On Tue, Dec 05, 2017 at 01:51:52PM +0800, Peter Xu wrote:
> Set maximum QMP request queue length to 8.  If queue full, instead of
> queue the command, we directly return a "request-dropped" event, telling
> client that specific command is dropped.
> 
> Note that this flow control mechanism is only valid if OOB is enabled.
> If it's not, the effective queue length will always be 1, which strictly
> follows original behavior of QMP command handling (which never drop
> messages).
> 
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  monitor.c | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/monitor.c b/monitor.c
> index c20e659740..f7923c4590 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -4029,6 +4029,8 @@ static void monitor_qmp_bh_dispatcher(void *data)
>      }
>  }
>  
> +#define  QMP_REQ_QUEUE_LEN_MAX  (8)
> +
>  static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens,
>                                 void *opaque)
>  {
> @@ -4061,6 +4063,19 @@ static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens,
>      req_obj->id = id;
>      req_obj->req = req;
>  
> +    if (qmp_oob_enabled(mon)) {
> +        /* Drop the request if queue is full. */
> +        if (mon->qmp.qmp_requests->length >= QMP_REQ_QUEUE_LEN_MAX) {

qmp_queue_lock must be held.  Perhaps it's simplest to move this check
into the qmp_queue_lock critical section below and unlock before calling
qapi_event_send_request_dropped()...

> +            qapi_event_send_request_dropped(id,
> +                                            REQUEST_DROP_REASON_QUEUE_FULL,
> +                                            NULL);
> +            qobject_decref(id);
> +            qobject_decref(req);
> +            g_free(req_obj);
> +            return;
> +        }
> +    }
> +
>      /*
>       * Put the request to the end of queue so that requests will be
>       * handled in time order.  Ownership for req_obj, req, id,

... down here.