The netdev reports NETDEV_VHOST_USER_CONNECTED event when
the chardev is connected, and NETDEV_VHOST_USER_DISCONNECTED
when it is disconnected.

The NETDEV_VHOST_USER_CONNECTED event includes the chardev id.

This allows a system manager like libvirt to detect when the server
fails.

For instance with passt:

{ 'execute': 'qmp_capabilities' }
{ "return": { } }

[killing passt here]

{ "timestamp": { "seconds": 1739538634, "microseconds": 920450 },
  "event": "NETDEV_VHOST_USER_DISCONNECTED",
  "data": { "netdev-id": "netdev0" } }

[automatic reconnection with reconnect-ms]

{ "timestamp": { "seconds": 1739538638, "microseconds": 354181 },
  "event": "NETDEV_VHOST_USER_CONNECTED",
  "data": { "netdev-id": "netdev0", "chardev-id": "chr0" } }

Tested-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---

Notes:
    v3:
      - remove ChardevInfo, provides only the chardev id
    
    v2:
      - remove duplicate line info.frontend_open

 qapi/char.json   | 40 ++++++++++++++++++++++++++++++++++++++++
 net/vhost-user.c |  3 +++
 2 files changed, 43 insertions(+)

diff --git a/qapi/char.json b/qapi/char.json
index f02b66c06b3e..ef4fa95eec39 100644
--- a/qapi/char.json
+++ b/qapi/char.json
@@ -902,3 +902,43 @@
 { 'event': 'VSERPORT_CHANGE',
   'data': { 'id': 'str',
             'open': 'bool' } }
+
+##
+# @NETDEV_VHOST_USER_CONNECTED:
+#
+# Emitted when the vhost-user chardev is connected
+#
+# @netdev-id: QEMU netdev id that is connected
+#
+# @chardev-id: The character device id used by the QEMU netdev
+#
+# Since: 10.0
+#
+# .. qmp-example::
+#
+#     <- { "timestamp": {"seconds": 1739538638, "microseconds": 354181 },
+#          "event": "NETDEV_VHOST_USER_CONNECTED",
+#          "data": { "netdev-id": "netdev0", "chardev-id": "chr0" } }
+#
+##
+{ 'event': 'NETDEV_VHOST_USER_CONNECTED',
+  'data': { 'netdev-id': 'str', 'chardev-id': 'str' } }
+
+##
+# @NETDEV_VHOST_USER_DISCONNECTED:
+#
+# Emitted when the vhost-user chardev is disconnected
+#
+# @netdev-id: QEMU netdev id that is disconnected
+#
+# Since: 10.0
+#
+# .. qmp-example::
+#
+#     <- { "timestamp": { "seconds": 1739538634, "microseconds": 920450 },
+#          "event": "NETDEV_VHOST_USER_DISCONNECTED",
+#          "data": { "netdev-id": "netdev0" } }
+#
+##
+{ 'event': 'NETDEV_VHOST_USER_DISCONNECTED',
+  'data': { 'netdev-id': 'str' } }
diff --git a/net/vhost-user.c b/net/vhost-user.c
index 12555518e838..d754e4239e7f 100644
--- a/net/vhost-user.c
+++ b/net/vhost-user.c
@@ -16,6 +16,7 @@
 #include "chardev/char-fe.h"
 #include "qapi/error.h"
 #include "qapi/qapi-commands-net.h"
+#include "qapi/qapi-events-char.h"
 #include "qemu/config-file.h"
 #include "qemu/error-report.h"
 #include "qemu/option.h"
@@ -271,6 +272,7 @@ static void chr_closed_bh(void *opaque)
     if (err) {
         error_report_err(err);
     }
+    qapi_event_send_netdev_vhost_user_disconnected(name);
 }
 
 static void net_vhost_user_event(void *opaque, QEMUChrEvent event)
@@ -300,6 +302,7 @@ static void net_vhost_user_event(void *opaque, QEMUChrEvent event)
                                          net_vhost_user_watch, s);
         qmp_set_link(name, true, &err);
         s->started = true;
+        qapi_event_send_netdev_vhost_user_connected(name, chr->label);
         break;
     case CHR_EVENT_CLOSED:
         /* a close event may happen during a read/write, but vhost
-- 
2.48.1