make pull --run-cache work with local remote having relative path (#4670)

* Fixes #4637

* use PathInfo instead of Path

* catch ValueError with PathInfo.relative_to and fallback to dvc.utils.relpath

* cleanup

* add test

Co-authored-by: Peter Rowlands <[email protected]>

dvc/path_info.py

@@ -75,6 +75,17 @@ class PathInfo(pathlib.PurePath, _BasePath):
         n = len(other._cparts)
         return len(self._cparts) > n and self._cparts[:n] == other._cparts
 
+    def relative_to(self, other):  # pylint: disable=arguments-differ
+        # pathlib relative_to raises exception when one path is not a direct
+        # descendant of the other when os.path.relpath would return abspath.
+        # For DVC PathInfo we only need the relpath behavior.
+        # See: https://bugs.python.org/issue40358
+        try:
+            path = super().relative_to(other)
+        except ValueError:
+            path = relpath(self, other)
+        return self.__class__(path)
+
 
 class WindowsPathInfo(PathInfo, pathlib.PureWindowsPath):
     pass

tests/unit/test_path_info.py

@@ -1,4 +1,5 @@
 import copy
+import os
 import pathlib
 
 import pytest
@@ -102,3 +103,17 @@ def test_url_replace_path():
     assert u.params == new_u.params
     assert u.query == new_u.query
     assert u.fragment == new_u.fragment
+
+
+@pytest.mark.parametrize(
+    "p1, p2",
+    [
+        ("/foo/bar", "/baz"),
+        ("/baz", "/foo/bar"),
+        ("/foo/bar", "../baz"),
+        ("../baz", "/foo/bar"),
+    ],
+)
+def test_relative_to_different_subpath(p1, p2):
+    expected = PathInfo(os.path.relpath(p1, p2))
+    assert PathInfo(p1).relative_to(PathInfo(p2)) == expected