Ignore PermissionError when checking cwd during import

On macOS `getcwd(3)` can return EACCES if a path component isn't readable,
resulting in PermissionError. `PathFinder.find_spec()` now catches these and
ignores them - the same treatment as a missing/deleted cwd.

Introduces 2 new `test.support.os_helper` context managers

- `save_mode(path, ...)` - restores the mode of a path on exit
- `save_cwd(...)` - restores the current directory on exit

Unlike `change_cwd(path)` (which always performs a `chdir(path)`),
`save_cwd()` allows the code inside the `with` block to control this. The new
context manager just restores the original working directory afterward.

This is allows finer control of exception handling and robust environment
restoration across platforms in `FinderTests.test_permission_error_cwd()`.

Doc/reference/import.rst

@@ -871,10 +871,10 @@ module.
 
 The current working directory -- denoted by an empty string -- is handled
 slightly differently from other entries on :data:`sys.path`. First, if the
-current working directory is found to not exist, no value is stored in
-:data:`sys.path_importer_cache`. Second, the value for the current working
-directory is looked up fresh for each module lookup. Third, the path used for
-:data:`sys.path_importer_cache` and returned by
+current working directory cannot be determined or is found to not exist, no
+value is stored in :data:`sys.path_importer_cache`. Second, the value for the
+current working directory is looked up fresh for each module lookup. Third,
+the path used for :data:`sys.path_importer_cache` and returned by
 :meth:`importlib.machinery.PathFinder.find_spec` will be the actual current
 working directory and not the empty string.
 

Lib/importlib/_bootstrap_external.py

@@ -1506,7 +1506,7 @@ def _path_importer_cache(cls, path):
         if path == '':
             try:
                 path = _os.getcwd()
-            except FileNotFoundError:
+            except (FileNotFoundError, PermissionError):
                 # Don't cache the failure as the cwd can easily change to
                 # a valid directory later on.
                 return None

Lib/test/support/os_helper.py

@@ -294,6 +294,33 @@ def skip_unless_working_chmod(test):
     return test if ok else unittest.skip(msg)(test)
 
 
+@contextlib.contextmanager
+def save_mode(path, *, quiet=False):
+    """Context manager that restores the mode (permissions) of path on exit.
+
+    Arguments:
+
+      path: Path of the file to restore mode of.
+
+      quiet: if False (the default), the context manager raises an exception
+        on error.  Otherwise, it issues only a warning and keeps the current
+        working directory the same.
+
+    """
+    saved_mode = os.stat(path)
+    try:
+        yield
+    finally:
+        try:
+            os.chmod(path, saved_mode.st_mode)
+        except OSError as exc:
+            if not quiet:
+                raise
+            warnings.warn(f'tests may fail, unable to restore the mode of '
+                          f'{path!r} to {saved_mode.st_mode}: {exc}',
+                          RuntimeWarning, stacklevel=3)
+
+
 # Check whether the current effective user has the capability to override
 # DAC (discretionary access control). Typically user root is able to
 # bypass file read, write, and execute permission checks. The capability
@@ -508,9 +535,36 @@ def temp_dir(path=None, quiet=False):
             rmtree(path)
 
 
+@contextlib.contextmanager
+def save_cwd(*, quiet=False):
+    """Return a context manager that restores the current working directory on
+    exit.
+
+    Arguments:
+
+      quiet: if False (the default), the context manager raises an exception
+        on error.  Otherwise, it issues only a warning and keeps the current
+        working directory the same.
+
+    """
+    saved_dir = os.getcwd()
+    try:
+        yield
+    finally:
+        try:
+            os.chdir(saved_dir)
+        except OSError as exc:
+            if not quiet:
+                raise
+            warnings.warn(f'tests may fail, unable to restore the current '
+                          f'working directory to {saved_dir!r}: {exc}',
+                          RuntimeWarning, stacklevel=3)
+
+
 @contextlib.contextmanager
 def change_cwd(path, quiet=False):
-    """Return a context manager that changes the current working directory.
+    """Return a context manager that changes the current working directory on
+    entry, and restores it on exit.
 
     Arguments:
 

Lib/test/test_importlib/import_/test_path.py

@@ -1,3 +1,4 @@
+from test.support import os_helper
 from test.test_importlib import util
 
 importlib = util.import_importlib('importlib')
@@ -153,6 +154,25 @@ def test_deleted_cwd(self):
             # Do not want FileNotFoundError raised.
             self.assertIsNone(self.machinery.PathFinder.find_spec('whatever'))
 
+    @os_helper.skip_unless_working_chmod
+    def test_permission_error_cwd(self):
+        # gh-115911
+        with (
+            os_helper.temp_dir() as new_dir,
+            os_helper.save_mode(new_dir),
+            os_helper.save_cwd(),
+            util.import_state(path=['']),
+        ):
+            os.chdir(new_dir)
+            try:
+                os.chmod(new_dir, 0o000)
+            except OSError:
+                self.skipTest("platform does not allow "
+                              "changing mode of the cwd")
+
+            # Do not want PermissionError raised.
+            self.assertIsNone(self.machinery.PathFinder.find_spec('whatever'))
+
     def test_invalidate_caches_finders(self):
         # Finders with an invalidate_caches() method have it called.
         class FakeFinder:

Misc/NEWS.d/next/Core and Builtins/2024-02-29-16-55-52.gh-issue-115911.Vnkue_.rst

@@ -0,0 +1,3 @@
+If the current working directory cannot be determined due to permissions,
+then import will no longer raise :exc:`PermissionError`. Patch by Alex
+Willmer.