handle_system_exit() flushs files to warranty the output order

PyObject_Print() writes into the C object stderr, whereas PySys_WriteStderr()
writes into the Python object sys.stderr. Each object has its own buffer, so
call sys.stderr.flush() and fflush(stderr).

Lib/test/test_sys.py

@@ -86,6 +86,8 @@ class SysModuleTest(unittest.TestCase):
     # Python/pythonrun.c::PyErr_PrintEx() is tricky.
 
     def test_exit(self):
+        import subprocess
+
         self.assertRaises(TypeError, sys.exit, 42, 42)
 
         # call without argument
@@ -140,20 +142,28 @@ class SysModuleTest(unittest.TestCase):
             self.fail("no exception")
 
         # test that the exit machinery handles SystemExits properly
-        import subprocess
         rc = subprocess.call([sys.executable, "-c",
                               "raise SystemExit(47)"])
         self.assertEqual(rc, 47)
 
+        def check_exit_message(code, expected):
+            process = subprocess.Popen([sys.executable, "-c", code],
+                                       stderr=subprocess.PIPE)
+            stdout, stderr = process.communicate()
+            self.assertEqual(process.returncode, 1)
+            self.assertTrue(stderr.startswith(expected), stderr)
+
+        # test that stderr buffer if flushed before the exit message is written
+        # into stderr
+        check_exit_message(
+            r'import sys; sys.stderr.write("unflushed,"); sys.exit("message")',
+            b"unflushed,message")
+
         # test that the exit message is written with backslashreplace error
         # handler to stderr
-        import subprocess
-        code = r'import sys; sys.exit("surrogates:\uDCFF")'
-        process = subprocess.Popen([sys.executable, "-c", code],
-                                   stderr=subprocess.PIPE)
-        stdout, stderr = process.communicate()
-        self.assertEqual(process.returncode, 1)
-        self.assertTrue(stderr.startswith(b"surrogates:\\udcff"), stderr)
+        check_exit_message(
+            r'import sys; sys.exit("surrogates:\uDCFF")',
+            b"surrogates:\\udcff")
 
     def test_getdefaultencoding(self):
         self.assertRaises(TypeError, sys.getdefaultencoding, 42)