feat(hello_handler.py): add HELLO message handler

src/judas_server/backend/handler/hello_handler.py

@@ -0,0 +1,64 @@
+# -*- coding: utf-8 -*-
+from __future__ import annotations
+
+from typing import TYPE_CHECKING, override
+
+from judas_protocol import Category, ControlAction, Message
+
+from judas_server.backend.client import ClientStatus
+from judas_server.backend.handler import BaseHandler
+
+if TYPE_CHECKING:
+    from judas_server.backend.backend_server import BackendServer
+    from judas_server.backend.client import Client
+
+
+class HelloHandler(BaseHandler):
+    def __init__(self, backend_server: BackendServer) -> None:
+        super().__init__(backend_server)
+
+    @override
+    def handle(self, client: Client, message: Message) -> None:
+        if client.id is not None:
+            return
+
+        if (
+            message.category != Category.CONTROL
+            or message.action != ControlAction.HELLO
+        ):
+            self.logger.error(
+                f"Expected HELLO message from {client}, got {message}, disconnecting client..."
+            )
+            self.backend_server._disconnect(client)
+            return
+
+        if message.payload.get("id") is None:
+            self.logger.error(
+                f"HELLO message from {client} missing 'id' field, disconnecting client..."
+            )
+            self.backend_server._disconnect(client)
+            return
+
+        client.id = message.payload["id"]
+
+        # check if client already connected, if so disconnect old client and register new one
+        if (
+            client.id in self.backend_server.clients
+            and self.backend_server.clients[client.id].status == "connected"
+        ):
+            old_client: Client = self.backend_server.clients[client.id]
+            self.backend_server.logger.warning(
+                f"Client {client.id} is already connected from {old_client.addr}, disconnecting old client..."
+            )
+            self.backend_server.send_close(old_client)
+            return
+
+        self.backend_server.clients[client.id] = client  # type: ignore
+        self.backend_server.known_clients[client.id] = {  # type: ignore
+            "last_seen": client.last_seen
+        }
+
+        self.backend_server._save_known_clients()
+        client.status = ClientStatus.ONLINE
+
+        self.logger.info(f"[+] Registered new client {client}")