fix(proxy): remove invalid savings tracker flock

headroom/proxy/savings_tracker.py

@@ -21,19 +21,6 @@ from typing import Any
 
 from headroom import paths as _paths
 
-# fcntl is Unix-only; on Windows file locking is skipped (best-effort).
-# Typed Any so attribute access on flock/LOCK_EX/LOCK_UN doesn't fail on
-# platforms where the type stub is absent.
-_fcntl: Any = None
-_HAS_FCNTL = False
-try:
-    import fcntl as _fcntl_impl  # noqa: E402
-
-    _fcntl = _fcntl_impl
-    _HAS_FCNTL = True
-except ImportError:
-    pass
-
 logger = logging.getLogger(__name__)
 
 HEADROOM_SAVINGS_PATH_ENV_VAR = _paths.HEADROOM_SAVINGS_PATH_ENV
@@ -825,18 +812,7 @@ class SavingsTracker:
                     f.write(json_data)
                     f.flush()
                     os.fsync(f.fileno())
-                # Acquire an exclusive cross-process lock on the target file
-                # before the atomic rename so concurrent workers don't clobber
-                # each other's increments. Unix only; Windows skips gracefully.
-                if _HAS_FCNTL and self._path.exists():
-                    with open(self._path, "r+b") as lock_fh:
-                        _fcntl.flock(lock_fh, _fcntl.LOCK_EX)
-                        try:
-                            Path(tmp_path).replace(self._path)
-                        finally:
-                            _fcntl.flock(lock_fh, _fcntl.LOCK_UN)
-                else:
-                    Path(tmp_path).replace(self._path)
+                Path(tmp_path).replace(self._path)
             except Exception:
                 try:
                     Path(tmp_path).unlink()

tests/test_proxy_savings_history.py

@@ -211,6 +211,42 @@ def test_record_compression_savings_skips_empty_updates_and_normalizes_timestamp
     assert persisted["history"][-1]["timestamp"] == "2026-03-27T12:34:00Z"
 
 
+def test_savings_tracker_save_does_not_flock_target_inode_before_replace(tmp_path, monkeypatch):
+    path = tmp_path / "proxy_savings.json"
+    tracker = SavingsTracker(path=str(path))
+
+    tracker.record_request(
+        model="gpt-4o",
+        input_tokens=120,
+        tokens_saved=10,
+        timestamp="2026-03-27T09:00:00Z",
+    )
+    assert path.exists()
+
+    flock_calls: list[int] = []
+
+    class _FcntlSpy:
+        LOCK_EX = 1
+        LOCK_UN = 2
+
+        def flock(self, _fh, operation: int) -> None:
+            flock_calls.append(operation)
+
+    monkeypatch.setattr(savings_tracker_module, "_HAS_FCNTL", True, raising=False)
+    monkeypatch.setattr(savings_tracker_module, "_fcntl", _FcntlSpy(), raising=False)
+
+    tracker.record_request(
+        model="gpt-4o",
+        input_tokens=80,
+        tokens_saved=5,
+        timestamp="2026-03-27T09:10:00Z",
+    )
+
+    assert flock_calls == []
+    persisted = json.loads(path.read_text(encoding="utf-8"))
+    assert persisted["lifetime"]["tokens_saved"] == 15
+
+
 def test_litellm_resolution_and_savings_estimation_fallbacks(monkeypatch):
     def fake_cost_per_token(*, model, prompt_tokens, completion_tokens):
         if model in {"gpt-4o", "anthropic/claude-sonnet-4-6"}: