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

[Markus Armbruster <armbru@redhat.com>]

Eric Blake <eblake@redhat.com> writes:

> On 06/27/2018 07:07 AM, Peter Xu wrote:
>
>>>> 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.
>>
>
>>
>> My current plan is that I can touch up scripts/qapi/events.py and
>> related stuff to allow QMPEventFuncEmit to take a monitor parameter,
>> then we pass in NULL when we want to send the event to all monitors.
>>
>> Would that work?
>
> Makes sense to me. Also, right now, ALL callers of qapi_event_send_*
> pass &error_abort as their final parameter. If you're refactoring
> everything anyways, you could get rid of that parameter on the
> presumption that it doesn't buy us anything.

Let me try to turn presumption into fact :)

The generated qapi_event_send_FOO() can detect the following error
conditions:

* Visitor fails.  But the QObject output visitor can't actually fail.

  Since the errp parameter is part of the abstract visitor interface, we
  can't eliminate it.  We can pass &error_abort.

* emit() function fails.  Can't happen, either.

  Let's eliminate the unused errp parameter of QMPEventFuncEmit.

  If we can eliminate the somewhat silly indirection through
  qmp_event_get_func_emit(), even better.