gh-159630: Fix os.truncate() not raising OSError on Windows when truncation fails

Lib/test/test_os/test_os.py

@@ -201,6 +201,12 @@ def test_access(self):
         os.close(f)
         self.assertTrue(os.access(os_helper.TESTFN, os.W_OK))
 
+    @unittest.skipUnless(hasattr(os, 'truncate'), 'test needs os.truncate()')
+    def test_truncate_negative_length(self):
+        create_file(os_helper.TESTFN, b'abc')
+        with self.assertRaises(OSError):
+            os.truncate(os_helper.TESTFN, -1)
+
     @unittest.skipIf(
         support.is_wasi, "WASI does not support dup."
     )

Misc/NEWS.d/next/Library/2026-10-18-02-26-57.gh-issue-159630.lvoH2i.rst

@@ -0,0 +1,2 @@
+Fix :func:`os.truncate` not raising :exc:`OSError` on Windows when
+truncation fails.

Modules/posixmodule.c

@@ -13650,8 +13650,10 @@ os_truncate_impl(PyObject *module, path_t *path, Py_off_t length)
     else {
         result = _chsize_s(fd, length);
         close(fd);
-        if (result < 0)
+        if (result != 0) {
             errno = result;
+            result = -1;
+        }
     }
 #else
     result = truncate(path->narrow, length);