Re: [PATCH] monitor/qmp: cleanup socket listener sources early to avoid fd handling race

[Jie Song <mail@jiesong.me>]

Hi Daniel,

Thank you for your review and valuable feedback.

You're absolutely right about the concerns. Let me clarify the scenario 
this patch addresses:
The remove_fd_in_watch() function handles the client-side connection case. 
However, when the chardev is configured in server mode 
(e.g., -qmp unix:/var/lib/libvirt/qemu/qmp-xxx/qmp.monitor,server=on,wait=off), 
there's listener that needs cleanup. The socket_chr_listener_cleanup() 
is specifically intended to handle this server-side listener to prevent the 
race condition between the main thread and IOThread monitoring the same listener fd.

I apologize for the unsafe assumption that the chardev would always be a SocketChardev.
You're correct that this could cause crashes with other chardev types. 
To fix this properly, I’m considering a more general design. 
Would the following approach be acceptable?

  1.Add a chr_listener_cleanup callback to the ChardevClass structure
  2.Implement this callback in SocketChardev
  3.Register it in char_socket_class_init()
  4.In monitor/qmp.c, call it through the class method
    remove_fd_in_watch(chr);
    ChardevClass *cc = CHARDEV_GET_CLASS(chr);
    if (cc->chr_listener_cleanup) {
        cc->chr_listener_cleanup(chr);
    }

This would maintain type safety while keeping the fix properly abstracted
at the chardev layer. Would this fix make sense?

Looking forward to your guidance.

Best regards,
Jie Song