Re: [PATCH v7 32/35] monitor: add support for auto-deleting monitors upon close

[Markus Armbruster <armbru@redhat.com>]

Daniel P. Berrangé <berrange@redhat.com> writes:

> The default monitor is usually a long lived object that will exist for
> the entire lifetime of the VM. A monitor can only service a single
> client at a time though, and so it might be desirable to hotplug
> additional monitors at runtime for specific tasks. If doing that,
> however, there is a need to remove the monitor when it is no longer
> needed.
>
> A use case for hotplugging a monitor can involved a user wishing to

Typo: involve

> spawn an adhoc script that uses a temporary monitor. The script can

ad hoc

> ask the management application to hotplug a monitor and pass back a
> pre-opened FD using SCM_RIGHTS. In this case the lifetime of the
> script is not tied to the management application and thus it is
> desirable to have automatic cleanup when the script exits.
>
> Allowing a client to run "object-del" against its own monitor adds
> complex edge cases, as it would be desirable to send the QMP response
> despite the monitor sending it being deleted. Doing "object-del" alone
> will also result in orphaning a character device backend instance, as
> there is no opportunity to run the companion "chardev-del" command.
>
> A simpler way to ensure cleanup is to add the concept of auto-deleting
> monitor objects. Specifically when the "CHR_EVENT_CLOSED" event is
> emitted, the equivalent of "object-del" + "chardev-del" can be run
> internally. Since the transient client has already droppped its
> monitor connection, there is no synchronization to be concerned about
> with sending QMP replies. There is still some internal synchronization
> needed, however, between the character device event callback and the
> bottom-half that runs the delete. There is a chance that an incoming
> client connection may arise before the bottom-half runs, which has
> to be checked. Once the monitor object is deleted, the event callback
> is unregistered from the character device, eliminating any further
> races before the character device is fully deleted.
>
> This is implemented via a new "close-action=none|delete" property on
> the 'monitor-qmp' object. This concept could be extended with further
> actions in future, for example:
>
>  * close-action=shutdown - graceful guest shutdown
>  * close-action=terminate - immediate guest poweroff
>  * close-action=stop - pause guest CPUs while the monitor is not
>                        connected to any client
>
> This is left as an exercise for future interested contributors.
>
> Tested-by: Peter Krempa <pkrempa@redhat.com>
> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Acked-by: Markus Armbruster <armbru@redhat.com>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Happy to fix typos without a respin.