Re: [Qemu-devel] monitor: enable OOB by default

[Daniel P . Berrangé]

On Wed, Jun 27, 2018 at 10:41:38AM +0200, Markus Armbruster wrote:
> Markus Armbruster <address@hidden> writes:
> 
> > Markus Armbruster <address@hidden> writes:
> >
> >> I fooled around a bit, and I think there are a few lose ends.
> > [...]
> >> Talking to a QMP monitor that supports OOB:
> >>
> >>     $ socat UNIX:test-qmp READLINE,history=$HOME/.qmp_history,prompt='QMP> 
> >> '
> >>     {"QMP": {"version": {"qemu": {"micro": 50, "minor": 12, "major": 2}, 
> >> "package": "v2.12.0-1703-gb909799463"}, "capabilities": ["oob"]}}
> >>     QMP> { "execute": "qmp_capabilities", "arguments": { "oob": true } }
> >>     {"error": {"class": "GenericError", "desc": "Parameter 'oob' is 
> >> unexpected"}}
> >>     QMP> { "execute": "qmp_capabilities", "arguments": { "enable": ["oob"] 
> >> } }
> >>     {"return": {}}
> >>     QMP> { "execute": "query-qmp-schema" }
> >>     {"error": {"class": "GenericError", "desc": "Out-Of-Band capability 
> >> requires that every command contains an 'id' field"}}
> >>
> >> Why does every command require 'id'?
> >
> > I found one reason: event COMMAND_DROPPED wants it.  Any other reason?
> >
> > [...]
> 
> Apropos COMMAND_DROPPED: we send an event rather than an error response
> because we may send it out-of-order.  Makes sense.
> 
> However, broadcasting it to all monitors doesn't make sense.  We could
> use a way to send an event to just one monitor.

Worse than that - broadcasting to all monitors is categorically broken.
Different monitors make use the same "id" formatting scheme, so if you
broadcast COMMAND_DROPPED to a different monitor you might have clashing
"id" and thus incorrectly tell a client its command was dropped when in
fact it was processed. You'd have to be fairly unlucky in timing, but
it could happen.

Regards,
Daniel
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