Fix compile on -r relative paths outside of cwd

When `-r` is used to pass a path, we are normalizing with
`pathlib.Path.relative_to`. This fails when the input is not a subpath
of the current working directory.

In Python 3.12+ pathlib supports this usage (`walk_up=True`), but on
older Pythons we need to fallback to using `os.path.relpath`.

In order to support this usage and give a clear indication of how we
should upgrade when older versions are dropped, a small compat wrapper
is here defined, `_relative_to_walk_up` which uses
`relative_to(..., walk_up=True)` when it is available, and only uses
`os.path.relpath` when necessary. After we drop Python 3.11 in a few
years, the helper can be removed.

A new regression test reproduces #2231 and is fixed with this patch.
As part of writing that test, I wanted to leverage the
`TestFilesCollection` helper, so this was extended in a small way to
support usage without an explicit name.

Author: sirosen

changelog.d/2231.bugfix.md

@@ -0,0 +1,2 @@
+Fix `pip-compile` handling of relative path includes which are not subpaths of
+the current working directory.

piptools/_compat/pip_compat.py

@@ -1,7 +1,9 @@
 from __future__ import annotations
 
 import optparse
+import os.path
 import pathlib
+import sys
 import urllib.parse
 from dataclasses import dataclass
 from typing import TYPE_CHECKING, Callable, Iterable, Iterator, Set, cast
@@ -152,7 +154,7 @@ def _relativize_comes_from_location(original_comes_from: str, /) -> str:
         return f"{prefix} {file_path.as_posix()}"
 
     # make it relative to the current working dir
-    suffix = file_path.relative_to(pathlib.Path.cwd()).as_posix()
+    suffix = _relative_to_walk_up(file_path, pathlib.Path.cwd()).as_posix()
     return f"{prefix}{space_sep}{suffix}"
 
 
@@ -210,3 +212,18 @@ def get_dev_pkgs() -> set[str]:
     from pip._internal.commands.freeze import _dev_pkgs
 
     return cast(Set[str], _dev_pkgs())
+
+
+def _relative_to_walk_up(path: pathlib.Path, start: pathlib.Path) -> pathlib.Path:
+    """
+    Compute a relative path allowing for the input to not be a subpath of the start.
+
+    This is a compatibility helper for ``pathlib.Path.relative_to(..., walk_up=True)``
+    on all Python versions. (``walk_up: bool`` is Python 3.12+)
+    """
+    # prefer `pathlib.Path.relative_to` where available
+    if sys.version_info >= (3, 12):
+        return path.relative_to(start, walk_up=True)
+
+    str_result = os.path.relpath(path, start=start)
+    return pathlib.Path(str_result)

tests/test_cli_compile.py

@@ -65,9 +65,11 @@ class TestFilesCollection:
     """
 
     # the name for the collection of files
-    name: str
+    name: str = "<unnamed test file collection>"
     # static or computed contents
-    contents: dict[str, str | typing.Callable[[pathlib.Path], str]]
+    contents: dict[str, str | typing.Callable[[pathlib.Path], str]] = dataclasses.field(
+        default_factory=dict
+    )
 
     def __str__(self) -> str:
         return self.name
@@ -4047,6 +4049,50 @@ def test_second_order_requirements_relative_path_in_separate_dir(
     )
 
 
+def test_second_order_requirements_can_be_in_parent_of_cwd(
+    pip_conf,
+    runner,
+    tmp_path,
+    monkeypatch,
+    pip_produces_absolute_paths,
+):
+    """
+    Test handling of ``-r`` includes when the included requirements file is in the
+    parent of the current working directory.
+    """
+    test_files_collection = TestFilesCollection(
+        contents={
+            "subdir1/requirements.in": "-r ../requirements2.in\n",
+            "requirements2.in": "small-fake-a\n",
+        }
+    )
+    test_files_collection.populate(tmp_path)
+
+    with monkeypatch.context() as revertable_ctx:
+        # cd into the subdir where the initial requirements are
+        revertable_ctx.chdir(tmp_path / "subdir1")
+        out = runner.invoke(
+            cli,
+            [
+                "--output-file",
+                "-",
+                "--quiet",
+                "--no-header",
+                "--no-emit-options",
+                "-r",
+                "requirements.in",
+            ],
+        )
+
+    assert out.exit_code == 0
+    assert out.stdout == dedent(
+        """\
+        small-fake-a==0.2
+            # via -r ../requirements2.in
+        """
+    )
+
+
 @pytest.mark.parametrize(
     "input_path_absolute", (True, False), ids=("absolute-input", "relative-input")
 )