Issue #23430: Stop socketserver from catching SystemExit etc from handlers

Also make handle_error() consistently output to stderr, and fix the
documentation.

Lib/test/test_socketserver.py

@@ -58,6 +58,7 @@ if HAVE_UNIX_SOCKETS:
 
 @contextlib.contextmanager
 def simple_subprocess(testcase):
+    """Tests that a custom child process is not waited on (Issue 1540386)"""
     pid = os.fork()
     if pid == 0:
         # Don't raise an exception; it would be caught by the test harness.
@@ -281,6 +282,97 @@ class SocketServerTest(unittest.TestCase):
                                        socketserver.StreamRequestHandler)
 
 
+class ErrorHandlerTest(unittest.TestCase):
+    """Test that the servers pass normal exceptions from the handler to
+    handle_error(), and that exiting exceptions like SystemExit and
+    KeyboardInterrupt are not passed."""
+
+    def tearDown(self):
+        test.support.unlink(test.support.TESTFN)
+
+    def test_sync_handled(self):
+        BaseErrorTestServer(ValueError)
+        self.check_result(handled=True)
+
+    def test_sync_not_handled(self):
+        with self.assertRaises(SystemExit):
+            BaseErrorTestServer(SystemExit)
+        self.check_result(handled=False)
+
+    @unittest.skipUnless(threading, 'Threading required for this test.')
+    def test_threading_handled(self):
+        ThreadingErrorTestServer(ValueError)
+        self.check_result(handled=True)
+
+    @unittest.skipUnless(threading, 'Threading required for this test.')
+    def test_threading_not_handled(self):
+        ThreadingErrorTestServer(SystemExit)
+        self.check_result(handled=False)
+
+    @requires_forking
+    def test_forking_handled(self):
+        ForkingErrorTestServer(ValueError)
+        self.check_result(handled=True)
+
+    @requires_forking
+    def test_forking_not_handled(self):
+        ForkingErrorTestServer(SystemExit)
+        self.check_result(handled=False)
+
+    def check_result(self, handled):
+        with open(test.support.TESTFN) as log:
+            expected = 'Handler called\n' + 'Error handled\n' * handled
+            self.assertEqual(log.read(), expected)
+
+
+class BaseErrorTestServer(socketserver.TCPServer):
+    def __init__(self, exception):
+        self.exception = exception
+        super().__init__((HOST, 0), BadHandler)
+        with socket.create_connection(self.server_address):
+            pass
+        try:
+            self.handle_request()
+        finally:
+            self.server_close()
+        self.wait_done()
+
+    def handle_error(self, request, client_address):
+        with open(test.support.TESTFN, 'a') as log:
+            log.write('Error handled\n')
+
+    def wait_done(self):
+        pass
+
+
+class BadHandler(socketserver.BaseRequestHandler):
+    def handle(self):
+        with open(test.support.TESTFN, 'a') as log:
+            log.write('Handler called\n')
+        raise self.server.exception('Test error')
+
+
+class ThreadingErrorTestServer(socketserver.ThreadingMixIn,
+        BaseErrorTestServer):
+    def __init__(self, *pos, **kw):
+        self.done = threading.Event()
+        super().__init__(*pos, **kw)
+
+    def shutdown_request(self, *pos, **kw):
+        super().shutdown_request(*pos, **kw)
+        self.done.set()
+
+    def wait_done(self):
+        self.done.wait()
+
+
+class ForkingErrorTestServer(socketserver.ForkingMixIn, BaseErrorTestServer):
+    def wait_done(self):
+        [child] = self.active_children
+        os.waitpid(child, 0)
+        self.active_children.clear()
+
+
 class MiscTestCase(unittest.TestCase):
 
     def test_all(self):