Warn instead of failing when a forwarded host port is in use in the Windows 11 launcher

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

run-windows11.py

@@ -4,6 +4,7 @@
 import os
 import platform
 import shutil
+import socket
 import string
 import subprocess
 import sys
@@ -33,6 +34,17 @@ def capture(command):
     return result.stdout.rstrip("\n")
 
 
+def port_in_use(port):
+    """True if a host TCP port cannot be bound, as QEMU does for a hostfwd rule."""
+    with socket.socket(socket.AF_INET, socket.SOCK_STREAM) as probe:
+        probe.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        try:
+            probe.bind(("", int(port)))
+        except OSError:
+            return True
+    return False
+
+
 def is_executable(path):
     return path.is_file() and os.access(path, os.X_OK)
 
@@ -296,6 +308,19 @@ except (KeyError, ValueError) as error:
     die(f"invalid placeholder in {qemu_config_template}: {error}")
 qemu_config.write_text(qemu_config_text)
 
+# QEMU refuses to start when a forwarded host port is taken, so warn and leave
+# that forward out instead.
+forwards = []
+forwarded_ports = []
+for label, host_port, guest_port in (("RDP", rdp_port, 3389), ("SSH", ssh_port, 22)):
+    in_use = port_in_use(host_port)
+    forwarded_ports.append((label, host_port, not in_use))
+    if in_use:
+        print(f"Warning: host port {host_port} is already in use; "
+              f"{label} will not be forwarded to the guest.", file=sys.stderr)
+    else:
+        forwards.append(f"hostfwd=tcp::{host_port}-:{guest_port}")
+
 qemu_args = [
     "-readconfig", str(qemu_config),
     "-readconfig", str(qemu_arch_config),
@@ -304,7 +329,7 @@ qemu_args = [
     # A config file section cannot hold two hostfwd entries, so the network
     # that the NIC in the config file uses is defined here. -nic none stops
     # QEMU from adding its default NIC as well.
-    "-netdev", f"user,id=net0,hostfwd=tcp::{rdp_port}-:3389,hostfwd=tcp::{ssh_port}-:22",
+    "-netdev", ",".join(["user,id=net0", *forwards]),
     "-nic", "none",
     "-display", display_backend,
     # "-display", "none", "-spice", "port=5900,addr=127.0.0.1,disable-ticketing=on,agent-mouse=off", "-device", "virtio-vga",
@@ -324,8 +349,9 @@ elif installing:
     print("Starting Windows 11 from the QCOW2 base image.")
 else:
     print("Starting Windows 11 normally.")
-print(f"Account: guofang / password; RDP: localhost:{rdp_port}; SSH: localhost:{ssh_port}",
+print("Account: guofang / password; " + "; ".join(
-      flush=True)
+    f"{label}: localhost:{port}" for label, port, forwarded in forwarded_ports if forwarded),
+    flush=True)
 
 if daemonize:
     log_file.write_text("")