fix: handle empty/windows paths correctly in base-path-aware path fixer

- the regular path fixer early-exits if the passed-in path is `""`
- if the regular path fixer doesn't succeed, the base-aware path fixer retries with some prefixes (one of them being `./`)
- the regular path fixer is retried with the passed-in path of `""`, so it does not early-exit
- however, the next line strips `./` so the path is _turned into_ `""`
- `os.path.relpath()` gets mad

services/path_fixer/__init__.py

@@ -1,6 +1,6 @@
 import logging
 import os.path
-from pathlib import PurePath
+from pathlib import PurePosixPath, PureWindowsPath
 from typing import Sequence
 
 import sentry_sdk
@@ -126,7 +126,19 @@ def __init__(self, original_path_fixer, base_path) -> None:
         # base_path argument is the file path after the "# path=" in the report containing report location, if provided.
         # to get the base path we use, strip the coverage report from the path to get the base path
         # e.g.: "path/to/coverage.xml" --> "path/to/"
-        self.base_path = [PurePath(base_path).parent] if base_path is not None else []
+
+        self.base_path = []
+
+        if base_path:
+            # We want to use a `PurePath`, but we have to handle both Windows
+            # and POSIX paths. The cleanest way to do that is:
+            # - start by assuming it's a Windows path
+            # - if it doesn't have a drive letter like C:\, convert to POSIX
+            pure_path = PureWindowsPath(base_path)
+            if not pure_path.drive:
+                pure_path = PurePosixPath(pure_path.as_posix())
+
+            self.base_path = [pure_path.parent]
 
     def _try_fix_path(self, path: str, bases_to_try: Sequence[str]) -> str | None:
         original_path_fixer_result = self.original_path_fixer(path)
@@ -153,6 +165,8 @@ def _try_fix_path(self, path: str, bases_to_try: Sequence[str]) -> str | None:
     def __call__(
         self, path: str, bases_to_try: Sequence[str] | None = None
     ) -> str | None:
+        if not path:
+            return None
         bases_to_try = bases_to_try or tuple()
         key = (path, bases_to_try)
 

services/path_fixer/tests/unit/test_path_fixer.py

@@ -1,3 +1,5 @@
+from pathlib import PurePosixPath, PureWindowsPath
+
 from shared.yaml import UserYaml
 
 from services.path_fixer import PathFixer, invert_pattern
@@ -128,6 +130,32 @@ def test_basepath_uses_own_result_if_main_is_none_multuple_base_paths(self):
             == "project/__init__.py"
         )
 
+    def test_basepath_does_not_resolve_empty_paths(self):
+        toc = ["project/__init__.py", "tests/__init__.py", "tests/test_project.py"]
+        pf = PathFixer.init_from_user_yaml({}, toc, [])
+        coverage_file = "/some/coverage.xml"
+        base_aware_pf = pf.get_relative_path_aware_pathfixer(coverage_file)
+
+        assert base_aware_pf("") is None
+
+    def test_basepath_with_win_and_posix_paths(self):
+        toc = ["project/__init__.py", "tests/__init__.py", "tests/test_project.py"]
+        pf = PathFixer.init_from_user_yaml({}, toc, [])
+
+        posix_coverage_file_path = "/posix_base_path/coverage.xml"
+        posix_base_aware_pf = pf.get_relative_path_aware_pathfixer(
+            posix_coverage_file_path
+        )
+        assert posix_base_aware_pf.base_path == [PurePosixPath("/posix_base_path")]
+
+        windows_coverage_file_path = "C:\\windows_base_path\\coverage.xml"
+        windows_base_aware_pf = pf.get_relative_path_aware_pathfixer(
+            windows_coverage_file_path
+        )
+        assert windows_base_aware_pf.base_path == [
+            PureWindowsPath("C:\\windows_base_path")
+        ]
+
 
 def test_ambiguous_paths():
     toc = [