raise if server address is updated on non-closed connections

mitmproxy/connection.py

@@ -293,6 +293,11 @@ class Server(Connection):
             local_port = ""
         return f"Server({human.format_address(self.address)}, state={self.state.name.lower()}{tls_state}{local_port})"
 
+    def __setattr__(self, name, value):
+        if name == "address" and self.__dict__.get("state", ConnectionState.CLOSED) is ConnectionState.OPEN:
+            raise RuntimeError("Cannot change server address on open connection.")
+        return super().__setattr__(name, value)
+
     def get_state(self):
         return {
             'address': self.address,

test/mitmproxy/proxy/layers/test_tls.py

@@ -226,8 +226,8 @@ class TestServerTLS:
 
     def test_simple(self, tctx):
         playbook = tutils.Playbook(tls.ServerTLSLayer(tctx))
-        tctx.server.state = ConnectionState.OPEN
         tctx.server.address = ("example.mitmproxy.org", 443)
+        tctx.server.state = ConnectionState.OPEN
         tctx.server.sni = "example.mitmproxy.org"
 
         tssl = SSLTest(server_side=True)
@@ -345,7 +345,7 @@ def make_client_tls_layer(
     playbook = tutils.Playbook(server_layer)
 
     # Add some server config, this is needed anyways.
-    tctx.server.address = ("example.mitmproxy.org", 443)
+    tctx.server.__dict__["address"] = ("example.mitmproxy.org", 443)  # .address fails because connection is open
     tctx.server.sni = "example.mitmproxy.org"
 
     tssl_client = SSLTest(**kwargs)

test/mitmproxy/test_connection.py

@@ -1,3 +1,5 @@
+import pytest
+
 from mitmproxy.connection import Server, Client, ConnectionState
 from mitmproxy.test.tflow import tclient_conn, tserver_conn
 
@@ -76,3 +78,10 @@ class TestServer:
         assert c2.get_state() != c.get_state()
         c.id = c2.id = "foo"
         assert c2.get_state() == c.get_state()
+
+    def test_address(self):
+        s = Server(("address", 22))
+        s.address = ("example.com", 443)
+        s.state = ConnectionState.OPEN
+        with pytest.raises(RuntimeError):
+            s.address = ("example.com", 80)