Re: [PATCH v5 32/35] monitor: add support for auto-deleting monitors upon close

[Markus Armbruster <armbru@redhat.com>]

marcandre.lureau@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.

Whatever adds the additional monitor can also delete it.  The fact that
you propose other means suggests you believe this would be cumbersome in
practice.  Why?

>> 
>> 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.

If object-del or chardev-del fail, there's no way to report the error.
Can they fail?

Do we always want to delete both monitor and character device?

>> 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

I'm not sure these would be useful and safe.  Mentioning them here is
okay regardless.

>> 
>> This is left as an exercise for future interested contributors.
>> 
>> Tested-by: Peter Krempa <pkrempa@redhat.com>
>> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
>> Message-ID: <20260624173752.2928717-33-berrange@redhat.com>
>>
>> diff --git a/qapi/qom.json b/qapi/qom.json
>> index 6affb70a594..b4c49a2e8b9 100644
>> --- a/qapi/qom.json
>> +++ b/qapi/qom.json
>> @@ -1213,18 +1213,37 @@
>>    'base': 'MonitorProperties',
>>    'data': { '*readline': 'bool' } }
>>  
>> +
>> +##
>> +# @MonitorQMPCloseAction:
>> +#
>> +# Action to take when the character device backend is

No line break here.

>> +# closed.
>> +#
>> +# @none: take no action (the default)

The type cannot specify a default, only its users can.  Scratch (the
default).

Blank line between member descriptions, please.

>> +# @delete: delete both the 'monitor-qmp' object and its associated
>> +#          character device backend object

Format like

    # @delete: delete both the 'monitor-qmp' object and its associated
    #     character device backend object

>> +#
>> +# Since 11.1
>
> should be "Since: 11.1" (worth a new check perhaps)

docs/devel/qapi-code-gen.rst:

    Section tags are case-sensitive and end with a colon.  They are only
    recognized after a blank line.  Good example::

     #
     # Since: 7.1

    Bad examples (all ordinary paragraphs)::

     # since: 7.1

     # Since 7.1

     # Since : 7.1

There are a few bad ones in master.  I posted a patch:

    Subject: [PATCH] qapi: Fix misspelled section tags in doc comments
    Date: Wed, 1 Jul 2026 08:11:36 +0200
    Message-ID: <20260701061136.798815-1-armbru@redhat.com>

Code to detect and reject such errors would have to carefully avoid
false negatives.

>> +##
>> +{ 'enum' : 'MonitorQMPCloseAction',
>
> extra space, vs the usual '{ 'enum':'.

Yes.

>> +  'data': ['none', 'delete'] }
>> +
>>  ##
>>  # @MonitorQMPProperties:
>>  #
>>  # Properties for the QMP monitor
>>  #
>>  # @pretty: whether to pretty print JSON responses (default: disabled)

(default: false)

See my review of PATCH 22.

Blank line between member descriptions, please.

>> +# @close-action: action to take when the character device backend
>> +#                is closed (default: none)

Format like

    # @close-action: action to take when the character device backend is
    #     closed (default: none)

>>  #
>>  # Since: 11.1
>>  ##
>>  { 'struct': 'MonitorQMPProperties',
>>    'base': 'MonitorProperties',
>> -  'data': { '*pretty': 'bool' } }
>> +  'data': { '*pretty': 'bool',
>> +            '*close-action': 'MonitorQMPCloseAction' } }
>>  
>>  ##
>>  # @ObjectType:
>> diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
>> index 5522e05464b..23829f32f9a 100644
>> --- a/monitor/monitor-internal.h
>> +++ b/monitor/monitor-internal.h
>> @@ -28,6 +28,7 @@
>>  #include "chardev/char-fe.h"
>>  #include "monitor/monitor.h"
>>  #include "qapi/qapi-types-control.h"
>> +#include "qapi/qapi-types-qom.h"
>>  #include "qapi/qmp-registry.h"
>>  #include "qobject/json-parser.h"
>>  #include "qemu/readline.h"
>> @@ -178,7 +179,9 @@ struct MonitorQMP {
>>      Monitor parent_obj;
>>      JSONMessageParser parser;
>>      bool pretty;
>> +    MonitorQMPCloseAction close_action;
>>      bool setup_pending; /* iothread BH has not yet set up chardev handlers */
>> +    bool delete_pending; /* close_action has started 'delete' process */
>>      /*
>>       * When a client connects, we're in capabilities negotiation mode.
>>       * @commands is &qmp_cap_negotiation_commands then.  When command
>> diff --git a/monitor/qmp.c b/monitor/qmp.c
>> index 5301927f09f..1057b0d12ba 100644
>> --- a/monitor/qmp.c
>> +++ b/monitor/qmp.c
>> @@ -28,6 +28,7 @@
>>  #include "monitor-internal.h"
>>  #include "qapi/error.h"
>>  #include "qapi/qapi-commands-control.h"
>> +#include "qapi/qapi-commands-char.h"
>>  #include "qobject/qdict.h"
>>  #include "qobject/qjson.h"
>>  #include "qobject/qlist.h"
>> @@ -103,6 +104,20 @@ static void monitor_qmp_set_pretty(Object *obj, bool val, Error **errp)
>>      mon->pretty = val;
>>  }
>>  
>> +static int monitor_qmp_get_close_action(Object *obj, Error **errp)
>> +{
>> +    MonitorQMP *mon = MONITOR_QMP(obj);
>> +
>> +    return mon->close_action;
>> +}
>> +
>> +static void monitor_qmp_set_close_action(Object *obj, int val, Error **errp)
>> +{
>> +    MonitorQMP *mon = MONITOR_QMP(obj);
>> +
>> +    mon->close_action = val;
>> +}
>> +
>>  static void monitor_qmp_emit_event(Monitor *mon, QAPIEvent event, QDict *qdict);
>>  static bool monitor_qmp_requires_iothread(const Monitor *mon);
>>  static void monitor_qmp_complete(UserCreatable *uc, Error **errp);
>> @@ -117,6 +132,11 @@ static void monitor_qmp_class_init(ObjectClass *cls, const void *data)
>>      object_class_property_add_bool(cls, "pretty",
>>                                     monitor_qmp_get_pretty,
>>                                     monitor_qmp_set_pretty);
>> +    object_class_property_add_enum(cls, "close-action",
>> +                                   "MonitorQMPCloseAction",
>> +                                   &MonitorQMPCloseAction_lookup,
>> +                                   monitor_qmp_get_close_action,
>> +                                   monitor_qmp_set_close_action);
>>  
>>      moncls->emit_event = monitor_qmp_emit_event;
>>      moncls->requires_iothread = monitor_qmp_requires_iothread;
>> @@ -550,11 +570,45 @@ static QDict *qmp_greeting(MonitorQMP *mon)
>>          ver, cap_list);
>>  }
>>  
>> +static void monitor_qmp_self_delete_bh(void *opaque)
>> +{
>> +    MonitorQMP *mon = opaque;
>> +    g_autofree char *mon_id = object_property_get_child_name(
>> +        object_get_objects_root(), OBJECT(mon));
>> +    g_autofree char *chardev_id = g_strdup(mon->parent_obj.chardev_id);
>> +    Error *local_error = NULL;
>> +
>> +    if (!mon_id) {
>> +        /* Another monitor raced & ran 'object-del' on 'mon'
>> +         * before this BH got scheduled, so we have a ref on
>> +         * mon but it is already unparented.
>> +         */
>> +        object_unref(mon);
>> +        return;
>> +    }
>> +
>> +    user_creatable_del(mon_id, &local_error);
>> +    object_unref(mon);

This object_unref() and the one above pair with the object_ref() in
monitor_qmp_event() case MONITOR_QMP_CLOSE_ACTION_DELETE.  Correct?

Is this worth a comment?

>> +    if (local_error != NULL) {
>> +        error_report_err(local_error);
>> +    } else {
>> +        qmp_chardev_remove(chardev_id, NULL);
>> +    }
>> +}
>> +
>>  static void monitor_qmp_event(void *opaque, QEMUChrEvent event)
>>  {
>>      QDict *data;
>>      MonitorQMP *mon = opaque;
>>  
>> +    /* Protect against race if a client drops & quickly
>> +     * reconnects - we'll have the delete BH scheduled
>> +     * so must not honour a new open request
>> +     */
>> +    if (mon->delete_pending) {
>> +        return;
>> +    }

Consider "something happens between setting @delete_pending and monitor
destruction that triggers the event" and "same thing happens after
destruction".  Any observable differences in behavior?

Are there any other monitor interactions that might need similar
special-casing when @delete_pending?

>> +
>>      switch (event) {
>>      case CHR_EVENT_OPENED:
>>          WITH_QEMU_LOCK_GUARD(&mon->parent_obj.mon_lock) {
>> @@ -577,6 +631,27 @@ static void monitor_qmp_event(void *opaque, QEMUChrEvent event)
>>          json_message_parser_init(&mon->parser, handle_qmp_command,
>>                                   mon, NULL);
>>          monitor_fdsets_cleanup();
>> +        switch (mon->close_action) {
>> +        case MONITOR_QMP_CLOSE_ACTION_NONE:
>> +            break; /* nada */

Useless comment :)

>> +        case MONITOR_QMP_CLOSE_ACTION_DELETE:
>> +            mon->delete_pending = true;
>> +            /*
>> +             * Do NOT run in the AIO context associated with the
>> +             * monitor. We need to run in the default AIO context
>> +             * which is the same context in which 'qmp_object_del'
>> +             * will execute

I believe you.  I don't have the headspace right now to confirm this
myself.

>> +             *
>> +             * Hold an extra ref in case a separate monitor races
>> +             * with the BH by processing an explicit 'object-del'

The extra ref ensures the object still lives when the BH runs.  Correct?

>> +             */
>> +            object_ref(mon);

This is the object_ref() I referred to above.

>> +            aio_bh_schedule_oneshot(qemu_get_aio_context(),
>> +                                    monitor_qmp_self_delete_bh, mon);
>> +            break;
>> +        default:
>> +            g_assert_not_reached();
>> +        }
>>          break;
>>      case CHR_EVENT_BREAK:
>>      case CHR_EVENT_MUX_IN:

[...]

> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>