system: use linkat instead of link

`os.link` is broken across platforms when dealing with symlinks.

See https://bugs.python.org/issue41355 for more info.

dvc/system.py

@@ -2,14 +2,42 @@ import errno
 import logging
 import os
 import platform
+import stat
+import sys
 
 logger = logging.getLogger(__name__)
 
 
+if os.name == "nt" and sys.version_info < (3, 8):
+    # NOTE: using backports for `os.path.realpath`
+    # See https://bugs.python.org/issue9949 for more info.
+    # pylint: disable=import-error, no-name-in-module
+    from jaraco.windows.filesystem.backports import realpath as _realpath
+
+    def realpath(path):
+        return _realpath(os.fspath(path))
+
+
+else:
+    realpath = os.path.realpath
+
+
 class System:
     @staticmethod
     def hardlink(source, link_name):
-        os.link(source, link_name)
+        # NOTE: we should really be using `os.link()` here with
+        # `follow_symlinks=True`, but unfortunately the implementation is
+        # buggy across platforms, so until it is fixed, we just dereference
+        # the symlink ourselves here.
+        #
+        # See https://bugs.python.org/issue41355 for more info.
+        st = os.lstat(source)
+        if stat.S_ISLNK(st.st_mode):
+            src = realpath(source)
+        else:
+            src = source
+
+        os.link(src, link_name)
 
     @staticmethod
     def symlink(source, link_name):

setup.cfg

@@ -75,6 +75,7 @@ install_requires =
     fsspec[http]>=2021.10.1
     aiohttp-retry>=2.4.5
     diskcache>=5.2.1
+    jaraco.windows>=5.7.0; python_version < '3.8' and sys_platform == 'win32'
 
 [options.extras_require]
 # gssapi should not be included in all_remotes, because it doesn't have wheels
@@ -139,7 +140,6 @@ tests =
     Pygments==2.10.0
     collective.checkdocs==0.2
     pydocstyle==6.1.1
-    jaraco.windows==5.7.0
     # pylint requirements
     pylint==2.11.1
     # we use this to suppress pytest-related false positives in our tests.