gh-140774: Fix clearing the read-only file attribute on Windows

Lib/test/test_os/test_windows.py

@@ -454,6 +454,59 @@ def test_unlink_removes_junction(self):
         self.assertFalse(os.path.exists(self.junction))
 
 
+class Win32ChmodTests(unittest.TestCase):
+    # gh-140774: os.chmod(follow_symlinks=True) and os.fchmod() go through
+    # SetFileInformationByHandle(), which interprets FileAttributes == 0 as
+    # "leave the attributes unchanged".  Clearing the read-only flag of a
+    # file with no other attribute set must substitute FILE_ATTRIBUTE_NORMAL
+    # for 0 to take effect.
+
+    def setUp(self):
+        self.fname = os.path.abspath(os_helper.TESTFN)
+        self.addCleanup(os_helper.unlink, self.fname)
+        self.addCleanup(self._make_writable)
+        create_file(self.fname)
+
+    def _make_writable(self):
+        try:
+            os.chmod(self.fname, stat.S_IWRITE | stat.S_IREAD,
+                     follow_symlinks=False)
+        except OSError:
+            pass
+
+    def _set_readonly_only_attribute(self):
+        # Make FILE_ATTRIBUTE_READONLY the only attribute of the file
+        # (in particular, clear FILE_ATTRIBUTE_ARCHIVE).
+        ctypes = import_helper.import_module('ctypes')
+        kernel32 = ctypes.WinDLL('Kernel32.dll', use_last_error=True)
+        if not kernel32.SetFileAttributesW(self.fname,
+                                           stat.FILE_ATTRIBUTE_READONLY):
+            raise ctypes.WinError(ctypes.get_last_error())
+        attributes = os.stat(self.fname).st_file_attributes
+        if attributes != stat.FILE_ATTRIBUTE_READONLY:
+            self.skipTest('file system reports extra file attributes '
+                          f'{attributes:#x}')
+
+    def _assert_writable(self):
+        self.assertEqual(os.stat(self.fname).st_file_attributes
+                         & stat.FILE_ATTRIBUTE_READONLY, 0)
+
+    def test_chmod_clears_readonly_without_other_attributes(self):
+        self._set_readonly_only_attribute()
+        os.chmod(self.fname, stat.S_IWRITE | stat.S_IREAD,
+                 follow_symlinks=True)
+        self._assert_writable()
+
+    def test_fchmod_clears_readonly_without_other_attributes(self):
+        fd = os.open(self.fname, os.O_RDWR)
+        try:
+            self._set_readonly_only_attribute()
+            os.fchmod(fd, stat.S_IWRITE | stat.S_IREAD)
+        finally:
+            os.close(fd)
+        self._assert_writable()
+
+
 class Win32NtTests(unittest.TestCase):
     def test_getfinalpathname_handles(self):
         nt = import_helper.import_module('nt')

Misc/NEWS.d/next/Windows/2026-07-29-13-05-00.gh-issue-140774.wA3ttr.rst

@@ -0,0 +1,3 @@
+Fix :func:`os.chmod` with ``follow_symlinks=True``, :func:`os.fchmod` and
+:meth:`pathlib.Path.chmod` on Windows: clearing the read-only attribute of a
+file which has no other attribute set is no longer silently ignored.

Modules/posixmodule.c

@@ -4021,6 +4021,12 @@ win32_hchmod(HANDLE hfile, int mode)
     }
     if (mode & _S_IWRITE) {
         info.FileAttributes &= ~FILE_ATTRIBUTE_READONLY;
+        if (info.FileAttributes == 0) {
+            /* SetFileInformationByHandle() interprets FileAttributes == 0
+               as "leave the attributes unchanged"; FILE_ATTRIBUTE_NORMAL
+               is the documented way to clear all attributes. */
+            info.FileAttributes = FILE_ATTRIBUTE_NORMAL;
+        }
     }
     else {
         info.FileAttributes |= FILE_ATTRIBUTE_READONLY;