[Qemu-devel] [RFC v5 18/26] monitor: send event when request queue full

[Peter Xu <peterx@redhat.com>]

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) {
+            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,
-- 
2.14.3