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

[Markus Armbruster]

Daniel P. Berrangé <address@hidden> writes:

> 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.

Right.  Must fix bug.

I'm glad I went over this one more time, and in public!