bpo-46426: Improve tests for the dir_fd argument (pythonGH-30668)

Ensure that directory file descriptors refer to directories different
from the current directory, and that src_dir_fd and dst_dir_fd refer
to different directories.

Add context manager open_dir_fd() in test.support.os_helper.

Author: serhiy-storchaka

Lib/test/support/os_helper.py

@@ -455,6 +455,17 @@ def create_empty_file(filename):
     os.close(fd)
 
 
+@contextlib.contextmanager
+def open_dir_fd(path):
+    """Open a file descriptor to a directory."""
+    assert os.path.isdir(path)
+    dir_fd = os.open(path, os.O_RDONLY)
+    try:
+        yield dir_fd
+    finally:
+        os.close(dir_fd)
+
+
 def fs_is_case_insensitive(directory):
     """Detects if the file system for the specified directory
     is case-insensitive."""

Lib/test/test_os.py

@@ -848,12 +848,9 @@ def set_time(filename, ns):
     def test_utime_dir_fd(self):
         def set_time(filename, ns):
             dirname, name = os.path.split(filename)
-            dirfd = os.open(dirname, os.O_RDONLY)
-            try:
+            with os_helper.open_dir_fd(dirname) as dirfd:
                 # pass dir_fd to test utimensat(timespec) or futimesat(timeval)
                 os.utime(name, dir_fd=dirfd, ns=ns)
-            finally:
-                os.close(dirfd)
         self._test_utime(set_time)
 
     def test_utime_directory(self):
@@ -4341,8 +4338,7 @@ def test_fd(self):
             os.symlink('file.txt', os.path.join(self.path, 'link'))
             expected_names.append('link')
 
-        fd = os.open(self.path, os.O_RDONLY)
-        try:
+        with os_helper.open_dir_fd(self.path) as fd:
             with os.scandir(fd) as it:
                 entries = list(it)
             names = [entry.name for entry in entries]
@@ -4357,8 +4353,6 @@ def test_fd(self):
                     self.assertEqual(entry.stat(), st)
                     st = os.stat(entry.name, dir_fd=fd, follow_symlinks=False)
                     self.assertEqual(entry.stat(follow_symlinks=False), st)
-        finally:
-            os.close(fd)
 
     def test_empty_path(self):
         self.assertRaises(FileNotFoundError, os.scandir, '')