gh-89708: use fds when possible in contextlib.chdir

Lib/contextlib.py

@@ -805,10 +805,38 @@ class chdir(AbstractContextManager):
     def __init__(self, path):
         self.path = path
         self._old_cwd = []
+        if self._supports_fd():
+            self._getcwd = self._getcwd_fd
+            self._close = os.close
+
+    @staticmethod
+    def _supports_fd():
+        return os.chdir in os.supports_fd
+
+    @staticmethod
+    def _getcwd():
+        return os.getcwd()
+
+    @staticmethod
+    def _close(dir):
+        pass
+
+    @staticmethod
+    def _getcwd_fd():
+        return os.open('.', os.O_RDONLY)
 
     def __enter__(self):
-        self._old_cwd.append(os.getcwd())
-        os.chdir(self.path)
+        dir = self._getcwd()
+        try:
+            os.chdir(self.path)
+        except:
+            self._close(dir)
+            raise
+        self._old_cwd.append(dir)
 
     def __exit__(self, *excinfo):
-        os.chdir(self._old_cwd.pop())
+        dir = self._old_cwd.pop()
+        try:
+            os.chdir(dir)
+        finally:
+            self._close(dir)

Lib/test/test_contextlib.py

@@ -1361,6 +1361,63 @@ def test_exception(self):
             self.assertEqual(str(re), "boom")
         self.assertEqual(os.getcwd(), old_cwd)
 
+    def test_failed_chdir(self):
+        old_cwd = os.getcwd()
+        target = self.make_relative_path('non_existent_directory')
+        self.assertNotEqual(old_cwd, target)
+        ctx = chdir(target)
+        with self.assertRaises(OSError):
+            with ctx:
+                self.fail("chdir should have raised an exception")
+        self.assertFalse(ctx._old_cwd)
+        self.assertEqual(os.getcwd(), old_cwd)
+
+    @unittest.skipUnless(chdir._supports_fd(), "chdir requires fd support")
+    def test_chdir_to_fd(self):
+        old_cwd = os.getcwd()
+        target = self.make_relative_path('data')
+        fd = os.open(target, os.O_RDONLY)
+        try:
+            with chdir(fd):
+                self.assertEqual(os.getcwd(), target)
+        finally:
+            os.close(fd)
+        self.assertEqual(os.getcwd(), old_cwd)
+
+    @unittest.skipUnless(chdir._supports_fd(),
+                         "chdir requires fd support for long path")
+    @unittest.skipUnless(os_helper.can_symlink(), "need symlink support")
+    def test_original_path_too_long(self):
+        with tempfile.TemporaryDirectory() as dir:
+            with chdir(dir):
+                count = 0
+                big_name = 'a' * os.pathconf('.', 'PC_NAME_MAX')
+                path_max = os.pathconf('.', 'PC_PATH_MAX')
+                while len(dir) < path_max + 1:
+                    dir = os.path.join(dir, big_name)
+                    os.mkdir(big_name)
+                    os.symlink(big_name, 'a', target_is_directory=True)
+                    os.chdir('a')
+                    count += 1
+
+                self.assertTrue(count > 0)
+                os.mkdir('a')
+                with chdir('a'):
+                    self.assertTrue(len(os.getcwd()) > path_max)
+
+    @unittest.skipUnless(chdir._supports_fd(),
+                         "chdir requires fd support for deleted dir")
+    def test_original_path_deleted(self):
+        original = os.getcwd()
+        dir = tempfile.TemporaryDirectory()
+        try:
+            os.chdir(dir.name)
+            with chdir(original):
+                dir.cleanup()
+        finally:
+            os.chdir(original)
+            dir.cleanup()
+
 
 if __name__ == "__main__":
     unittest.main()

Misc/NEWS.d/next/Library/2025-04-26-12-10-32.gh-issue-89708.haEUh3.rst

@@ -0,0 +1,3 @@
+Use fds in contextlib.chdir where supported. This fixes bugs if the original
+working directory is greater than PATH_MAX and/or is deleted before the
+context manager exits.