feat: take the host probe lock per run

core_probe takes probe.lock itself around one execution, load check included, and releases it when the run ends. A campaign pass never holds it between runs, so two passes interleave. RETROBIOS_PROBE_LOCK names another file, RETROBIOS_PROBE_LOCK_HELD says the caller holds it. Tests use their own lock file.

tests/test_core_probe.py

@@ -504,6 +504,32 @@ class FakeCoreTests(unittest.TestCase):
         sealed = measurement_records.seal({**record, "emulator": "fake", "subject": "s"})
         self.assertEqual([e.message for e in Draft202012Validator(schema).iter_errors(sealed)], [])
 
+    def test_a_run_waits_for_the_host_lock_and_frees_it(self) -> None:
+        import fcntl
+        import time
+
+        lock = self.tmp / "host.lock"
+        argv = [sys.executable, str(REPO_ROOT / "scripts" / "core_probe.py"), "run", "--core",
+                str(self.core), "--frames", "1", "--no-trace", "--work-dir", str(self.tmp / "work")]
+        env = {**os.environ, "RETROBIOS_PROBE_LOCK": str(lock)}
+        with lock.open("a") as held:
+            fcntl.flock(held, fcntl.LOCK_EX)
+            waiting = subprocess.Popen(argv, env=env, stdout=subprocess.DEVNULL, stderr=subprocess.PIPE,
+                                       text=True)
+            time.sleep(1.5)
+            self.assertIsNone(waiting.poll(), "the run went ahead while another held the lock")
+            # A caller that holds the lock for this run says so.
+            held_run = subprocess.run(argv, env={**env, "RETROBIOS_PROBE_LOCK_HELD": "1"},
+                                      capture_output=True, text=True, timeout=60, check=False)
+            self.assertEqual(held_run.returncode, 0, held_run.stderr)
+            fcntl.flock(held, fcntl.LOCK_UN)
+        _, err = waiting.communicate(timeout=60)
+        self.assertEqual(waiting.returncode, 0, err)
+        self.assertIn("another measurement is running", err)
+        # Released at the end of the run: the lock is free again.
+        with lock.open("a") as again:
+            fcntl.flock(again, fcntl.LOCK_EX | fcntl.LOCK_NB)
+
     def test_second_run_is_appended_beside_the_first(self) -> None:
         record_dir = checkout(self.tmp / "appended")
         argv = ["run", "--core", str(self.core), "--frames", "1", "--no-trace", "--max-load", "1000",