This removes the need for using monitor_is_qmp() to check the
subclass type, which is an anti-pattern.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dave@treblig.org>
Tested-by: Peter Krempa <pkrempa@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 monitor/hmp.c              | 13 +++++++++++++
 monitor/monitor-internal.h |  7 +++++++
 monitor/monitor.c          | 11 ++++-------
 3 files changed, 24 insertions(+), 7 deletions(-)

diff --git a/monitor/hmp.c b/monitor/hmp.c
index c03b91782c..f9252387c4 100644
--- a/monitor/hmp.c
+++ b/monitor/hmp.c
@@ -68,11 +68,18 @@ static void monitor_hmp_set_readline(Object *obj, bool val, Error **errp)
     mon->use_readline = val;
 }
 
+int monitor_hmp_vprintf(Monitor *mon, const char *fmt, va_list ap)
+    G_GNUC_PRINTF(2, 0);
+
 static void monitor_hmp_class_init(ObjectClass *cls, const void *data)
 {
+    MonitorClass *moncls = MONITOR_CLASS(cls);
+
     object_class_property_add_bool(cls, "readline",
                                    monitor_hmp_get_readline,
                                    monitor_hmp_set_readline);
+
+    moncls->vprintf = monitor_hmp_vprintf;
 }
 
 static void monitor_hmp_init(Object *obj)
@@ -87,6 +94,12 @@ static void monitor_hmp_init(Object *obj)
     hmp->use_readline = true;
 }
 
+int monitor_hmp_vprintf(Monitor *mon, const char *fmt, va_list ap)
+{
+    g_autofree char *buf = g_strdup_vprintf(fmt, ap);
+    return monitor_puts(mon, buf);
+}
+
 static void monitor_command_cb(void *opaque, const char *cmdline,
                                void *readline_opaque)
 {
diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 6a4c3b0f37..23cef68153 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -104,6 +104,13 @@ typedef struct HMPCommand {
 
 struct MonitorClass {
     ObjectClass parent_class;
+
+    /*
+     * If non-NULL, the monitor is able to print messages
+     * for attention of the client user
+     */
+    int (*vprintf)(Monitor *mon, const char *fmt, va_list ap)
+        G_GNUC_PRINTF(2, 0);
 };
 
 struct Monitor {
diff --git a/monitor/monitor.c b/monitor/monitor.c
index 9fe131983d..0cb0930bf4 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -264,21 +264,18 @@ int monitor_puts(Monitor *mon, const char *str)
 
 int monitor_vprintf(Monitor *mon, const char *fmt, va_list ap)
 {
-    char *buf;
-    int n;
+    MonitorClass *moncls;
 
     if (!mon) {
         return -1;
     }
 
-    if (monitor_is_qmp(mon)) {
+    moncls = MONITOR_GET_CLASS(mon);
+    if (!moncls->vprintf) {
         return -1;
     }
 
-    buf = g_strdup_vprintf(fmt, ap);
-    n = monitor_puts(mon, buf);
-    g_free(buf);
-    return n;
+    return moncls->vprintf(mon, fmt, ap);
 }
 
 int monitor_printf(Monitor *mon, const char *fmt, ...)
-- 
2.55.0