Fix CEFS extraction for root paths by standardizing on None

Change get_extraction_path_from_symlink to return None instead of Path('.')
when extracting from root. This prevents the bug where temp_dir/Path('.')
equals temp_dir itself, causing rmtree to fail after the directory was renamed.

This standardizes the codebase on using None to indicate 'extract everything'
as previously discussed.

Author: mattgodbolt

bin/lib/cefs/paths.py

@@ -217,26 +217,26 @@ def parse_cefs_target(cefs_target: Path, cefs_image_dir: Path, mount_point: Path
     return cefs_image_path, is_already_consolidated
 
 
-def get_extraction_path_from_symlink(symlink_target: Path, mount_point: Path) -> Path:
+def get_extraction_path_from_symlink(symlink_target: Path, mount_point: Path) -> Path | None:
     """Determine what to extract from a CEFS image based on symlink target.
 
-    Returns the relative path after {mount_point}/XX/HASH/ or Path(".") if at root.
+    Returns the relative path after {mount_point}/XX/HASH/ or None if at root.
 
     Args:
         symlink_target: The symlink target path
         mount_point: CEFS mount point (e.g., /cefs)
 
     Examples (assuming mount_point=/cefs):
         /cefs/ab/abcd1234567890abcdef12/content → Path("content")
-        /cefs/ab/abcd1234567890abcdef12 → Path(".")
+        /cefs/ab/abcd1234567890abcdef12 → None
         /cefs/ab/abcd1234567890abcdef12/gcc-4.5 → Path("gcc-4.5")
         /cefs/ab/abcd1234567890abcdef12/libs/boost → Path("libs/boost")
     """
     parts = symlink_target.parts
     mount_parts = mount_point.parts
     # Need at least mount_point + XX + HASH to have any relative path
     if len(parts) <= len(mount_parts) + 2:
-        return Path(".")
+        return None
 
     relative_parts = parts[len(mount_parts) + 2 :]
     return Path(*relative_parts)

bin/test/cefs/paths_test.py

@@ -67,7 +67,7 @@ def test_parse_cefs_target_variations(tmp_path):
 def test_get_extraction_path_from_symlink():
     mount_point = Path("/cefs")
     test_cases = [
-        (Path("/cefs/ab/abcdef1234567890abcdef12"), Path(".")),
+        (Path("/cefs/ab/abcdef1234567890abcdef12"), None),
         (Path("/cefs/ab/abcdef1234567890abcdef12/content"), Path("content")),
         (Path("/cefs/ab/abcdef1234567890abcdef12/gcc-4.5"), Path("gcc-4.5")),
         (Path("/cefs/ab/abcdef1234567890abcdef12/libs/boost"), Path("libs/boost")),
@@ -201,7 +201,7 @@ def test_get_extraction_path_with_custom_mount():
     custom_mount = Path("/test/cefs")
 
     test_cases = [
-        (custom_mount / "ab" / "abcd1234", Path(".")),
+        (custom_mount / "ab" / "abcd1234", None),
         (custom_mount / "ab" / "abcd1234" / "content", Path("content")),
         (custom_mount / "ab" / "abcd1234" / "deep" / "path", Path("deep/path")),
     ]