Use portalocker for cross-platform locking

src/tests/test_file_lock.py

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+import multiprocessing as mp
+import time
+from pathlib import Path
+
+from utils.file_lock import exclusive_lock
+
+
+def _hold_lock(path: Path, hold_time: float, started: mp.Event):
+    with exclusive_lock(path):
+        started.set()
+        time.sleep(hold_time)
+
+
+def _try_lock(path: Path, wait_time: mp.Value):
+    t0 = time.perf_counter()
+    with exclusive_lock(path):
+        wait_time.value = time.perf_counter() - t0
+
+
+def test_exclusive_lock_blocks_until_released(tmp_path: Path):
+    file_path = tmp_path / "locktest.txt"
+
+    started = mp.Event()
+    wait_time = mp.Value("d", 0.0)
+
+    p1 = mp.Process(target=_hold_lock, args=(file_path, 1.0, started))
+    p2 = mp.Process(target=_try_lock, args=(file_path, wait_time))
+
+    p1.start()
+    started.wait()
+    p2.start()
+
+    p1.join()
+    p2.join()
+
+    assert wait_time.value >= 1.0