Avoid crashing when the user has no homedir (#2814)

CHANGES.md

@@ -20,6 +20,7 @@
 
 - Do not format `__pypackages__` directories by default (#2836)
 - Add support for specifying stable version with `--required-version` (#2832).
+- Avoid crashing when the user has no homedir (#2814)
 
 ### Documentation
 

src/black/files.py

@@ -87,7 +87,7 @@ def find_pyproject_toml(path_search_start: Tuple[str, ...]) -> Optional[str]:
             if path_user_pyproject_toml.is_file()
             else None
         )
-    except PermissionError as e:
+    except (PermissionError, RuntimeError) as e:
         # We do not have access to the user-level config directory, so ignore it.
         err(f"Ignoring user configuration directory due to {e!r}")
         return None
@@ -111,6 +111,10 @@ def find_user_pyproject_toml() -> Path:
 
     This looks for ~\.black on Windows and ~/.config/black on Linux and other
     Unix systems.
+
+    May raise:
+    - RuntimeError: if the current user has no homedir
+    - PermissionError: if the current process cannot access the user's homedir
     """
     if sys.platform == "win32":
         # Windows

tests/test_black.py

@@ -10,7 +10,7 @@
 import types
 import unittest
 from concurrent.futures import ThreadPoolExecutor
-from contextlib import contextmanager
+from contextlib import contextmanager, redirect_stderr
 from dataclasses import replace
 from io import BytesIO
 from pathlib import Path
@@ -1358,6 +1358,21 @@ def test_find_project_root(self) -> None:
                 (src_dir.resolve(), "pyproject.toml"),
             )
 
+    @patch(
+        "black.files.find_user_pyproject_toml",
+    )
+    def test_find_pyproject_toml(self, find_user_pyproject_toml: MagicMock) -> None:
+        find_user_pyproject_toml.side_effect = RuntimeError()
+
+        with redirect_stderr(io.StringIO()) as stderr:
+            result = black.files.find_pyproject_toml(
+                path_search_start=(str(Path.cwd().root),)
+            )
+
+        assert result is None
+        err = stderr.getvalue()
+        assert "Ignoring user configuration" in err
+
     @patch(
         "black.files.find_user_pyproject_toml",
         black.files.find_user_pyproject_toml.__wrapped__,