GH-89727: Fix FD leak on os.fwalk() generator finalization. (#119766)

Follow-up to 3c890b50. Ensure we `os.close()` open file descriptors when
the `os.fwalk()` generator is finalized.

Lib/test/test_os.py

@@ -1685,6 +1685,27 @@ class FwalkTests(WalkTests):
         self.addCleanup(os.close, newfd)
         self.assertEqual(newfd, minfd)
 
+    @unittest.skipIf(
+        support.is_emscripten, "Cannot dup stdout on Emscripten"
+    )
+    @unittest.skipIf(
+        support.is_android, "dup return value is unpredictable on Android"
+    )
+    def test_fd_finalization(self):
+        # Check that close()ing the fwalk() generator closes FDs
+        def getfd():
+            fd = os.dup(1)
+            os.close(fd)
+            return fd
+        for topdown in (False, True):
+            old_fd = getfd()
+            it = self.fwalk(os_helper.TESTFN, topdown=topdown)
+            self.assertEqual(getfd(), old_fd)
+            next(it)
+            self.assertGreater(getfd(), old_fd)
+            it.close()
+            self.assertEqual(getfd(), old_fd)
+
     # fwalk() keeps file descriptors open
     test_walk_many_open_files = None