fix(offline): mutate cached HF constants + threadsafe refcount

Review feedback on the initial fix surfaced two real issues:

1. ``os.environ`` toggles alone don't flip offline mode.
   ``huggingface_hub.constants.HF_HUB_OFFLINE`` is read once at import
   time into a module-level bool; ``transformers.utils.hub._is_offline_mode``
   mirrors that bool at its own import time. The hot paths
   (``_http._default_backend_factory`` in huggingface_hub,
   ``is_offline_mode`` in transformers) read the cached bools — not the
   env — so mutating only ``os.environ`` was a no-op.

2. Race condition on concurrent inference. Two threads running inside
   ``force_offline_if_cached`` via ``asyncio.to_thread`` could have
   thread A's ``finally`` strip thread B's offline protection mid-run.

Rewrite the helper to:
  - mutate ``huggingface_hub.constants.HF_HUB_OFFLINE`` and
    ``transformers.utils.hub._is_offline_mode`` directly
  - refcount concurrent users under a single ``threading.RLock`` so a
    shared offline window is restored only when the last caller exits
  - still write ``os.environ`` for anything that reads it dynamically

Also addresses the unused-variable ruff flag on the Whisper transcribe
path (``audio, sr`` → ``audio, _sr``).

New unit tests cover the cached-constant mutation, env propagation,
no-op on ``is_cached=False``, nested contexts, and a threaded race
where a slow thread must retain offline mode after a peer exits.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/backends/pytorch_backend.py

@@ -347,7 +347,7 @@ class PyTorchSTTBackend:
         def _transcribe_sync():
             """Run synchronous transcription in thread pool."""
             # Load audio
-            audio, sr = load_audio(audio_path, sample_rate=16000)
+            audio, _sr = load_audio(audio_path, sample_rate=16000)
 
             # Model is loaded → weights are on disk. Force offline so
             # `get_decoder_prompt_ids` and any lazy tokenizer lookups

backend/tests/test_offline_guard.py

@@ -0,0 +1,109 @@
+"""
+Unit tests for the ``force_offline_if_cached`` helper.
+
+Verifies that the helper mutates the cached module constants in
+``huggingface_hub.constants`` and ``transformers.utils.hub`` — not just
+``os.environ`` — and that concurrent users are refcount-coordinated so
+one thread's exit can't strip another thread's offline protection.
+"""
+
+import os
+import sys
+import threading
+import time
+from pathlib import Path
+
+import pytest
+
+sys.path.insert(0, str(Path(__file__).parent.parent))
+
+from utils.hf_offline_patch import force_offline_if_cached  # noqa: E402
+
+
+def _hf_const():
+    import huggingface_hub.constants as hf_const
+
+    return hf_const
+
+
+def _tf_hub():
+    import transformers.utils.hub as tf_hub
+
+    return tf_hub
+
+
+def test_mutates_cached_huggingface_hub_constant():
+    original = _hf_const().HF_HUB_OFFLINE
+    with force_offline_if_cached(True, "t"):
+        assert _hf_const().HF_HUB_OFFLINE is True
+    assert _hf_const().HF_HUB_OFFLINE == original
+
+
+def test_mutates_cached_transformers_constant():
+    original = _tf_hub()._is_offline_mode
+    with force_offline_if_cached(True, "t"):
+        assert _tf_hub()._is_offline_mode is True
+    assert _tf_hub()._is_offline_mode == original
+
+
+def test_sets_env_variable():
+    original = os.environ.get("HF_HUB_OFFLINE")
+    with force_offline_if_cached(True, "t"):
+        assert os.environ.get("HF_HUB_OFFLINE") == "1"
+    assert os.environ.get("HF_HUB_OFFLINE") == original
+
+
+def test_noop_when_not_cached():
+    before = _hf_const().HF_HUB_OFFLINE
+    with force_offline_if_cached(False, "t"):
+        assert _hf_const().HF_HUB_OFFLINE == before
+
+
+def test_nested_contexts_respect_refcount():
+    original = _hf_const().HF_HUB_OFFLINE
+    with force_offline_if_cached(True, "outer"):
+        assert _hf_const().HF_HUB_OFFLINE is True
+        with force_offline_if_cached(True, "inner"):
+            assert _hf_const().HF_HUB_OFFLINE is True
+        # inner exit must not restore while outer is still active
+        assert _hf_const().HF_HUB_OFFLINE is True
+    assert _hf_const().HF_HUB_OFFLINE == original
+
+
+def test_concurrent_threads_share_offline_window():
+    """A slow thread must keep seeing offline mode even if a peer exits first."""
+    original = _hf_const().HF_HUB_OFFLINE
+    observations: list[bool] = []
+    errors: list[Exception] = []
+    barrier = threading.Barrier(2)
+
+    def slow():
+        try:
+            with force_offline_if_cached(True, "slow"):
+                barrier.wait()  # sync with fast
+                time.sleep(0.15)  # fast will exit during this sleep
+                observations.append(_hf_const().HF_HUB_OFFLINE)
+        except Exception as exc:  # noqa: BLE001
+            errors.append(exc)
+
+    def fast():
+        try:
+            with force_offline_if_cached(True, "fast"):
+                barrier.wait()
+        except Exception as exc:  # noqa: BLE001
+            errors.append(exc)
+
+    t_slow = threading.Thread(target=slow)
+    t_fast = threading.Thread(target=fast)
+    t_slow.start()
+    t_fast.start()
+    t_slow.join(timeout=5)
+    t_fast.join(timeout=5)
+
+    assert not errors, errors
+    assert observations == [True], "slow thread lost offline protection"
+    assert _hf_const().HF_HUB_OFFLINE == original
+
+
+if __name__ == "__main__":
+    pytest.main([__file__, "-v"])

backend/utils/hf_offline_patch.py

@@ -6,6 +6,7 @@ are already downloaded. Must be imported BEFORE mlx_audio.
 
 import logging
 import os
+import threading
 from contextlib import contextmanager
 from pathlib import Path
 from typing import Optional, Union
@@ -13,13 +14,33 @@ from typing import Optional, Union
 logger = logging.getLogger(__name__)
 
 
+# huggingface_hub reads ``HF_HUB_OFFLINE`` once at import time into
+# ``huggingface_hub.constants.HF_HUB_OFFLINE``; transformers mirrors that into
+# ``transformers.utils.hub._is_offline_mode`` at *its* import time. Toggling
+# ``os.environ`` after either module is imported does not flip those cached
+# bools, and the hot paths (``_http._default_backend_factory``,
+# ``transformers.utils.hub.is_offline_mode``) read the bools — not the env.
+# We mutate the cached constants directly, guarded by a refcount so
+# concurrent inference threads share a single offline window safely.
+
+_offline_lock = threading.RLock()
+_offline_refcount = 0
+_saved_env: Optional[str] = None
+_saved_hf_const: Optional[bool] = None
+_saved_transformers_const: Optional[bool] = None
+
+
 @contextmanager
 def force_offline_if_cached(is_cached: bool, model_label: str = ""):
-    """Context manager that sets ``HF_HUB_OFFLINE=1`` while loading a cached model.
+    """Force offline mode for the duration of a cached-model operation.
+
+    Flips ``HF_HUB_OFFLINE`` in the process env **and** in the cached bools
+    inside ``huggingface_hub.constants`` / ``transformers.utils.hub`` so HTTP
+    adapters and offline-mode checks actually see the change. Uses a refcount
+    so multiple concurrent inference threads share a single offline window
+    and the last one to exit restores state.
 
     If *is_cached* is ``False`` the block runs normally (network allowed).
-    If the offline load raises an error containing "offline" we automatically
-    retry with network access so a partially-cached model still works.
 
     Args:
         is_cached: Whether the model weights are already on disk.
@@ -29,34 +50,60 @@ def force_offline_if_cached(is_cached: bool, model_label: str = ""):
         yield
         return
 
-    original_value = os.environ.get("HF_HUB_OFFLINE")
-    os.environ["HF_HUB_OFFLINE"] = "1"
-    logger.info(
-        "[offline-guard] %s is cached — forcing HF_HUB_OFFLINE=1",
-        model_label or "model",
-    )
+    global _offline_refcount, _saved_env, _saved_hf_const, _saved_transformers_const
+
+    with _offline_lock:
+        if _offline_refcount == 0:
+            _saved_env = os.environ.get("HF_HUB_OFFLINE")
+            try:
+                import huggingface_hub.constants as hf_const
+
+                _saved_hf_const = hf_const.HF_HUB_OFFLINE
+                hf_const.HF_HUB_OFFLINE = True
+            except ImportError:
+                _saved_hf_const = None
+            try:
+                import transformers.utils.hub as tf_hub
+
+                _saved_transformers_const = tf_hub._is_offline_mode
+                tf_hub._is_offline_mode = True
+            except ImportError:
+                _saved_transformers_const = None
+
+            os.environ["HF_HUB_OFFLINE"] = "1"
+            logger.info(
+                "[offline-guard] %s is cached — forcing offline mode",
+                model_label or "model",
+            )
+        _offline_refcount += 1
 
     try:
         yield
-    except Exception as exc:
-        if "offline" in str(exc).lower():
-            logger.warning(
-                "[offline-guard] Offline load failed for %s, retrying with network: %s",
-                model_label or "model",
-                exc,
-            )
-            # Restore original env and retry — caller must wrap the load
-            # inside force_offline_if_cached so retrying here isn't possible.
-            # Instead, propagate a flag via the exception so the caller can
-            # decide.  For simplicity we just let it fall through to the
-            # finally block and re-raise.
-            raise
-        raise
     finally:
-        if original_value is not None:
-            os.environ["HF_HUB_OFFLINE"] = original_value
-        else:
-            os.environ.pop("HF_HUB_OFFLINE", None)
+        with _offline_lock:
+            _offline_refcount -= 1
+            if _offline_refcount == 0:
+                if _saved_env is not None:
+                    os.environ["HF_HUB_OFFLINE"] = _saved_env
+                else:
+                    os.environ.pop("HF_HUB_OFFLINE", None)
+                if _saved_hf_const is not None:
+                    try:
+                        import huggingface_hub.constants as hf_const
+
+                        hf_const.HF_HUB_OFFLINE = _saved_hf_const
+                    except ImportError:
+                        pass
+                if _saved_transformers_const is not None:
+                    try:
+                        import transformers.utils.hub as tf_hub
+
+                        tf_hub._is_offline_mode = _saved_transformers_const
+                    except ImportError:
+                        pass
+                _saved_env = None
+                _saved_hf_const = None
+                _saved_transformers_const = None
 
 
 def patch_huggingface_hub_offline():