[PATCH 2/2] Support monitor chardev hotswap with QMP

[Li Zhang <zhlcindy@gmail.com>]

From: Li Zhang <li.zhang@cloud.ionos.com>

For some scenarios, it needs to hot-add a monitor device.
But QEMU doesn't support hotplug yet. It also works by adding
a monitor with null backend by default and then change its
backend to socket by QMP command "chardev-change".

So this patch is to support monitor chardev hotswap with QMP.

Signed-off-by: Li Zhang <li.zhang@cloud.ionos.com>
---
 monitor/monitor-internal.h |  3 +++
 monitor/monitor.c          |  2 +-
 monitor/qmp.c              | 42 +++++++++++++++++++++++++++++++++++---
 3 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 40903d6386..2df6dd21de 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -186,4 +186,7 @@ int hmp_compare_cmd(const char *name, const char *list);
 void qmp_query_qmp_schema(QDict *qdict, QObject **ret_data,
                                  Error **errp);
 
+gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
+                               void *opaque);
+
 #endif
diff --git a/monitor/monitor.c b/monitor/monitor.c
index e94f532cf5..2d255bab18 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -157,7 +157,7 @@ static inline bool monitor_is_hmp_non_interactive(const Monitor *mon)
 
 static void monitor_flush_locked(Monitor *mon);
 
-static gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
+gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
                                   void *opaque)
 {
     Monitor *mon = opaque;
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 2326bd7f9b..55cfb230d9 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -44,6 +44,7 @@ struct QMPRequest {
     Error *err;
 };
 typedef struct QMPRequest QMPRequest;
+static void monitor_qmp_set_handlers_bh(void *opaque);
 
 QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
 
@@ -480,7 +481,35 @@ void monitor_data_destroy_qmp(MonitorQMP *mon)
     g_queue_free(mon->qmp_requests);
 }
 
-static void monitor_qmp_setup_handlers_bh(void *opaque)
+static int monitor_qmp_change(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+
+    mon->common.use_io_thread =
+        qemu_chr_has_feature(mon->common.chr.chr, QEMU_CHAR_FEATURE_GCONTEXT);
+
+    if (mon->common.use_io_thread) {
+        aio_bh_schedule_oneshot(iothread_get_aio_context(mon_iothread),
+                                monitor_qmp_set_handlers_bh, mon);
+    } else {
+        qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
+                                 monitor_qmp_read, monitor_qmp_event,
+                                 monitor_qmp_change, &mon->common, NULL, true);
+    }
+
+    if (mon->common.out_watch) {
+        g_source_remove(mon->common.out_watch);
+        qemu_mutex_lock(&mon->common.mon_lock);
+        mon->common.out_watch =
+        qemu_chr_fe_add_watch(&mon->common.chr, G_IO_OUT | G_IO_HUP,
+                               monitor_unblocked, &mon->common);
+        qemu_mutex_unlock(&mon->common.mon_lock);
+    }
+
+    return 0;
+}
+
+static void monitor_qmp_set_handlers_bh(void *opaque)
 {
     MonitorQMP *mon = opaque;
     GMainContext *context;
@@ -490,7 +519,14 @@ static void monitor_qmp_setup_handlers_bh(void *opaque)
     assert(context);
     qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
                              monitor_qmp_read, monitor_qmp_event,
-                             NULL, &mon->common, context, true);
+                             monitor_qmp_change, &mon->common, context, true);
+
+}
+
+static void monitor_qmp_setup_handlers_bh(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+    monitor_qmp_set_handlers_bh(mon);
     monitor_list_append(&mon->common);
 }
 
@@ -531,7 +567,7 @@ void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
     } else {
         qemu_chr_fe_set_handlers(&mon->common.chr, monitor_can_read,
                                  monitor_qmp_read, monitor_qmp_event,
-                                 NULL, &mon->common, NULL, true);
+                                 monitor_qmp_change, &mon->common, NULL, true);
         monitor_list_append(&mon->common);
     }
 }
-- 
2.25.1