GH-113528: Deoptimise `pathlib._abc.PurePathBase.relative_to()` (again) (#113882)

Restore full battle-tested implementations of `PurePath.[is_]relative_to()`. These were recently split up in 3375dfe and a15a773.

In `PurePathBase`, add entirely new implementations based on `_stack`, which itself calls `pathmod.split()` repeatedly to disassemble a path. These new implementations preserve features like trailing slashes where possible, while still observing that a `..` segment cannot be added to traverse an empty or `.` segment in *walk_up* mode. They do not rely on `parents` nor `__eq__()`, nor do they spin up temporary path objects.

Unfortunately calling `pathmod.relpath()` isn't an option, as it calls `abspath()` and in turn `os.getcwd()`, which is impure.

2 files changed, 42 insertions, 15 deletions

diff --git a/Lib/pathlib/__init__.py b/Lib/pathlib/__init__.py
index 26e14b3..ccdd9c3 100644
--- a/Lib/pathlib/__init__.py
+++ b/Lib/pathlib/__init__.py
@@ -11,6 +11,7 @@ import os
 import posixpath
 import sys
 import warnings
+from itertools import chain
 from _collections_abc import Sequence
 
 try:
@@ -254,10 +255,19 @@ class PurePath(_abc.PurePathBase):
                    "scheduled for removal in Python 3.14")
             warnings.warn(msg, DeprecationWarning, stacklevel=2)
             other = self.with_segments(other, *_deprecated)
-        path = _abc.PurePathBase.relative_to(self, other, walk_up=walk_up)
-        path._drv = path._root = ''
-        path._tail_cached = path._raw_paths.copy()
-        return path
+        elif not isinstance(other, PurePath):
+            other = self.with_segments(other)
+        for step, path in enumerate(chain([other], other.parents)):
+            if path == self or path in self.parents:
+                break
+            elif not walk_up:
+                raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")
+            elif path.name == '..':
+                raise ValueError(f"'..' segment in {str(other)!r} cannot be walked")
+        else:
+            raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
+        parts = ['..'] * step + self._tail[len(path._tail):]
+        return self._from_parsed_parts('', '', parts)
 
     def is_relative_to(self, other, /, *_deprecated):
         """Return True if the path is relative to another path or False.
@@ -268,7 +278,9 @@ class PurePath(_abc.PurePathBase):
                    "scheduled for removal in Python 3.14")
             warnings.warn(msg, DeprecationWarning, stacklevel=2)
             other = self.with_segments(other, *_deprecated)
-        return _abc.PurePathBase.is_relative_to(self, other)
+        elif not isinstance(other, PurePath):
+            other = self.with_segments(other)
+        return other == self or other in self.parents
 
     def as_uri(self):
         """Return the path as a URI."""
diff --git a/Lib/pathlib/_abc.py b/Lib/pathlib/_abc.py
index c16beca..5caad3c 100644
--- a/Lib/pathlib/_abc.py
+++ b/Lib/pathlib/_abc.py
@@ -3,7 +3,6 @@ import ntpath
 import posixpath
 import sys
 from errno import ENOENT, ENOTDIR, EBADF, ELOOP, EINVAL
-from itertools import chain
 from stat import S_ISDIR, S_ISLNK, S_ISREG, S_ISSOCK, S_ISBLK, S_ISCHR, S_ISFIFO
 
 #
@@ -358,24 +357,40 @@ class PurePathBase:
         """
         if not isinstance(other, PurePathBase):
             other = self.with_segments(other)
-        for step, path in enumerate(chain([other], other.parents)):
-            if path == self or path in self.parents:
-                break
+        anchor0, parts0 = self._stack
+        anchor1, parts1 = other._stack
+        if anchor0 != anchor1:
+            raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
+        while parts0 and parts1 and parts0[-1] == parts1[-1]:
+            parts0.pop()
+            parts1.pop()
+        for part in parts1:
+            if not part or part == '.':
+                pass
             elif not walk_up:
                 raise ValueError(f"{str(self)!r} is not in the subpath of {str(other)!r}")
-            elif path.name == '..':
+            elif part == '..':
                 raise ValueError(f"'..' segment in {str(other)!r} cannot be walked")
-        else:
-            raise ValueError(f"{str(self)!r} and {str(other)!r} have different anchors")
-        parts = ['..'] * step + self._tail[len(path._tail):]
-        return self.with_segments(*parts)
+            else:
+                parts0.append('..')
+        return self.with_segments('', *reversed(parts0))
 
     def is_relative_to(self, other):
         """Return True if the path is relative to another path or False.
         """
         if not isinstance(other, PurePathBase):
             other = self.with_segments(other)
-        return other == self or other in self.parents
+        anchor0, parts0 = self._stack
+        anchor1, parts1 = other._stack
+        if anchor0 != anchor1:
+            return False
+        while parts0 and parts1 and parts0[-1] == parts1[-1]:
+            parts0.pop()
+            parts1.pop()
+        for part in parts1:
+            if part and part != '.':
+                return False
+        return True
 
     @property
     def parts(self):