fs: test_links: create destination directory first

[gh2o]

dvc.fs.utils.test_links() fails with a CacheLinkError if the target to_path does not already exist, as is the case when "abc.dvc" exists (referring to a directory object) and we try to run "dvc pull abc/def". To avoid this issue, fully create to_path before attempting to create a link in it.

Example error log which this should fix:

2022-01-08 00:58:31,286 ERROR: failed to pull data from the cloud - No possible cache link types for '/<REPO>/<OBJECT>/<SUBDIR>'. See <https://dvc.org/doc/user-guide/troubleshooting#cache-types> for more information.
------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/dvc/command/data_sync.py", line 30, in run
    stats = self.repo.pull(
  File "/usr/local/lib/python3.8/dist-packages/dvc/repo/__init__.py", line 49, in wrapper
    return f(repo, *args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/dvc/repo/pull.py", line 40, in pull
    stats = self.checkout(
  File "/usr/local/lib/python3.8/dist-packages/dvc/repo/__init__.py", line 49, in wrapper
    return f(repo, *args, **kwargs)
  File "/usr/local/lib/python3.8/dist-packages/dvc/repo/checkout.py", line 98, in checkout
    result = stage.checkout(
  File "/usr/local/lib/python3.8/dist-packages/funcy/decorators.py", line 45, in wrapper
    return deco(call, *dargs, **dkwargs)
  File "/usr/local/lib/python3.8/dist-packages/dvc/stage/decorators.py", line 36, in rwlocked
    return call()
  File "/usr/local/lib/python3.8/dist-packages/funcy/decorators.py", line 66, in __call__
    return self._func(*self._args, **self._kwargs)
  File "/usr/local/lib/python3.8/dist-packages/dvc/stage/__init__.py", line 572, in checkout
    key, outs = self._checkout(
  File "/usr/local/lib/python3.8/dist-packages/dvc/stage/__init__.py", line 584, in _checkout
    result = out.checkout(**kwargs)
  File "/usr/local/lib/python3.8/dist-packages/dvc/output.py", line 756, in checkout
    modified = checkout(
  File "/usr/local/lib/python3.8/dist-packages/dvc/objects/checkout.py", line 241, in checkout
    _checkout(
  File "/usr/local/lib/python3.8/dist-packages/dvc/objects/checkout.py", line 164, in _checkout
    raise CacheLinkError([fs_path])
dvc.exceptions.CacheLinkError: No possible cache link types for '/<REPO>/<OBJECT>/<SUBDIR>'. See <https://dvc.org/doc/user-guide/troubleshooting#cache-types> for more information.

[efiop]

Thanks, @gh2o 🙏

[sjawhar]

Is a hotfix release on the schedule soon? This bugfix is what I need to be able to upgrade to 2.9.x (was actually about to make the fix myself 😄 )

[efiop]

@sjawhar We have a few release blockers that we need to sort out first https://github.com/iterative/dvc/issues?q=is%3Aopen+is%3Aissue+label%3Arelease-blocker Thank you for your patience 🙏