diff --git a/tests/concurrency/concutil.py b/tests/concurrency/concutil.py
new file mode 100644
index 0000000..374e747
--- /dev/null
+++ b/tests/concurrency/concutil.py
@@ -0,0 +1,108 @@
+"""Shared utilities for the real-kernel concurrency suite (imported by the test modules; the
+fixtures in conftest.py wrap these). No flock mocking anywhere — probes use real LOCK_NB."""
+
+from __future__ import annotations
+
+import contextlib
+import fcntl
+import os
+import signal
+import subprocess
+import sys
+import time
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from harness import lifecycle  # noqa: E402
+
+HELPERS = os.path.join(os.path.dirname(__file__), "helpers.py")
+DOMAIN = "test-abc123.ci.commoninternet.net"  # matches RUN_APP_RE
+
+
+class HelperPool:
+    """Spawns helpers.py subprocesses and GUARANTEES their cleanup (incl. recorded grandchild
+    pids from `hold-with-child`/`wrapper` markers) — no leaked children in the test VM."""
+
+    def __init__(self, out_dir: str):
+        self.out_dir = out_dir
+        self.procs: list[subprocess.Popen] = []
+        self.extra_pids: list[int] = []
+        self._n = 0
+
+    def spawn(self, *args: str, env_extra: dict | None = None) -> tuple[subprocess.Popen, str]:
+        """Start `helpers.py <args...>`; returns (proc, marker_file)."""
+        self._n += 1
+        out = os.path.join(self.out_dir, f"helper-{self._n}.out")
+        env = dict(os.environ, CCCI_HELPER_OUT=out, **(env_extra or {}))
+        p = subprocess.Popen(  # noqa: S603
+            [sys.executable, HELPERS, *args],
+            env=env,
+            stdout=subprocess.DEVNULL,
+            stderr=subprocess.STDOUT,
+        )
+        self.procs.append(p)
+        return p, out
+
+    def track_pid(self, pid: int) -> None:
+        self.extra_pids.append(pid)
+
+    def cleanup(self) -> None:
+        for p in self.procs:
+            if p.poll() is None:
+                p.kill()
+            with contextlib.suppress(subprocess.TimeoutExpired):
+                p.wait(timeout=10)
+        for pid in self.extra_pids:
+            with contextlib.suppress(OSError):
+                os.kill(pid, signal.SIGKILL)
+
+
+def wait_marker(out: str, token: str, timeout: float = 15.0) -> str | None:
+    """Poll a helper's marker file for a line containing `token`; returns the line or None."""
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        try:
+            with open(out) as f:
+                for line in f:
+                    if token in line:
+                        return line.strip()
+        except OSError:
+            pass
+        time.sleep(0.1)
+    return None
+
+
+def lock_state(domain: str) -> str:
+    """'held' | 'free' | 'absent' for the domain's lockfile, probed with a REAL LOCK_NB."""
+    path = lifecycle._app_lock_path(domain)  # noqa: SLF001
+    if not os.path.exists(path):
+        return "absent"
+    with open(path, "a") as f:
+        try:
+            fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            return "free"
+        except BlockingIOError:
+            return "held"
+
+
+def wait_lock_state(domain: str, want: str, timeout: float = 10.0) -> str:
+    """Poll until lock_state(domain) == want (kernel release on process death is fast, but give
+    the scheduler room). Returns the final observed state."""
+    deadline = time.time() + timeout
+    state = lock_state(domain)
+    while state != want and time.time() < deadline:
+        time.sleep(0.1)
+        state = lock_state(domain)
+    return state
+
+
+def pid_alive(pid: int) -> bool:
+    return os.path.exists(f"/proc/{pid}")
+
+
+def wait_pid_gone(pid: int, timeout: float = 15.0) -> bool:
+    deadline = time.time() + timeout
+    while time.time() < deadline:
+        if not pid_alive(pid):
+            return True
+        time.sleep(0.1)
+    return False
diff --git a/tests/concurrency/conftest.py b/tests/concurrency/conftest.py
new file mode 100644
index 0000000..ab7be03
--- /dev/null
+++ b/tests/concurrency/conftest.py
@@ -0,0 +1,34 @@
+"""Fixtures for the real-kernel concurrency suite (concurrency-restructure plan, 19 cases).
+
+NOT part of the default `pytest tests/unit` gate — run explicitly with `pytest tests/concurrency
+-q` (docs/concurrency.md). Locks live in a per-test tmp dir (CCCI_APP_LOCK_DIR); helper
+subprocesses hold REAL flocks / install the REAL prctl+signal guards and are always reaped in
+fixture finalizers (no leaked children in the test VM).
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+
+import pytest
+
+sys.path.insert(0, os.path.dirname(__file__))
+from concutil import HelperPool  # noqa: E402
+
+
+@pytest.fixture
+def lock_dir(tmp_path, monkeypatch):
+    """Sandbox lock dir, exported so BOTH this process's lifecycle calls and helper subprocesses
+    (which inherit os.environ) resolve their lockfiles here — never /run/lock."""
+    d = tmp_path / "locks"
+    d.mkdir()
+    monkeypatch.setenv("CCCI_APP_LOCK_DIR", str(d))
+    return str(d)
+
+
+@pytest.fixture
+def pool(tmp_path):
+    hp = HelperPool(str(tmp_path))
+    yield hp
+    hp.cleanup()
diff --git a/tests/concurrency/helpers.py b/tests/concurrency/helpers.py
new file mode 100644
index 0000000..559801f
--- /dev/null
+++ b/tests/concurrency/helpers.py
@@ -0,0 +1,120 @@
+#!/usr/bin/env python3
+"""Subprocess helpers for tests/concurrency — REAL kernel locks and the REAL lifetime guards in
+separate processes (flock/prctl are never mocked; tests assert on actual kernel behavior).
+
+Invoked as:  python3 helpers.py <command> <args...>
+
+Env contract (set by the spawning test):
+  CCCI_APP_LOCK_DIR  sandbox lock dir (never /run/lock in tests)
+  CCCI_HELPER_OUT    marker file this helper APPENDS progress lines to (ACQUIRED/READY/...)
+
+Commands:
+  hold <domain>                 acquire the app lock, mark `ACQUIRED <ts>`, sleep forever
+  hold-with-child <domain>      acquire the lock, spawn a plain sleeping subprocess child, mark
+                                `ACQUIRED <ts>` + `CHILD <pid>` (PEP 446: the child must NOT
+                                inherit the lock fd), sleep forever
+  guarded <domain> <deadline>   install the REAL lifetime guards (alarm=<deadline>s), acquire the
+                                lock, mark `READY`; when the teardown funnel runs (`finally:`),
+                                mark `TEARDOWN` before exiting
+  wrapper <domain>              spawn `guarded <domain> 3600` as MY child, mark `WRAPPED <pid>`,
+                                sleep — the test kills me to prove PDEATHSIG TERMs the child
+  orphan-probe                  wait (bounded) until reparented (ppid==1), then install the
+                                guards; mark `REFUSED` if they exit (expected) or `GUARDS_OK`
+  fetch-checkout <recipe> <ref> run run_recipe_ci.fetch_recipe (the test sets CCCI_SKIP_FETCH=1
+                                + a per-"run" ABRA_DIR), git-checkout <ref>, mark
+                                `RESULT <head> <data.txt content>`
+"""
+
+from __future__ import annotations
+
+import os
+import subprocess
+import sys
+import time
+
+sys.path.insert(0, os.path.join(os.path.dirname(os.path.abspath(__file__)), "..", "..", "runner"))
+from harness import abra, lifecycle, lifetime  # noqa: E402
+
+OUT = os.environ.get("CCCI_HELPER_OUT")
+
+
+def mark(line: str) -> None:
+    if OUT:
+        with open(OUT, "a") as f:
+            f.write(line + "\n")
+            f.flush()
+    print(line, flush=True)
+
+
+def cmd_hold(domain: str) -> None:
+    lifecycle.acquire_app_lock(domain)
+    mark(f"ACQUIRED {time.time()}")
+    time.sleep(3600)
+
+
+def cmd_hold_with_child(domain: str) -> None:
+    lifecycle.acquire_app_lock(domain)
+    child = subprocess.Popen([sys.executable, "-c", "import time; time.sleep(3600)"])
+    mark(f"ACQUIRED {time.time()}")
+    mark(f"CHILD {child.pid}")
+    time.sleep(3600)
+
+
+def cmd_guarded(domain: str, deadline: str) -> None:
+    lifetime.install_lifetime_guards(deadline_seconds=int(deadline))
+    lifecycle.acquire_app_lock(domain)
+    mark("READY")
+    try:
+        time.sleep(3600)
+    finally:
+        mark("TEARDOWN")
+
+
+def cmd_wrapper(domain: str) -> None:
+    p = subprocess.Popen(  # noqa: S603
+        [sys.executable, os.path.abspath(__file__), "guarded", domain, "3600"],
+        env=os.environ.copy(),
+    )
+    mark(f"WRAPPED {p.pid}")
+    time.sleep(3600)
+
+
+def cmd_orphan_probe() -> None:
+    # Our spawner exits immediately after fork; wait (bounded) until we are reparented so the
+    # prctl is installed with the parent ALREADY dead — the exact race the ppid check closes.
+    for _ in range(200):
+        if os.getppid() == 1:
+            break
+        time.sleep(0.05)
+    else:
+        mark("NEVER_REPARENTED")  # e.g. a subreaper environment — test will fail visibly
+        return
+    try:
+        lifetime.install_lifetime_guards()
+    except SystemExit:
+        mark("REFUSED")
+        raise
+    mark("GUARDS_OK")
+
+
+def cmd_fetch_checkout(recipe: str, ref: str) -> None:
+    import run_recipe_ci
+
+    run_recipe_ci.fetch_recipe(recipe, None, None)
+    abra.recipe_checkout(recipe, ref)
+    head = abra.recipe_head_commit(recipe)
+    with open(os.path.join(abra.recipe_dir(recipe), "data.txt")) as f:
+        content = f.read().strip()
+    mark(f"RESULT {head} {content}")
+
+
+if __name__ == "__main__":
+    cmd, *args = sys.argv[1:]
+    {
+        "hold": cmd_hold,
+        "hold-with-child": cmd_hold_with_child,
+        "guarded": cmd_guarded,
+        "wrapper": cmd_wrapper,
+        "orphan-probe": cmd_orphan_probe,
+        "fetch-checkout": cmd_fetch_checkout,
+    }[cmd](*args)
diff --git a/tests/concurrency/test_abra_dir.py b/tests/concurrency/test_abra_dir.py
new file mode 100644
index 0000000..9867fe1
--- /dev/null
+++ b/tests/concurrency/test_abra_dir.py
@@ -0,0 +1,175 @@
+"""Per-run ABRA_DIR isolation (concurrency-restructure plan, cases 17-19). Real directories,
+real symlinks, real git — abra itself is replaced by a recording stub where a CLI call is
+involved (case 17), because these cases test OUR dir/env plumbing, not abra."""
+
+from __future__ import annotations
+
+import os
+import stat
+import subprocess
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+import run_recipe_ci  # noqa: E402
+from concutil import wait_marker  # noqa: E402
+from harness import abra  # noqa: E402
+
+RECIPE = "fakerecipe"
+
+
+def _git(cwd, *args):
+    subprocess.run(
+        ["git", "-c", "user.email=t@t", "-c", "user.name=t", *args],
+        cwd=cwd,
+        check=True,
+        capture_output=True,
+    )
+
+
+def _make_fake_home(tmp_path):
+    """A fake $HOME with a canonical ~/.abra: servers/default + catalogue dirs, and a recipe git
+    repo with two tags whose data.txt differs (v1 -> 'one', v2 -> 'two', HEAD at v2)."""
+    home = tmp_path / "home"
+    (home / ".abra" / "servers" / "default").mkdir(parents=True)
+    (home / ".abra" / "catalogue").mkdir(parents=True)
+    repo = home / ".abra" / "recipes" / RECIPE
+    repo.mkdir(parents=True)
+    _git(repo, "init", "-q")
+    (repo / "data.txt").write_text("one\n")
+    _git(repo, "add", "data.txt")
+    _git(repo, "commit", "-qm", "v1")
+    _git(repo, "tag", "v1")
+    (repo / "data.txt").write_text("two\n")
+    _git(repo, "add", "data.txt")
+    _git(repo, "commit", "-qm", "v2")
+    _git(repo, "tag", "v2")
+    return home
+
+
+def test_17_per_run_dir_built_and_exported_before_abra(tmp_path, monkeypatch):
+    """Case 17: setup_run_abra_dir builds the per-run dir correctly (servers/catalogue symlinks
+    resolve to the canonical tree, recipes/ empty + writable) and $ABRA_DIR is exported before
+    the first abra call — proven by a stub `abra` on PATH that records the env it saw."""
+    home = _make_fake_home(tmp_path)
+    monkeypatch.setenv("HOME", str(home))
+    monkeypatch.setenv("CCCI_RUNS_DIR", str(tmp_path / "runs"))
+    monkeypatch.setenv("DRONE_BUILD_NUMBER", "777")
+    monkeypatch.setenv("ABRA_DIR", "sentinel-to-be-overwritten")  # so monkeypatch restores it
+
+    d = run_recipe_ci.setup_run_abra_dir()
+    assert d == str(tmp_path / "runs" / "777" / "abra")
+    assert os.environ["ABRA_DIR"] == d
+    assert os.readlink(os.path.join(d, "servers")) == str(home / ".abra" / "servers")
+    assert os.readlink(os.path.join(d, "catalogue")) == str(home / ".abra" / "catalogue")
+    # symlinks RESOLVE (targets exist) and recipes/ is empty + writable
+    assert os.path.isdir(os.path.join(d, "servers", "default"))
+    assert os.path.isdir(os.path.join(d, "catalogue"))
+    assert os.listdir(os.path.join(d, "recipes")) == []
+    probe = os.path.join(d, "recipes", ".write-probe")
+    open(probe, "w").close()
+    os.remove(probe)
+    # idempotent re-entry (Drone build-number retry): must not raise on existing symlinks
+    assert run_recipe_ci.setup_run_abra_dir() == d
+
+    # stub abra records $ABRA_DIR at call time; fetch_recipe's catalogue branch invokes it
+    stub_dir = tmp_path / "bin"
+    stub_dir.mkdir()
+    log = tmp_path / "abra-env.log"
+    stub = stub_dir / "abra"
+    stub.write_text(f'#!/bin/sh\necho "$ABRA_DIR" >> {log}\nexit 0\n')
+    stub.chmod(stub.stat().st_mode | stat.S_IEXEC)
+    monkeypatch.setenv("PATH", f"{stub_dir}{os.pathsep}{os.environ['PATH']}")
+    monkeypatch.delenv("CCCI_SKIP_FETCH", raising=False)
+    run_recipe_ci.fetch_recipe(RECIPE, None, None)
+    assert log.read_text().strip() == d, "abra was called without the per-run ABRA_DIR exported"
+
+
+def test_18_concurrent_same_recipe_fetch_no_cross_talk(tmp_path, monkeypatch, pool):
+    """Case 18: two CONCURRENT fetch+checkout flows of the SAME recipe into different ABRA_DIRs
+    produce two correct, divergent trees (v1 vs v2) — the old shared-tree corruption scenario,
+    now structurally safe with no lock. The canonical staged clone is untouched."""
+    home = _make_fake_home(tmp_path)
+    canonical_repo = home / ".abra" / "recipes" / RECIPE
+    head_before = subprocess.run(
+        ["git", "-C", canonical_repo, "rev-parse", "HEAD"], capture_output=True, text=True
+    ).stdout.strip()
+
+    runs = {}
+    for name, ref in (("runA", "v1"), ("runB", "v2")):
+        abra_dir = tmp_path / name / "abra"
+        abra_dir.mkdir(parents=True)
+        _, out = pool.spawn(
+            "fetch-checkout",
+            RECIPE,
+            ref,
+            env_extra={
+                "HOME": str(home),
+                "ABRA_DIR": str(abra_dir),
+                "CCCI_SKIP_FETCH": "1",
+            },
+        )
+        runs[name] = (out, ref, abra_dir)
+
+    expect = {"v1": "one", "v2": "two"}
+    for name, (out, ref, abra_dir) in runs.items():
+        line = wait_marker(out, "RESULT", timeout=30)
+        assert line, f"{name} never produced a RESULT"
+        _, head, content = line.split()
+        assert content == expect[ref], f"{name}@{ref}: tree content {content!r}"
+        tree = abra_dir / "recipes" / RECIPE
+        assert (tree / "data.txt").read_text().strip() == expect[ref]
+        assert (
+            head
+            == subprocess.run(
+                ["git", "-C", tree, "rev-parse", "HEAD"], capture_output=True, text=True
+            ).stdout.strip()
+        )
+
+    # the two trees genuinely diverge AND the canonical staged clone is untouched
+    a = (runs["runA"][2] / "recipes" / RECIPE / "data.txt").read_text()
+    b = (runs["runB"][2] / "recipes" / RECIPE / "data.txt").read_text()
+    assert a != b
+    head_after = subprocess.run(
+        ["git", "-C", canonical_repo, "rev-parse", "HEAD"], capture_output=True, text=True
+    ).stdout.strip()
+    assert head_after == head_before, "canonical clone must not be touched by per-run fetches"
+
+
+def test_19_env_written_through_servers_symlink_lands_canonical(tmp_path, monkeypatch):
+    """Case 19: an app .env written through the per-run servers/ symlink (what abra does under
+    $ABRA_DIR) lands in the CANONICAL shared path — so janitor discovery and every
+    expanduser('~/.abra/servers/...') reader keep working unchanged."""
+    home = _make_fake_home(tmp_path)
+    monkeypatch.setenv("HOME", str(home))
+    monkeypatch.setenv("CCCI_RUNS_DIR", str(tmp_path / "runs"))
+    monkeypatch.setenv("DRONE_BUILD_NUMBER", "778")
+    monkeypatch.setenv("ABRA_DIR", "sentinel-to-be-overwritten")
+    d = run_recipe_ci.setup_run_abra_dir()
+
+    domain = "test-abc123.ci.commoninternet.net"
+    via_symlink = os.path.join(d, "servers", "default", f"{domain}.env")
+    with open(via_symlink, "w") as f:
+        f.write("TYPE=fakerecipe:1.0.0\nDOMAIN=placeholder\n")
+
+    canonical = home / ".abra" / "servers" / "default" / f"{domain}.env"
+    assert canonical.is_file(), ".env written via the symlink must land in the canonical path"
+    # the canonical-path readers/writers (abra.env_get/env_set use ~/.abra) see the same file
+    assert abra.env_get(domain, "TYPE") == "fakerecipe:1.0.0"
+    abra.env_set(domain, "DOMAIN", domain)
+    with open(via_symlink) as f:
+        assert f"DOMAIN={domain}" in f.read()
+
+
+def test_18b_run_id_manual_fallback_is_per_process(tmp_path, monkeypatch):
+    """Companion to case 18: two concurrent MANUAL runs (no DRONE_BUILD_NUMBER) must not share an
+    abra dir either — the manual fallback is pid-suffixed."""
+    home = _make_fake_home(tmp_path)
+    monkeypatch.setenv("HOME", str(home))
+    monkeypatch.setenv("CCCI_RUNS_DIR", str(tmp_path / "runs"))
+    monkeypatch.delenv("DRONE_BUILD_NUMBER", raising=False)
+    monkeypatch.delenv("CCCI_APP_DOMAIN", raising=False)
+    monkeypatch.delenv("CCCI_RUN_ID", raising=False)
+    monkeypatch.setenv("ABRA_DIR", "sentinel-to-be-overwritten")
+    d = run_recipe_ci.setup_run_abra_dir()
+    assert f"manual-{os.getpid()}" in d
diff --git a/tests/concurrency/test_janitor.py b/tests/concurrency/test_janitor.py
new file mode 100644
index 0000000..bdf61b9
--- /dev/null
+++ b/tests/concurrency/test_janitor.py
@@ -0,0 +1,189 @@
+"""Janitor / flock-probe semantics (concurrency-restructure plan, cases 5-12).
+
+The janitor runs IN-PROCESS with its discovery monkeypatched (candidates injected via a stubbed
+abra.app_ls + empty docker sweep) and teardown_app stubbed to record calls — but the LOCKS are
+real kernel flocks, held by real helper subprocesses where a live owner is needed."""
+
+from __future__ import annotations
+
+import os
+import sys
+import threading
+import time
+
+sys.path.insert(0, os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from concutil import DOMAIN, lock_state, wait_marker  # noqa: E402
+from harness import lifecycle  # noqa: E402
+
+
+def _inject_candidates(monkeypatch, domains):
+    """Point janitor discovery at exactly `domains`: abra lists them, docker sweep is empty.
+    teardown_app is stubbed to a recorder; returns the calls list."""
+    calls = []
+    monkeypatch.setattr(lifecycle.abra, "app_ls", lambda: [{"appName": d} for d in domains])
+    monkeypatch.setattr(lifecycle, "_docker_names", lambda kind, stack: [])
+    monkeypatch.setattr(lifecycle, "teardown_app", lambda d, verify=True: calls.append(d))
+    return calls
+
+
+def test_5_orphan_reaped_lockfile_unlinked(lock_dir, pool, monkeypatch):
+    """Case 5: an orphan (lockfile exists, no holder — its run was SIGKILL'd) is reaped exactly
+    once and its lockfile unlinked."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    p.kill()
+    p.wait(timeout=10)
+    calls = _inject_candidates(monkeypatch, [DOMAIN])
+    lifecycle.janitor()
+    assert calls == [DOMAIN], f"teardown calls: {calls} (expected exactly one)"
+    assert lock_state(DOMAIN) == "absent", "reaped orphan's lockfile must be unlinked"
+
+
+def test_6_live_run_never_reaped(lock_dir, pool, monkeypatch, capsys):
+    """Case 6: a held lock (live helper) is never reaped and is logged as live."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    calls = _inject_candidates(monkeypatch, [DOMAIN])
+    lifecycle.janitor()
+    assert calls == []
+    assert "live concurrent run" in capsys.readouterr().out
+    assert lock_state(DOMAIN) == "held"
+
+
+def test_7_new_run_blocks_until_reap_finishes(lock_dir, pool, monkeypatch):
+    """Case 7: the janitor reaps WHILE HOLDING the probe lock, so a new run of the same domain
+    blocks in acquire_app_lock until the reap completes — no window where a fresh app coexists
+    with a half-reaped one."""
+    # Make an orphan.
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    p.kill()
+    p.wait(timeout=10)
+
+    state = {"teardown_end": None, "acquirer_out": None}
+
+    def slow_teardown(domain, verify=True):
+        # While the janitor holds the probe lock mid-reap, a new run starts acquiring.
+        _, aout = pool.spawn("hold", DOMAIN)
+        state["acquirer_out"] = aout
+        time.sleep(2.0)
+        state["teardown_end"] = time.time()
+
+    monkeypatch.setattr(lifecycle.abra, "app_ls", lambda: [{"appName": DOMAIN}])
+    monkeypatch.setattr(lifecycle, "_docker_names", lambda kind, stack: [])
+    monkeypatch.setattr(lifecycle, "teardown_app", slow_teardown)
+    lifecycle.janitor()
+
+    line = wait_marker(state["acquirer_out"], "ACQUIRED", timeout=15)
+    assert line, "new run never acquired after the reap"
+    acquired_ts = float(line.split()[1])
+    assert (
+        acquired_ts >= state["teardown_end"]
+    ), f"new run acquired at {acquired_ts} BEFORE the reap finished at {state['teardown_end']}"
+    # The new run must hold a lock the next probe can SEE (fresh inode at the path).
+    assert lock_state(DOMAIN) == "held"
+
+
+def test_8_two_janitors_exactly_one_reaps(lock_dir, pool, monkeypatch):
+    """Case 8: two concurrent janitors arbitrate on the probe flock — exactly one reaps (the
+    other sees 'held' and leaves). Teardown is slowed so the runs genuinely overlap."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    p.kill()
+    p.wait(timeout=10)
+
+    calls = []
+    calls_lock = threading.Lock()
+
+    def slow_teardown(domain, verify=True):
+        with calls_lock:
+            calls.append(domain)
+        time.sleep(2.0)
+
+    monkeypatch.setattr(lifecycle.abra, "app_ls", lambda: [{"appName": DOMAIN}])
+    monkeypatch.setattr(lifecycle, "_docker_names", lambda kind, stack: [])
+    monkeypatch.setattr(lifecycle, "teardown_app", slow_teardown)
+
+    barrier = threading.Barrier(2)
+
+    def run_janitor():
+        barrier.wait()
+        lifecycle.janitor()
+
+    t1, t2 = threading.Thread(target=run_janitor), threading.Thread(target=run_janitor)
+    t1.start(), t2.start()
+    t1.join(timeout=30), t2.join(timeout=30)
+    assert calls == [DOMAIN], f"expected exactly one reap, got {calls}"
+    assert lock_state(DOMAIN) == "absent"
+
+
+def test_9_reboot_lockfile_absent_reaped_immediately(lock_dir, monkeypatch):
+    """Case 9: post-reboot simulation — the app exists but its lockfile is gone (/run/lock is
+    tmpfs). The probe trivially acquires -> immediate reap, NO age threshold (improvement over
+    the old 2h fallback)."""
+    assert lock_state(DOMAIN) == "absent"
+    calls = _inject_candidates(monkeypatch, [DOMAIN])
+    t0 = time.time()
+    lifecycle.janitor()
+    assert calls == [DOMAIN]
+    assert time.time() - t0 < 5, "reap must be immediate (no age wait)"
+
+
+def test_10_long_held_lock_flagged_never_stolen(lock_dir, pool, monkeypatch, capsys):
+    """Case 10: a lock held with mtime older than 120min is flagged as a possible leaked run —
+    and NOT reaped (never steal a held lock)."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    path = lifecycle._app_lock_path(DOMAIN)  # noqa: SLF001
+    backdate = time.time() - (130 * 60)
+    os.utime(path, (backdate, backdate))
+    calls = _inject_candidates(monkeypatch, [DOMAIN])
+    lifecycle.janitor()
+    assert calls == []
+    out_text = capsys.readouterr().out
+    assert "possible leaked run" in out_text and "lslocks" in out_text
+    assert lock_state(DOMAIN) == "held"
+
+
+def test_11_warm_canonical_names_never_probed(lock_dir, monkeypatch):
+    """Case 11: RUN_APP_RE allowlist — warm/canonical-shaped names never become candidates, so
+    they are never probed (no lockfile is even created for them) and never reaped."""
+    warmish = [
+        "warm-keycloak.ci.commoninternet.net",
+        "keycloak.ci.commoninternet.net",
+        "warm-hedgedoc.ci.commoninternet.net",
+        "drone.ci.commoninternet.net",
+    ]
+    calls = []
+    monkeypatch.setattr(lifecycle.abra, "app_ls", lambda: [{"appName": d} for d in warmish])
+    monkeypatch.setattr(
+        lifecycle,
+        "_docker_names",
+        lambda kind, stack: ["warm-keycloak_ci_commoninternet_net_app"]
+        if kind == "service"
+        else [],
+    )
+    monkeypatch.setattr(lifecycle, "teardown_app", lambda d, verify=True: calls.append(d))
+    lifecycle.janitor()
+    assert calls == []
+    lockdir = os.environ["CCCI_APP_LOCK_DIR"]
+    assert [
+        f for f in os.listdir(lockdir) if f.startswith("cc-ci-app-")
+    ] == [], "janitor must not create lockfiles for non-run-app names"
+
+
+def test_12_degrades_safely_on_bad_lockfile_and_missing_dir(lock_dir, monkeypatch, capsys):
+    """Case 12: a garbled/unopenable lockfile (here: a DIRECTORY at the lockfile path) is skipped
+    with a log line; a missing lock dir doesn't crash the janitor either. Never a crash."""
+    path = lifecycle._app_lock_path(DOMAIN)  # noqa: SLF001
+    os.makedirs(path)  # open(path, "a") -> IsADirectoryError (an OSError)
+    calls = _inject_candidates(monkeypatch, [DOMAIN])
+    lifecycle.janitor()  # must not raise
+    assert calls == []
+    assert "skipping" in capsys.readouterr().out
+
+    os.rmdir(path)
+    monkeypatch.setenv("CCCI_APP_LOCK_DIR", os.path.join(os.environ["CCCI_APP_LOCK_DIR"], "gone"))
+    lifecycle.janitor()  # missing dir: probe open fails -> skip; tidy glob -> empty. No crash.
+    assert calls == []
diff --git a/tests/concurrency/test_lifetime.py b/tests/concurrency/test_lifetime.py
new file mode 100644
index 0000000..1b23c69
--- /dev/null
+++ b/tests/concurrency/test_lifetime.py
@@ -0,0 +1,82 @@
+"""Lifetime hardening (concurrency-restructure plan, cases 13-16): the REAL prctl/signal/alarm
+guards installed by helper subprocesses; tests assert teardown ran, exit was non-zero, and the
+lock was released."""
+
+from __future__ import annotations
+
+import os
+import signal
+import sys
+
+sys.path.insert(0, os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from concutil import (  # noqa: E402
+    DOMAIN,
+    wait_lock_state,
+    wait_marker,
+    wait_pid_gone,
+)
+
+
+def test_13_pdeathsig_parent_kill_terms_harness(lock_dir, pool):
+    """Case 13: wrapper-parent spawns a guarded harness-child; the parent is SIGKILL'd (the
+    harness gets no courtesy signal) -> the kernel's PDEATHSIG TERMs the child, its teardown
+    funnel runs, it exits, and the lock is released."""
+    p, out = pool.spawn("wrapper", DOMAIN)
+    line = wait_marker(out, "WRAPPED")
+    assert line, "wrapper never spawned its child"
+    child_pid = int(line.split()[1])
+    pool.track_pid(child_pid)
+    assert wait_marker(out, "READY"), "guarded child never got ready"
+
+    p.kill()  # parent dies WITHOUT signalling the child — only PDEATHSIG can save us
+    p.wait(timeout=10)
+    assert wait_pid_gone(child_pid), "guarded child must exit on parent death (PDEATHSIG)"
+    assert wait_marker(out, "TEARDOWN", timeout=5), "teardown funnel did not run"
+    assert wait_lock_state(DOMAIN, "free") == "free"
+
+
+def test_14_already_orphaned_helper_refuses_to_run(lock_dir, pool):
+    """Case 14 (ppid race): a helper whose parent died BEFORE the prctl was armed (it starts
+    already reparented to pid 1) must refuse to run — PDEATHSIG would never fire for it."""
+    # Spawn an intermediate parent that forks orphan-probe and exits immediately.
+    import subprocess
+
+    out = os.path.join(pool.out_dir, "orphan.out")
+    intermediate = (
+        "import subprocess, sys, os; "
+        "subprocess.Popen([sys.executable, os.environ['CCCI_HELPERS'], 'orphan-probe']); "
+    )
+    env = dict(
+        os.environ,
+        CCCI_HELPER_OUT=out,
+        CCCI_HELPERS=os.path.join(os.path.dirname(__file__), "helpers.py"),
+    )
+    subprocess.run([sys.executable, "-c", intermediate], env=env, timeout=15, check=True)
+    line = wait_marker(out, "REFUSED", timeout=20)
+    assert line, "orphaned helper did not refuse to run (or never reparented to pid 1)"
+
+
+def test_15_deadline_alarm_fires_teardown_and_releases(lock_dir, pool):
+    """Case 15: the self-deadline (alarm). A guarded helper with a 2s deadline tears down via
+    the funnel (finally: ran), exits NON-zero, and its lock is released."""
+    p, out = pool.spawn("guarded", DOMAIN, "2")
+    assert wait_marker(out, "READY")
+    rc = p.wait(timeout=20)
+    assert rc != 0, f"deadline exit must be non-zero (got {rc})"
+    assert rc == 128 + signal.SIGALRM, f"expected 142 (128+SIGALRM), got {rc}"
+    assert wait_marker(out, "TEARDOWN", timeout=5), "teardown funnel did not run on deadline"
+    assert wait_lock_state(DOMAIN, "free") == "free"
+
+
+def test_16_sigterm_runs_teardown_funnel_and_releases(lock_dir, pool):
+    """Case 16: SIGTERM (drone cancel path) -> the finally: teardown funnel runs, exit is
+    non-zero, lock released."""
+    p, out = pool.spawn("guarded", DOMAIN, "3600")
+    assert wait_marker(out, "READY")
+    p.send_signal(signal.SIGTERM)
+    rc = p.wait(timeout=20)
+    assert rc != 0, f"SIGTERM exit must be non-zero (got {rc})"
+    assert rc == 128 + signal.SIGTERM, f"expected 143 (128+SIGTERM), got {rc}"
+    assert wait_marker(out, "TEARDOWN", timeout=5), "teardown funnel did not run on SIGTERM"
+    assert wait_lock_state(DOMAIN, "free") == "free"
diff --git a/tests/concurrency/test_locks.py b/tests/concurrency/test_locks.py
new file mode 100644
index 0000000..8059dbb
--- /dev/null
+++ b/tests/concurrency/test_locks.py
@@ -0,0 +1,85 @@
+"""Lock fundamentals (concurrency-restructure plan, cases 1-4). Real kernel flocks held by real
+subprocesses — nothing mocked."""
+
+from __future__ import annotations
+
+import fcntl
+import os
+import sys
+import time
+
+sys.path.insert(0, os.path.dirname(__file__))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "runner"))
+from concutil import (  # noqa: E402
+    DOMAIN,
+    lock_state,
+    wait_lock_state,
+    wait_marker,
+)
+from harness import lifecycle  # noqa: E402
+
+
+def test_1_sigkill_releases_lock(lock_dir, pool):
+    """Case 1: acquire -> holder SIGKILL'd -> lock immediately acquirable (kernel auto-release).
+    The exact property the old pidfile registry approximated with /proc checks."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED"), "holder never acquired"
+    assert lock_state(DOMAIN) == "held"
+    p.kill()
+    p.wait(timeout=10)
+    assert wait_lock_state(DOMAIN, "free") == "free"
+
+
+def test_2_nb_probe_held_vs_unheld(lock_dir, pool):
+    """Case 2: LOCK_NB probe raises BlockingIOError against a held lock; succeeds when unheld."""
+    p, out = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    path = lifecycle._app_lock_path(DOMAIN)  # noqa: SLF001
+    with open(path, "a") as f:
+        try:
+            fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+            raise AssertionError("LOCK_NB succeeded against a held lock")
+        except BlockingIOError:
+            pass
+    p.kill()
+    p.wait(timeout=10)
+    assert wait_lock_state(DOMAIN, "free") == "free"
+    with open(path, "a") as f:
+        fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)  # must not raise now
+
+
+def test_3_lock_fd_not_inherited_by_children(lock_dir, pool):
+    """Case 3 (PEP 446): the holder spawns a subprocess child, the holder dies, the child lives —
+    and the lock is STILL released (the child never inherited the lock fd). This is what makes
+    'held lock == live HARNESS owner' sound even though runs spawn abra/docker/pytest children."""
+    p, out = pool.spawn("hold-with-child", DOMAIN)
+    assert wait_marker(out, "ACQUIRED")
+    child_line = wait_marker(out, "CHILD")
+    assert child_line, "holder never reported its child pid"
+    child_pid = int(child_line.split()[1])
+    pool.track_pid(child_pid)
+    p.kill()
+    p.wait(timeout=10)
+    assert os.path.exists(f"/proc/{child_pid}"), "child should outlive the holder"
+    assert (
+        wait_lock_state(DOMAIN, "free") == "free"
+    ), "lock must release on holder death even with a live child (PEP 446 non-inheritable fd)"
+
+
+def test_4_second_acquire_blocks_until_first_exits(lock_dir, pool):
+    """Case 4: a second same-domain acquire blocks until the first holder exits — the
+    double-!testme serialisation property."""
+    p1, out1 = pool.spawn("hold", DOMAIN)
+    assert wait_marker(out1, "ACQUIRED")
+    p2, out2 = pool.spawn("hold", DOMAIN)
+    # p2 must NOT acquire while p1 holds.
+    time.sleep(1.5)
+    assert wait_marker(out2, "ACQUIRED", timeout=0.1) is None, "second acquire did not block"
+    t_kill = time.time()
+    p1.kill()
+    p1.wait(timeout=10)
+    line = wait_marker(out2, "ACQUIRED", timeout=15)
+    assert line, "second acquire never completed after first holder exited"
+    acquired_ts = float(line.split()[1])
+    assert acquired_ts >= t_kill - 0.05, "second holder acquired before the first exited"
+    assert lock_state(DOMAIN) == "held"