Simplify check for symlinks that resolve outside root (#4221)

This PR does not change any behaviour.

There have been 1-2 issues about symlinks recently. Both over and under
resolving can cause problems. This makes a case where we resolve more
explicit and prevent a resolved path from leaking out via the return.

src/black/__init__.py

@@ -50,9 +50,9 @@
     gen_python_files,
     get_gitignore,
     get_root_relative_path,
-    normalize_path_maybe_ignore,
     parse_pyproject_toml,
     path_is_excluded,
+    resolves_outside_root_or_cannot_stat,
     wrap_stream_for_windows,
 )
 from black.handle_ipynb_magics import (
@@ -763,12 +763,9 @@ def get_sources(
                 )
                 continue
 
-            normalized_path: Optional[str] = normalize_path_maybe_ignore(
-                path, root, report
-            )
-            if normalized_path is None:
+            if resolves_outside_root_or_cannot_stat(path, root, report):
                 if verbose:
-                    out(f'Skipping invalid source: "{normalized_path}"', fg="red")
+                    out(f'Skipping invalid source: "{path}"', fg="red")
                 continue
 
             if is_stdin:
@@ -780,7 +777,7 @@ def get_sources(
                 continue
 
             if verbose:
-                out(f'Found input source: "{normalized_path}"', fg="blue")
+                out(f'Found input source: "{path}"', fg="blue")
             sources.add(path)
         elif path.is_dir():
             path = root / (path.resolve().relative_to(root))

src/black/files.py

@@ -254,26 +254,24 @@ def get_gitignore(root: Path) -> PathSpec:
         raise
 
 
-def normalize_path_maybe_ignore(
+def resolves_outside_root_or_cannot_stat(
     path: Path,
     root: Path,
     report: Optional[Report] = None,
-) -> Optional[str]:
-    """Normalize `path`. May return `None` if `path` was ignored.
-
-    `report` is where "path ignored" output goes.
+) -> bool:
+    """
+    Returns whether the path is a symbolic link that points outside the
+    root directory. Also returns True if we failed to resolve the path.
     """
     try:
-        abspath = path if path.is_absolute() else Path.cwd() / path
-        normalized_path = abspath.resolve()
-        root_relative_path = get_root_relative_path(normalized_path, root, report)
-
+        if sys.version_info < (3, 8, 6):
+            path = path.absolute()  # https://bugs.python.org/issue33660
+        resolved_path = path.resolve()
+        return get_root_relative_path(resolved_path, root, report) is None
     except OSError as e:
         if report:
             report.path_ignored(path, f"cannot be read because {e}")
-        return None
-
-    return root_relative_path
+        return True
 
 
 def get_root_relative_path(
@@ -369,8 +367,7 @@ def gen_python_files(
             report.path_ignored(child, "matches the --force-exclude regular expression")
             continue
 
-        normalized_path = normalize_path_maybe_ignore(child, root, report)
-        if normalized_path is None:
+        if resolves_outside_root_or_cannot_stat(child, root, report):
             continue
 
         if child.is_dir():

tests/test_black.py

@@ -1760,12 +1760,15 @@ def test_bpo_33660_workaround(self) -> None:
             return
 
         # https://bugs.python.org/issue33660
+        # Can be removed when we drop support for Python 3.8.5
         root = Path("/")
         with change_directory(root):
             path = Path("workspace") / "project"
             report = black.Report(verbose=True)
-            normalized_path = black.normalize_path_maybe_ignore(path, root, report)
-            self.assertEqual(normalized_path, "workspace/project")
+            resolves_outside = black.resolves_outside_root_or_cannot_stat(
+                path, root, report
+            )
+            self.assertIs(resolves_outside, False)
 
     def test_normalize_path_ignore_windows_junctions_outside_of_root(self) -> None:
         if system() != "Windows":
@@ -1778,13 +1781,13 @@ def test_normalize_path_ignore_windows_junctions_outside_of_root(self) -> None:
             os.system(f"mklink /J {junction_dir} {junction_target_outside_of_root}")
 
             report = black.Report(verbose=True)
-            normalized_path = black.normalize_path_maybe_ignore(
+            resolves_outside = black.resolves_outside_root_or_cannot_stat(
                 junction_dir, root, report
             )
             # Manually delete for Python < 3.8
             os.system(f"rmdir {junction_dir}")
 
-            self.assertEqual(normalized_path, None)
+            self.assertIs(resolves_outside, True)
 
     def test_newline_comment_interaction(self) -> None:
         source = "class A:\\\r\n# type: ignore\n pass\n"