Re: [Qemu-devel] [PATCH 2/7] qemu-char: Allow a chardev to reconnect if disconnected

[Corey Minyard]

On 03/06/2014 01:43 AM, Michael S. Tsirkin wrote:
>  
> address@hidden specifies that if the client socket does not connect at
> +startup, or if the client socket is closed for some reason (like the other
> +end exited), wait the given number of seconds and attempt to reconnect.
> +
>  TCP and unix socket options are given below:
>  
>  @table @option
> This looks useful but it bothers me that this
> new option can't be used together with others:
> server and nowait, which limits its usefulness
> and would make it harder to extend in the future.
>
> - I think that specifying a time-out should be separate from the option to
> retry.
> E.g. it might be quite reasonable to retry connecting immediately.

Maybe, but that would basically disable qemu until the connection came
back, because it would just be spinning trying to connect.  That doesn't
seem like a good idea to me.

> - it seems to me that if we don't specify nowait, this
> should stop the VM until reconnect.

That's an interesting thought.  Currently the option basically implies
nowait.  It's probably easy to add this, but I'm not sure of the use
case.  Anyone else have any opinions on this?

> - I think this can co-exist with server option.
> E.g. if we detect that the other side closed the socket,
> start listening again. Or even listen all the time
> and accept if disconnected.

IIRC, that's what it does already, even without this patch.  If in
server mode, I believe you can disconnect and then reconnect a socket
and it works fine.  So I see no need for this function in server mode.

>
> I'd like to suggest we generalize this a bit:
>
> When we detect a disconnect, three thinkable things to
> do would be:
>     - keep going as if nothing happened
>         (default for all sockets except monitor)
>     - exit qemu
>         (default for monitor)
>     - retry
There's also
    - stop the VM until the socket reconnects.

I'm not sure why you would want to single out monitor as special. 
Especially if it is mux-ed into another connection.

>
> When waiting for retry it seems reasonable to
> either stop the VM or keep going (nowait option).
>
> When retrying, it seems reasonable to specify a
> timeout to make it fail.

I just don't see why you would want to do this.  It means that the
connection is gone until the VM starts back up again.

One thing I did think of was to add a monitor command to try to
reconnect.  Then I could understand giving up, since you can get into
the monitor and kick it back off.

Thanks,

-corey


> All of the above seem to apply to server
> and client sockets, with or without nowait option.
>
> For clients only, it might be somewhat useful to
> limit the network traffic caused by reconnects.
>
> Thanks,
> MST
>
>> -- 
>> 1.8.3.1