Stop test_dataset_cache_safe leaking the Hub no-symlink switch into later tests (#12973)

test_pre_1_9_hub_stops_re_probing_unknown_cache_dirs and the other tests
that reach _disable_hf_symlinks_for_process left HF_HUB_DISABLE_SYMLINKS=1
and huggingface_hub.constants.HF_HUB_DISABLE_SYMLINKS=True set for the rest
of the xdist worker: monkeypatch.delenv on an unset variable records nothing
to undo, and that test never patched the constant. Any later test in the
same worker then cached regular files instead of symlinks, which is how
test_gguf_header_delta's blob-layout test failed on the a-k shard. Running
the two files in one process reproduces it every time.

The product code is right: switching the whole worker to regular files is
the intended Windows recovery. Restore the env var, the constant and the
capability map after every test in that file, and have the GGUF header delta
tests pin a symlink-capable Hub, since they build a symlinked cache
themselves.

studio/backend/tests/test_dataset_cache_safe.py

@@ -32,6 +32,36 @@ def _install_fake_datasets(monkeypatch, load_dataset):
     monkeypatch.setattr("loggers.config.quiet_third_party_progress_bars", lambda: None)
 
 
+_UNSET = object()
+
+
+@pytest.fixture(autouse = True)
+def _restore_process_symlink_state():
+    """_disable_hf_symlinks_for_process changes the env var and Hub's live flag for the whole worker.
+
+    A test that calls it without patching both first left HF_HUB_DISABLE_SYMLINKS=1 and
+    constants.HF_HUB_DISABLE_SYMLINKS=True behind, so every later test in that xdist worker cached
+    regular files instead of symlinks (test_gguf_header_delta's blob layout failed on CI that way).
+    `monkeypatch.delenv(..., raising = False)` on an unset variable records nothing to undo, so
+    restore all three pieces here whatever the test did.
+    """
+    from huggingface_hub import constants, file_download
+
+    env = os.environ.get("HF_HUB_DISABLE_SYMLINKS", _UNSET)
+    flag = vars(constants).get("HF_HUB_DISABLE_SYMLINKS", _UNSET)
+    support = file_download._are_symlinks_supported_in_dir
+    yield
+    if env is _UNSET:
+        os.environ.pop("HF_HUB_DISABLE_SYMLINKS", None)
+    else:
+        os.environ["HF_HUB_DISABLE_SYMLINKS"] = env
+    if flag is _UNSET:
+        vars(constants).pop("HF_HUB_DISABLE_SYMLINKS", None)
+    else:
+        constants.HF_HUB_DISABLE_SYMLINKS = flag
+    file_download._are_symlinks_supported_in_dir = support
+
+
 def _isolate_hub_symlink_state(monkeypatch):
     """Keep _disable_hf_symlinks_for_process from leaking into other tests."""
     from huggingface_hub import constants, file_download

studio/backend/tests/test_gguf_header_delta.py

@@ -106,6 +106,12 @@ def cache_with_old(
 @pytest.fixture(autouse = True)
 def _small_ranges(monkeypatch, tmp_path):
     monkeypatch.delenv(delta.DELTA_ENV, raising = False)
+    # The blob-layout tests build a symlinked cache themselves, so Hub must agree symlinks work here,
+    # whatever an earlier test in this worker did to its process-wide symlink switch.
+    from huggingface_hub import constants, file_download
+
+    monkeypatch.setattr(constants, "HF_HUB_DISABLE_SYMLINKS", False, raising = False)
+    monkeypatch.setattr(file_download, "_are_symlinks_supported_in_dir", {})
     monkeypatch.setattr(delta, "_FIRST_RANGE", 64)
     monkeypatch.setattr(delta, "_HEADER_SLACK", 64)
     memo = tmp_path / "refused.json"