gh-99576: Fix cookiejar file that was not truncated for some classes (GH-99616)

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/test/test_http_cookiejar.py

@@ -397,6 +397,32 @@ class FileCookieJarTests(unittest.TestCase):
         finally:
             os_helper.unlink(filename)
 
+    @unittest.skipIf(mswindows, "windows file permissions are incompatible with file modes")
+    @os_helper.skip_unless_working_chmod
+    def test_cookie_files_are_truncated(self):
+        filename = os_helper.TESTFN
+        for cookiejar_class in (LWPCookieJar, MozillaCookieJar):
+            c = cookiejar_class(filename)
+
+            req = urllib.request.Request("http://www.acme.com/")
+            headers = ["Set-Cookie: pll_lang=en; Max-Age=31536000; path=/"]
+            res = FakeResponse(headers, "http://www.acme.com/")
+            c.extract_cookies(res, req)
+            self.assertEqual(len(c), 1)
+
+            try:
+                # Save the first version with contents:
+                c.save()
+                # Now, clear cookies and re-save:
+                c.clear()
+                c.save()
+                # Check that file was truncated:
+                c.load()
+            finally:
+                os_helper.unlink(filename)
+
+            self.assertEqual(len(c), 0)
+
     def test_bad_magic(self):
         # OSErrors (eg. file doesn't exist) are allowed to propagate
         filename = os_helper.TESTFN