unsloth/studio/backend/utils/hf_xet_fallback.py

# SPDX-License-Identifier: AGPL-3.0-only
# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0

"""Unsloth shim over the shared ``unsloth_zoo.hf_xet_fallback`` Xet -> HTTP stall fallback.

Re-exports the shared API and injects Unsloth's marker-aware cache purge
(``prepare_cache_for_transport``) so the download manager keeps its ``.transport``
marker semantics on the HTTP retry.

Import discipline: ``unsloth_zoo``'s ``__init__`` eagerly imports ``transformers``. The workers
import this shim at startup (to decide the per-worker Xet env flip) *before* activating the model's
``transformers`` sidecar. Activation only prepends the sidecar to ``sys.path``, so a ``transformers``
already cached in ``sys.modules`` (via an eager ``unsloth_zoo`` import here) wins -- pinning the
default 4.57.x and regressing Qwen3.5 / GLM-4.7 / gemma-4 training with
``Tokenizer class TokenizersBackend does not exist``. So the shared backend is loaded **lazily**
(``_load_shared``), only on first use of a heavy download helper, i.e. after the sidecar is active.
``child_should_disable_xet`` and the ``DEFAULT_*`` constants are defined locally so importing them
never triggers the heavy load.
"""

from __future__ import annotations

import threading
from typing import Any, Callable, Optional

# Defaults mirror unsloth_zoo.hf_xet_fallback; plain literals so they resolve (including as
# default args below) without importing unsloth_zoo/transformers.
DEFAULT_GRACE_PERIOD = 10.0
DEFAULT_HEARTBEAT_INTERVAL = 30.0
DEFAULT_STALL_TIMEOUT = 180.0

# --- lazy shared-backend loader ----------------------------------------------------------------
_shared: Any = None
_shared_available: Optional[bool] = None  # None = not yet attempted
_shared_import_error: Optional[BaseException] = None
_load_lock = threading.Lock()


def _load_shared() -> bool:
    """Import ``unsloth_zoo.hf_xet_fallback`` on demand; return True if available. Deferred so
    importing this module at worker startup does not pull transformers in before the sidecar is
    activated. Degrades (returns False) rather than crashing when unsloth_zoo is unavailable."""
    global _shared, _shared_available, _shared_import_error
    if _shared_available is not None:
        return _shared_available
    with _load_lock:
        if _shared_available is not None:
            return _shared_available
        try:
            import unsloth_zoo.hf_xet_fallback as shared

            _shared = shared
            _shared_available = True
            _shared_import_error = None
            return True
        except Exception as exc:  # noqa: BLE001 - any import failure must degrade, not crash
            # unsloth_zoo's __init__ runs torch/GPU detection, which raises on a torch-less/GPU-less
            # host. The download helper needs none of it, so retry via UNSLOTH_ZOO_DISABLE_GPU_INIT.
            _shared_import_error = exc
            import os as _os

            _prev_gpu_init = _os.environ.get("UNSLOTH_ZOO_DISABLE_GPU_INIT")
            _os.environ["UNSLOTH_ZOO_DISABLE_GPU_INIT"] = "1"
            try:
                import unsloth_zoo.hf_xet_fallback as shared

                _shared = shared
                _shared_available = True
                _shared_import_error = None
                return True
            except Exception as exc2:  # noqa: BLE001 - degrade so Unsloth still boots with plain HF
                _shared_import_error = exc2
                _shared_available = False
                import logging as _logging

                _logging.getLogger(__name__).warning(
                    "unsloth_zoo.hf_xet_fallback unavailable (%s); the Xet stall watchdog is "
                    "disabled. Install/upgrade unsloth_zoo (and its torch dependency) to "
                    "re-enable automatic Xet -> HTTP download recovery.",
                    _shared_import_error,
                )
                return False
            finally:
                if _prev_gpu_init is None:
                    _os.environ.pop("UNSLOTH_ZOO_DISABLE_GPU_INIT", None)
                else:
                    _os.environ["UNSLOTH_ZOO_DISABLE_GPU_INIT"] = _prev_gpu_init


def child_should_disable_xet(config: dict) -> bool:
    """Single source of truth for the per-worker Xet env flip (mirrors
    ``unsloth_zoo.hf_xet_fallback.child_should_disable_xet``). Deliberately lightweight: importing or
    calling it must NOT pull in unsloth_zoo/transformers, so the worker can decide before activating
    the transformers sidecar (see the module docstring)."""
    return bool(config.get("disable_xet"))


# --- degraded stubs (used only when unsloth_zoo is unavailable) -------------------------------
class _DegradedDownloadStallError(RuntimeError):
    """Stub mirror so callers' ``except`` clauses resolve; never raised in degraded mode."""


def _degraded_get_hf_download_state(*args: Any, **kwargs: Any) -> None:
    return None  # unmeasurable -> the (absent) watchdog never fires


def _degraded_start_watchdog(
    *,
    on_heartbeat: "Optional[Callable[[str], None]]" = None,
    interval: float = DEFAULT_HEARTBEAT_INTERVAL,
    xet_disabled: bool = False,
    **kwargs: Any,
) -> "threading.Event":
    # No stall detection, but keep emitting heartbeats so the orchestrator's inactivity deadline
    # is not tripped during a long download.
    stop = threading.Event()
    if on_heartbeat is None:
        return stop
    transport = "https" if xet_disabled else "xet"

    def _beat() -> None:
        while not stop.wait(interval):
            try:
                on_heartbeat(f"Downloading ({transport} transport)...")
            except Exception:
                pass

    threading.Thread(
        target = _beat,
        daemon = True,
        name = "hf-xet-degraded-heartbeat",
    ).start()
    return stop


def _degraded_cancelled(cancel_event: "Optional[threading.Event]") -> bool:
    return cancel_event is not None and cancel_event.is_set()


def _degraded_hf_hub_download_with_xet_fallback(
    repo_id: str,
    filename: str,
    token: Optional[str],
    *,
    repo_type: str = "model",
    revision: Optional[str] = None,
    cache_dir: Optional[str] = None,
    force_download: bool = False,
    cancel_event: "Optional[threading.Event]" = None,
    **_ignored: Any,
) -> str:
    # Keep the cancellation contract: do not start or return a download once cancelled.
    if _degraded_cancelled(cancel_event):
        raise RuntimeError("Cancelled")

    from huggingface_hub import hf_hub_download

    path = hf_hub_download(
        repo_id = repo_id,
        filename = filename,
        token = token,
        repo_type = repo_type,
        revision = revision,
        cache_dir = cache_dir,
        force_download = force_download,
    )
    if _degraded_cancelled(cancel_event):
        raise RuntimeError("Cancelled")
    return path


def _degraded_snapshot_download_with_xet_fallback(
    repo_id: str,
    *,
    revision: Optional[str] = None,
    token: Optional[str] = None,
    repo_type: str = "model",
    cache_dir: Optional[str] = None,
    allow_patterns: Optional[Any] = None,
    ignore_patterns: Optional[Any] = None,
    force_download: bool = False,
    cancel_event: "Optional[threading.Event]" = None,
    **_ignored: Any,
) -> str:
    if _degraded_cancelled(cancel_event):
        raise RuntimeError("Cancelled")

    from huggingface_hub import snapshot_download

    path = snapshot_download(
        repo_id = repo_id,
        repo_type = repo_type,
        revision = revision,
        token = token,
        cache_dir = cache_dir,
        allow_patterns = allow_patterns,
        ignore_patterns = ignore_patterns,
        force_download = force_download,
    )
    if _degraded_cancelled(cancel_event):
        raise RuntimeError("Cancelled")
    return path


# --- lazy attribute access for the heavy shared API -------------------------------------------
# ``DownloadStallError`` (class identity matters for ``except``), ``start_watchdog`` and
# ``get_hf_download_state`` come from the shared backend when available, else the degraded stubs.
# Resolved via PEP 562 ``__getattr__`` so ``from utils.hf_xet_fallback import X`` triggers the load
# only for these heavy names, not for ``child_should_disable_xet`` / ``DEFAULT_*``.
_DEGRADED_ATTRS = {
    "DownloadStallError": _DegradedDownloadStallError,
    "start_watchdog": _degraded_start_watchdog,
    "get_hf_download_state": _degraded_get_hf_download_state,
}

# Annotation-only declarations for the three names above: they bind NO value, so lookup still misses
# and PEP 562 ``__getattr__`` resolves them lazily -- but ruff/pyflakes see them as defined, so listing
# them in ``__all__`` does not trip F822 (while F822 still catches a real typo elsewhere in the list).
DownloadStallError: type
start_watchdog: Any
get_hf_download_state: Any


def __getattr__(name: str) -> Any:
    if name in _DEGRADED_ATTRS:
        if _load_shared():
            return getattr(_shared, name)
        return _DEGRADED_ATTRS[name]
    raise AttributeError(f"module {__name__!r} has no attribute {name!r}")


# Indirection seam the public wrappers call (and tests monkeypatch): lazy-load the shared backend,
# then dispatch to it or the degraded stub. The ``_shared_*`` names preserve the pre-refactor contract.
def _shared_hf_hub_download_with_xet_fallback(*args: Any, **kwargs: Any) -> str:
    impl = (
        _shared.hf_hub_download_with_xet_fallback
        if _load_shared()
        else _degraded_hf_hub_download_with_xet_fallback
    )
    return impl(*args, **kwargs)


def _shared_snapshot_download_with_xet_fallback(*args: Any, **kwargs: Any) -> str:
    impl = (
        _shared.snapshot_download_with_xet_fallback
        if _load_shared()
        else _degraded_snapshot_download_with_xet_fallback
    )
    return impl(*args, **kwargs)


__all__ = [
    "DEFAULT_GRACE_PERIOD",
    "DEFAULT_HEARTBEAT_INTERVAL",
    "DEFAULT_STALL_TIMEOUT",
    "DownloadStallError",
    "child_should_disable_xet",
    "get_hf_download_state",
    "start_watchdog",
    "hf_hub_download_with_xet_fallback",
    "snapshot_download_with_xet_fallback",
]


def _studio_prepare_for_http(repo_type: str, repo_id: str) -> None:
    """Unsloth's marker-aware purge before an HTTP resume, keeping the download manager's ``.transport``
    accounting consistent (vs unsloth_zoo's generic default). Guarded: a purge failure is logged,
    not fatal to the retry."""
    try:
        from hub.utils.download_registry import prepare_cache_for_transport
        prepare_cache_for_transport(repo_type, repo_id, "http")
    except Exception as exc:
        try:
            from loggers import get_logger
            get_logger(__name__).debug(
                "Unsloth prepare_cache_for_transport failed for %s: %s", repo_id, exc
            )
        except ModuleNotFoundError as logger_exc:
            if logger_exc.name != "loggers":
                raise


def hf_hub_download_with_xet_fallback(
    repo_id: str,
    filename: str,
    token: Optional[str],
    *,
    cancel_event: Optional[threading.Event] = None,
    repo_type: str = "model",
    revision: Optional[str] = None,
    stall_timeout: float = DEFAULT_STALL_TIMEOUT,
    interval: float = DEFAULT_HEARTBEAT_INTERVAL,
    grace_period: float = DEFAULT_GRACE_PERIOD,
    on_status: Optional[Callable[[str], None]] = None,
    force_download: bool = False,
) -> str:
    """Single-file download via the shared fallback with Unsloth's marker-aware HTTP-retry prep.
    ``force_download`` re-fetches a newer blob over a cached one (Unsloth's model-update path)."""
    return _shared_hf_hub_download_with_xet_fallback(
        repo_id,
        filename,
        token,
        cancel_event = cancel_event,
        repo_type = repo_type,
        revision = revision,
        stall_timeout = stall_timeout,
        interval = interval,
        grace_period = grace_period,
        on_status = on_status,
        force_download = force_download,
        prepare_for_http_fn = _studio_prepare_for_http,
    )


def snapshot_download_with_xet_fallback(repo_id: str, **kwargs: Any) -> str:
    """Whole-repo download via the shared fallback with Unsloth's marker-aware HTTP-retry prep."""
    kwargs.setdefault("prepare_for_http_fn", _studio_prepare_for_http)
    return _shared_snapshot_download_with_xet_fallback(repo_id, **kwargs)