fix(pymsg): 修复消息长度无上限导致 OOM DoS 漏洞

Python/Go 端 recv 缺失消息大小上限检查，恶意客户端发送 4 字节
超大长度头即可触发 OOM。新增 64 MiB 上限（与 C++ 端 Msg.hpp
对齐），在分配内存前拒绝并断开连接。新增 5 个单元测试覆盖。

pymsg/tests/test_msgsize.py

@@ -0,0 +1,123 @@
+"""消息大小上限（_MAX_MSG_SIZE）单元测试。
+
+验证 recv() 拒绝超大长度头、send() 拒绝超大载荷，
+防止恶意客户端通过 4 字节 header 声明超大长度导致 OOM。
+
+    cd pymsg
+    python -m unittest discover -s tests -v
+"""
+
+import socket
+import struct
+import threading
+import time
+import unittest
+import unittest.mock
+
+import pymsg
+from pymsg import MsgServer, recv, send, _MAX_MSG_SIZE
+
+
+def _free_port() -> int:
+    with socket.socket() as s:
+        s.bind(("127.0.0.1", 0))
+        return s.getsockname()[1]
+
+
+class _ServerFixture:
+    """后台线程运行 MsgServer。"""
+
+    def __init__(self, handler, workers=0):
+        self.port = _free_port()
+        self.server = MsgServer("127.0.0.1", self.port, workers=workers)
+        self.thread = threading.Thread(
+            target=self.server.start,
+            args=(lambda sock, addr, text: handler(self.server, sock, addr, text),),
+            daemon=True,
+        )
+
+    def __enter__(self):
+        self.thread.start()
+        deadline = time.time() + 5
+        while time.time() < deadline:
+            try:
+                with socket.create_connection(("127.0.0.1", self.port), timeout=0.2):
+                    return self
+            except OSError:
+                time.sleep(0.02)
+        raise RuntimeError("server did not start in time")
+
+    def __exit__(self, *exc):
+        self.server.stop()
+        self.thread.join(timeout=5)
+        return False
+
+
+class TestMaxMsgSize(unittest.TestCase):
+
+    def test_recv_rejects_oversized_header(self):
+        """服务端 recv() 收到超大长度头时应断开连接，不分配内存。"""
+        with _ServerFixture(lambda srv, sock, addr, text: srv.reply(sock, "ok")) as fx:
+            with socket.create_connection(("127.0.0.1", fx.port), timeout=5) as s:
+                # 发送合法 header 但声明长度为 _MAX_MSG_SIZE + 1
+                s.sendall(struct.pack("!I", _MAX_MSG_SIZE + 1))
+                # 服务端应断开连接（ConnectionError -> _remove_client）
+                # 客户端读到 EOF
+                data = s.recv(1024)
+                self.assertEqual(data, b"")
+
+    def test_recv_rejects_max_uint32(self):
+        """声明 0xFFFFFFFF（4 GiB）也应被拒绝。"""
+        with _ServerFixture(lambda srv, sock, addr, text: srv.reply(sock, "ok")) as fx:
+            with socket.create_connection(("127.0.0.1", fx.port), timeout=5) as s:
+                s.sendall(struct.pack("!I", 0xFFFFFFFF))
+                data = s.recv(1024)
+                self.assertEqual(data, b"")
+
+    def test_server_survives_oversized_message(self):
+        """拒绝超大消息后服务仍然存活，后续正常请求不受影响。"""
+        from pymsg import MsgClient
+
+        def handler(server, sock, addr, text):
+            server.reply(sock, f"ok:{text}")
+
+        with _ServerFixture(handler, workers=2) as fx:
+            # 发送恶意超大 header
+            with socket.create_connection(("127.0.0.1", fx.port), timeout=5) as s:
+                s.sendall(struct.pack("!I", _MAX_MSG_SIZE + 1))
+                s.recv(1024)  # 等待服务端断开
+
+            # 等待服务端清理连接
+            time.sleep(0.2)
+
+            # 后续正常 RPC 仍然工作
+            resp = MsgClient("127.0.0.1", fx.port, "t").call("ping", timeout=5)
+            self.assertEqual(resp, "ok:ping")
+
+    def test_send_rejects_oversized_payload(self):
+        """send() 对超大载荷应直接抛 ValueError，不触碰网络。"""
+        # 用 mock 将上限临时降到 100 字节，避免分配 64 MB 内存
+        with unittest.mock.patch.object(pymsg, "_MAX_MSG_SIZE", 100):
+            with _ServerFixture(lambda srv, sock, addr, text: None) as fx:
+                with socket.create_connection(("127.0.0.1", fx.port), timeout=5) as s:
+                    oversized = "x" * 101
+                    with self.assertRaises(ValueError) as cm:
+                        send(s, oversized)
+                    self.assertIn("too large", str(cm.exception))
+
+    def test_normal_messages_unaffected(self):
+        """合法大小的消息不受影响。"""
+        from pymsg import MsgClient
+
+        def handler(server, sock, addr, text):
+            server.reply(sock, f"echo:{text}")
+
+        with _ServerFixture(handler) as fx:
+            # 1 MB 消息（远小于 64 MiB 上限）
+            big = "A" * (1024 * 1024)
+            resp = MsgClient("127.0.0.1", fx.port, "t").call(big, timeout=10)
+            self.assertEqual(resp, f"echo:{big}")
+
+
+if __name__ == "__main__":
+    unittest.main()