turns: int, task_tim
     slug = candidate["slug"]
 
     canonical = canonical_output_path(repo, candidate)
-    tmp_path = canonical.with_name(canonical.name + ".tmp")
+    tmp_path = tmp_output_path(canonical)
 
     mapping = build_prompt_mapping(repo, dossier, slug, work_type, config, tmp_path)
     prompt_text = render_prompt(work_type, mapping)
@@ -872,14 +1003,34 @@ def run_task(repo: Path, config: dict, candidate: dict, max_turns: int, task_tim
     returncode = -1
     stderr = ""
     try:
-        proc = subprocess.run(
+        proc = subprocess.Popen(
-            argv, cwd=str(repo), env=env, input=prompt_text,
+            argv, cwd=str(repo), env=env,
-            capture_output=True, text=True, timeout=task_timeout,
+            stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE,
+            text=True, start_new_session=True,
         )
+    except OSError as e:
+        return {"outcome": "transient", "candidate": candidate, "reason": f"failed to start claude subprocess: {e}"}
+
+    # GAP 2a: own process group + tracked pid, so a killed dispatcher can
+    # take this (and any grandchildren it spawns) down with it instead of
+    # orphaning it. See "Child process tracking" section above.
+    register_child(proc.pid)
+    try:
+        try:
+            _, stderr = proc.communicate(input=prompt_text, timeout=task_timeout)
             returncode = proc.returncode
-        stderr = proc.stderr
         except subprocess.TimeoutExpired:
             timed_out = True
+            try:
+                os.killpg(proc.pid, signal.SIGKILL)
+            except (ProcessLookupError, PermissionError, OSError):
+                pass
+            try:
+                proc.communicate(timeout=5)
+            except Exception:
+                pass
+    finally:
+        unregister_child(proc.pid)
 
     failure = classify_failure(returncode, stderr, timed_out)
 
@@ -972,12 +1123,20 @@ def apply_failure_state(state: dict, candidate: dict, max_attempts: int) -> str
 
 def run_dispatch(args, config: dict, repo: Path, state_path: Path) -> int:
     rotate_log_if_needed(repo)
+    defaults = get_config_defaults(config)
     lock = None
     if not args.dryrun:
         lock = acquire_lock(repo)
         if lock is None:
             log_event(repo, "lock held by another invocation; exiting")
             return 0
+        # Held for the full dispatch loop, including while children run (see
+        # module-level "Child process tracking" section) -- a second cron-fired
+        # invocation exits 0 immediately per the check above, it never races
+        # this instance even if this instance runs for hours.
+        install_signal_handlers()
+        for swept in sweep_stale_tmp_files(repo, config["dossiers"], defaults["task_timeout"]):
+            log_event(repo, f"swept stale tmp file (orphaned litter): {swept}")
 
     try:
         state = load_state(state_path, repo)
@@ -985,7 +1144,6 @@ def run_dispatch(args, config: dict, repo: Path, state_path: Path) -> int:
         log_event(repo, f"FATAL: state validation failed: {e}")
         return 2
 
-    defaults = get_config_defaults(config)
     parallel = args.parallel or config.get("parallel", DEFAULT_PARALLEL)
 
     try:

specs/idle-draft.spec.md

@@ -65,6 +65,14 @@ lock), `flock` (`fcntl.flock`, `LOCK_EX | LOCK_NB`) on `<repo>/.idle-draft.lock`
 already locked by another invocation, log one line and exit **0** — a concurrent cron
 tick is not a failure.
 
+**Guarantee:** the lock is acquired before any other work and held for the *entire*
+dispatch loop, including while dispatched children are running — not released until the
+process is about to exit. A second cron-fired invocation therefore always exits 0
+immediately for as long as the first is alive, however long that is (a low-usage
+provider can let one instance run for hours "catching up"; the lock does not expire or
+time out). This is the primary concurrency guard; §13 covers what happens when the
+*lock-holding process itself* dies uncleanly rather than exiting normally.
+
 ### 2. Log rotation
 
 At startup, if `<repo>/idle-draft.log` exceeds 5 MB, rename it to `idle-draft.log.1`
@@ -216,6 +224,11 @@ For the selected `(item_or_dossier, work_type, provider)`:
 2. Render the template (`string.Template`, `$placeholder` substitution) with the
    resolved paths for that item (overview, agent, research, draft as applicable),
    dossier name, slug, style directory, source register path, and a temp output path.
+   The temp output path is `<canonical>.tmp.<dispatcher-pid>` — unique per dispatcher
+   *invocation* (all tasks within one dispatch run share the same suffix, since it's
+   this process's own pid), not per task. This is what makes tmp paths collision-proof
+   across dispatcher instances even when one instance dies leaving orphaned children
+   behind — see §13.
 3. Resolve the profile directory from `config["providers"][provider]["profile"]`
    (`~` expanded). Build the child environment: `CLAUDE_CONFIG_DIR=<profile>`, plus —
    if `<profile>/provider.env` exists — `ANTHROPIC_BASE_URL` (if set),
@@ -237,7 +250,11 @@ For the selected `(item_or_dossier, work_type, provider)`:
 5. Run the subprocess: `cwd=<repo>`, `env=<built env>`, prompt piped via **stdin**
    (not as an argv element — avoids `ARG_MAX` on large rendered prompts, same lesson
    `claude-profile` already applies to its system-prompt injection), timeout = per-task
-   timeout (1800s unless overridden).
+   timeout (1800s unless overridden). The child is started in its own process group
+   (`start_new_session=True`) and its pid is tracked in a live-children registry for the
+   duration of the call — see §13. On timeout, the whole process group is sent
+   `SIGKILL` (not just the immediate `claude` process), so any grandchild it spawned
+   dies too, before the task is classified as a transient failure.
 6. Classify the result:
    - **Timeout** → transient failure. Do not increment `attempts`.
    - **Non-zero exit** whose stderr matches a retryable signature (`429`, `5xx`,
@@ -265,7 +282,10 @@ For the selected `(item_or_dossier, work_type, provider)`:
    `<repo>/<dossier>/<NN-slug>.<work_type>.md.rejected` (kept for human inspection,
    never promoted, never committed to the canonical name; `topic_ideas` content
    failures are simply discarded — nothing is appended, nothing is blocked, since
-   `topic_ideas` has no per-item state entry to carry an attempt counter).
+   `topic_ideas` has no per-item state entry to carry an attempt counter). The
+   `.rejected` path is derived from the canonical path, not the tmp path, so it is
+   unaffected by the `.tmp.<dispatcher-pid>` suffix (§13) — always
+   `<canonical>.rejected`, never `<canonical>.rejected.<pid>`.
 10. On transient failure: temp file is discarded; no state change; no commit; the
     candidate may be retried on a later cycle.
 11. On success: write `idle-draft.state.json` atomically (temp file + `os.replace()` in
@@ -284,6 +304,53 @@ For the selected `(item_or_dossier, work_type, provider)`:
 One line per event (gate decision, dispatch, completion, failure, commit) appended to
 `<repo>/idle-draft.log` with an ISO-8601 timestamp, mirrored to stderr.
 
+### 13. Concurrency protection beyond the lock
+
+§1's flock is the primary guard and handles the common case: a second cron-fired
+invocation always sees the lock held and exits 0. This section covers the gap the lock
+alone cannot close — a dispatcher process that dies *without* releasing its children
+cleanly (`SIGTERM`, `SIGKILL`, OOM kill, machine reboot). The flock itself is released
+the instant the holding process's file descriptors close (on any death, clean or not),
+but a `claude` child spawned via `subprocess` is a separate OS process and does **not**
+die automatically when its parent does — left alone it becomes an orphan, reparented to
+init, still writing its output. A fresh cron-fired instance starting immediately after
+would then see the lock free, pick up the *same* ready item, and dispatch a *second*
+`claude` process against the *same* deterministic output path — two writers racing on
+the "same" canonical file. Two independent mechanisms close this:
+
+**a. Children die with the dispatcher (the common death path).** Every `claude` child is
+spawned with `start_new_session=True`, putting it (and any process it forks) in its own
+process group with the child's pid as the group id. The dispatcher tracks each live
+child's pid in an in-process registry for the duration of the call (`register_child` /
+`unregister_child`, guarded by a lock since tasks run in worker threads). On `SIGTERM` or
+`SIGINT`, an installed handler walks the registry and `os.killpg`s each group with
+`SIGKILL`, then exits — no child outlives a dispatcher that receives a catchable signal.
+The same cleanup also runs via `atexit`, so a normal unhandled-exception exit (no signal
+involved) still cannot leave children behind. **This cannot catch `SIGKILL` of the
+dispatcher itself** — no userspace handler can — which is exactly the case mechanism (b)
+exists for.
+
+**b. Tmp-path collisions are structurally impossible (the uncatchable-death path).** Each
+task's temp output file is named `<canonical>.tmp.<dispatcher-pid>` (§11 step 2), not
+the old bare `<canonical>.tmp`. Every dispatcher process has a distinct pid, so even an
+orphaned child that outlives a `SIGKILL`ed dispatcher is writing to a filename no other
+dispatcher instance — past, present, or future — will ever target. There is no race to
+resolve: the collision that used to be structurally possible (two writers, one path) is
+now structurally impossible (two writers, two paths).
+
+**Startup sweep.** After acquiring the lock, before touching config-driven state, a
+fresh dispatch instance globs each configured dossier directory for leftover
+`*.tmp.<pid>` files. Any whose mtime is older than the per-task timeout (1800s unless
+overridden) is deleted and logged as swept — it is orphaned litter from a dispatcher
+that died mid-task; since it was never promoted (only the exact `run_task()` call that
+created it ever calls `os.replace()` on it) it can never silently become the canonical
+file no matter how long it sits there. Files younger than the timeout are left alone —
+they may belong to a live orphan (mechanism (a) failed to reap it, e.g. it was itself
+`SIGKILL`ed independently, or belongs to another concurrent dispatch cycle that legitimately
+still has it in flight) still finishing up; touching them risks nothing since, again,
+nothing ever silently adopts a tmp file it didn't create — at worst a stale one sits
+unswept until the *next* startup sweep, once it ages past the timeout.
+
 ## Dryrun behaviour
 
 `--dryrun` runs the full probe (or reads `--probe-json`), computes all gates, builds
@@ -346,7 +413,9 @@ a lighter-touch check); `draft` and `review` carry no `allowed_tools` key.
 
 | Scenario | Handling |
 |---|---|
-| Lock already held | Log one line, exit 0 (not an error — another cron tick is running) |
+| Lock already held | Log one line, exit 0 (not an error — another cron tick is running), regardless of how long the holder has been running |
+| Dispatcher receives `SIGTERM`/`SIGINT` while children are running | Handler `os.killpg`s every live child's process group, then exits (§13a) |
+| Dispatcher is `SIGKILL`ed (uncatchable) | Children orphaned, but each was writing to a `.tmp.<dispatcher-pid>` path unique to that dead instance — no live instance can ever collide with it; stale ones swept once they age past the task timeout (§13b) |
 | `idle-draft.state.json` missing | Treated as `{"items": {}}`, not an error |
 | `idle-draft.state.json` present but invalid | Exit 2, loud message, run never starts |
 | Item's `.agent.md` missing | Item excluded from all dispatch (commissioning briefs exist today for every current item; this guards future additions) |

tests/test-idle-draft.sh

@@ -162,6 +162,50 @@ after_hash=$(cd "$FIXTURE_REPO" && git rev-parse HEAD)
 [[ "$before_hash" == "$after_hash" ]] && pass "dryrun makes no git commits" || fail "dryrun makes no git commits"
 [[ ! -f "$FIXTURE_REPO/ai/03-topic-three.review.md" ]] && pass "dryrun writes no output file" || fail "dryrun writes no output file"
 
+echo "-- concurrency: second invocation exits 0 when lock is held (GAP 1) --"
+# Holds .idle-draft.lock in a background helper (independent process, so the
+# flock is real -- fcntl locks are per-process/per-fd, a second flock from
+# this same bash process wouldn't contend). The helper touches a sentinel
+# file only *after* it has the lock, so we never race the flock call itself.
+LOCK_HELPER="$TMPDIR/lock_helper.py"
+cat > "$LOCK_HELPER" <<'PYEOF'
+import fcntl, sys, time
+lock_path, sentinel_path = sys.argv[1], sys.argv[2]
+f = open(lock_path, "w")
+fcntl.flock(f.fileno(), fcntl.LOCK_EX)
+with open(sentinel_path, "w") as s:
+    s.write("locked\n")
+time.sleep(30)
+PYEOF
+LOCK_SENTINEL="$TMPDIR/lock-sentinel"
+rm -f "$LOCK_SENTINEL"
+python3 "$LOCK_HELPER" "$FIXTURE_REPO/.idle-draft.lock" "$LOCK_SENTINEL" &
+HELPER_PID=$!
+
+waited=0
+while [[ ! -f "$LOCK_SENTINEL" && $waited -lt 50 ]]; do
+    sleep 0.1
+    waited=$((waited + 1))
+done
+
+if [[ ! -f "$LOCK_SENTINEL" ]]; then
+    fail "lock-contention test setup: helper never confirmed it holds the lock"
+else
+    before_hash=$(cd "$FIXTURE_REPO" && git rev-parse HEAD)
+    # Real dispatch mode (no --dryrun): a fixture with plenty of idle capacity,
+    # so absent the lock this would dispatch item 03's review task for real.
+    output=$("$SCRIPT" --config "$FIXTURE_CONFIG" --repo "$FIXTURE_REPO" --probe-json "$FIXTURE_PROBE" --once 2>&1)
+    code=$?
+    assert_exit_code "$code" 0 "second invocation with lock held exits 0"
+    assert_contains "$output" "lock held" "log line reports lock held"
+    after_hash=$(cd "$FIXTURE_REPO" && git rev-parse HEAD)
+    [[ "$before_hash" == "$after_hash" ]] && pass "no commit occurred while lock held" || fail "no commit occurred while lock held"
+    [[ ! -f "$FIXTURE_REPO/ai/03-topic-three.review.md" ]] && pass "no output file written while lock held" || fail "no output file written while lock held"
+fi
+
+kill "$HELPER_PID" 2>/dev/null
+wait "$HELPER_PID" 2>/dev/null
+
 echo "-- state validation failure exits 2 --"
 BADSTATE="$TMPDIR/badstate-repo"
 mkdir -p "$BADSTATE/ai"
@@ -547,6 +591,85 @@ except m.StateValidationError:
 m.validate_state({"items": {"ai/01-topic-one": {"human_edit_done": True, "attempts": {"draft": 1}}}}, repo)
 check("state validation: well-formed state passes", True)
 
+# --- GAP 2b: unique tmp path per invocation ---
+import os as _os
+import time as _time
+
+canonical = repo / "ai" / "03-topic-three.review.md"
+tmp_a = m.tmp_output_path(canonical, pid=11111)
+tmp_b = m.tmp_output_path(canonical, pid=22222)
+check("tmp path uniqueness: two different pids produce two different tmp paths",
+      tmp_a != tmp_b and tmp_a.name == "03-topic-three.review.md.tmp.11111"
+      and tmp_b.name == "03-topic-three.review.md.tmp.22222", (str(tmp_a), str(tmp_b)))
+
+tmp_default = m.tmp_output_path(canonical)
+check("tmp path uniqueness: default pid is os.getpid()",
+      tmp_default.name == f"03-topic-three.review.md.tmp.{_os.getpid()}", str(tmp_default))
+
+rejected = m.rejected_output_path(canonical)
+check("rejected-path derivation is unaffected by the new tmp naming (derived from canonical, not tmp)",
+      rejected.name == "03-topic-three.review.md.rejected", str(rejected))
+
+# --- GAP 2b: startup sweep of stale tmp files ---
+sweep_repo = tmpdir / "sweep-repo"
+(sweep_repo / "ai").mkdir(parents=True, exist_ok=True)
+old_tmp = sweep_repo / "ai" / "01-a.research.md.tmp.111"
+fresh_tmp = sweep_repo / "ai" / "02-b.research.md.tmp.222"
+old_tmp.write_text("stale orphan output\n")
+fresh_tmp.write_text("still-in-progress output\n")
+now = _time.time()
+task_timeout = 1800
+old_mtime = now - task_timeout - 100  # older than the timeout -> stale
+fresh_mtime = now - 10  # well within the timeout -> live orphan candidate, keep
+_os.utime(old_tmp, (old_mtime, old_mtime))
+_os.utime(fresh_tmp, (fresh_mtime, fresh_mtime))
+
+deleted = m.sweep_stale_tmp_files(sweep_repo, ["ai"], task_timeout, now=now)
+check("stale sweep: deletes a tmp file older than the task timeout",
+      old_tmp in deleted and not old_tmp.exists(), deleted)
+check("stale sweep: leaves a tmp file younger than the task timeout untouched",
+      fresh_tmp not in deleted and fresh_tmp.exists(), deleted)
+
+# A non-.tmp.<pid> file (e.g. the canonical output itself) must never be swept.
+canonical_lookalike = sweep_repo / "ai" / "01-a.research.md"
+canonical_lookalike.write_text("# Research: a\n")
+_os.utime(canonical_lookalike, (old_mtime, old_mtime))
+deleted2 = m.sweep_stale_tmp_files(sweep_repo, ["ai"], task_timeout, now=now)
+check("stale sweep: never touches a file without a .tmp.<pid> suffix, however old",
+      canonical_lookalike.exists() and canonical_lookalike not in deleted2, canonical_lookalike)
+
+# --- GAP 2a: child process-group tracking / signal handling (smoke tests) ---
+# No real subprocess is spawned here -- just the bookkeeping primitives that
+# the signal handler and run_task() rely on.
+fake_pid = 2**30  # astronomically unlikely to collide with a real pid/pgid
+m.register_child(fake_pid)
+check("child tracking: register_child adds the pid to the live set", fake_pid in m.live_children(), m.live_children())
+
+m.unregister_child(fake_pid)
+check("child tracking: unregister_child removes the pid", fake_pid not in m.live_children(), m.live_children())
+
+m.register_child(fake_pid)
+m.register_child(fake_pid + 1)
+try:
+    m.kill_all_children()
+    kill_all_ok = True
+except Exception as e:
+    kill_all_ok = False
+check("child tracking: kill_all_children on nonexistent pids raises nothing (ProcessLookupError swallowed)",
+      kill_all_ok, "")
+check("child tracking: kill_all_children clears the registry", m.live_children() == set(), m.live_children())
+
+m.install_signal_handlers()
+import signal as _signal
+check("signal handling: install_signal_handlers installs a non-default SIGTERM handler",
+      _signal.getsignal(_signal.SIGTERM) not in (_signal.SIG_DFL, _signal.SIG_IGN, None), "")
+check("signal handling: install_signal_handlers installs a non-default SIGINT handler",
+      _signal.getsignal(_signal.SIGINT) not in (_signal.SIG_DFL, _signal.SIG_IGN, None), "")
+# Restore defaults so this subprocess (which is about to exit anyway) doesn't
+# leave anything surprising behind for the interpreter teardown.
+_signal.signal(_signal.SIGTERM, _signal.SIG_DFL)
+_signal.signal(_signal.SIGINT, _signal.SIG_DFL)
+
 for status, name, detail in results:
     print(f"{status}: {name}", detail if detail else "")