[PATCH v3 RESEND 1/2] qmp: Support chardev-change

[Li Zhang]

For some scenarios, we'd like to hot-add a monitor device.  But QEMU
doesn't support that, yet.  It does support hot-swapping character
backends with QMP command chardev-change.  This lets us pre-add a
monitor with a null character backend, then chardev-change to a
socket backend.  Except the chardev-change fails with "Chardev user
does not support chardev hotswap" because monitors don't provide the
required callback.  Implement it for QMP monitors.

Signed-off-by: Li Zhang <li.zhang@ionos.com>
---
v3 -> v2: 
  * Rework the patch which is to add hotswap for QMP device
    https://lists.nongnu.org/archive/html/qemu-devel/2021-04/msg03167.html 
  * Refactor the functions qmp_chardev_add and qmp_chardev_change

 monitor/monitor-internal.h |  1 +
 monitor/monitor.c          |  4 +-
 monitor/qmp.c              | 84 +++++++++++++++++++++++++++-----------
 3 files changed, 63 insertions(+), 26 deletions(-)

diff --git a/monitor/monitor-internal.h b/monitor/monitor-internal.h
index 9c3a09cb01..162f73119b 100644
--- a/monitor/monitor-internal.h
+++ b/monitor/monitor-internal.h
@@ -182,5 +182,6 @@ int get_monitor_def(Monitor *mon, int64_t *pval, const char 
*name);
 void help_cmd(Monitor *mon, const char *name);
 void handle_hmp_command(MonitorHMP *mon, const char *cmdline);
 int hmp_compare_cmd(const char *name, const char *list);
+void monitor_flush_locked(Monitor *mon);
 
 #endif
diff --git a/monitor/monitor.c b/monitor/monitor.c
index b90c0f4051..1b05ef3bdb 100644
--- a/monitor/monitor.c
+++ b/monitor/monitor.c
@@ -154,8 +154,6 @@ static inline bool monitor_is_hmp_non_interactive(const 
Monitor *mon)
     return !monitor_uses_readline(container_of(mon, MonitorHMP, common));
 }
 
-static void monitor_flush_locked(Monitor *mon);
-
 static gboolean monitor_unblocked(GIOChannel *chan, GIOCondition cond,
                                   void *opaque)
 {
@@ -169,7 +167,7 @@ static gboolean monitor_unblocked(GIOChannel *chan, 
GIOCondition cond,
 }
 
 /* Caller must hold mon->mon_lock */
-static void monitor_flush_locked(Monitor *mon)
+void monitor_flush_locked(Monitor *mon)
 {
     int rc;
     size_t len;
diff --git a/monitor/qmp.c b/monitor/qmp.c
index 092c527b6f..29b49ee041 100644
--- a/monitor/qmp.c
+++ b/monitor/qmp.c
@@ -46,6 +46,8 @@ struct QMPRequest {
 typedef struct QMPRequest QMPRequest;
 
 QmpCommandList qmp_commands, qmp_cap_negotiation_commands;
+static void monitor_qmp_setup_handlers_bh(void *opaque);
+static void monitor_backend_init(MonitorQMP *mon, Chardev *chr);
 
 static bool qmp_oob_enabled(MonitorQMP *mon)
 {
@@ -481,6 +483,35 @@ void monitor_data_destroy_qmp(MonitorQMP *mon)
     g_queue_free(mon->qmp_requests);
 }
 
+static bool monitor_in_list(Monitor *mon)
+{
+    Monitor *mon_tmp;
+    QTAILQ_FOREACH(mon_tmp, &mon_list, entry) {
+        if (mon_tmp == mon) {
+            return true;
+        }
+    }
+    return false;
+}
+
+static int monitor_qmp_change(void *opaque)
+{
+    MonitorQMP *mon = opaque;
+
+    monitor_data_init(&mon->common, true, false,
+            qemu_chr_has_feature(mon->common.chr.chr,
+                                 QEMU_CHAR_FEATURE_GCONTEXT));
+    monitor_backend_init(mon, mon->common.chr.chr);
+    qemu_mutex_lock(&mon->common.mon_lock);
+    if (mon->common.out_watch) {
+        mon->common.out_watch = 0;
+        monitor_flush_locked(&mon->common);
+    }
+    qemu_mutex_unlock(&mon->common.mon_lock);
+
+    return 0;
+}
+
 static void monitor_qmp_setup_handlers_bh(void *opaque)
 {
     MonitorQMP *mon = opaque;
@@ -491,30 +522,15 @@ 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_list_append(&mon->common);
-}
+                             monitor_qmp_change, &mon->common, context, true);
 
-void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
-{
-    MonitorQMP *mon = g_new0(MonitorQMP, 1);
-
-    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
-        g_free(mon);
-        return;
+    if (!monitor_in_list(&mon->common)) {
+        monitor_list_append(&mon->common);
     }
-    qemu_chr_fe_set_echo(&mon->common.chr, true);
-
-    /* Note: we run QMP monitor in I/O thread when @chr supports that */
-    monitor_data_init(&mon->common, true, false,
-                      qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));
-
-    mon->pretty = pretty;
-
-    qemu_mutex_init(&mon->qmp_queue_lock);
-    mon->qmp_requests = g_queue_new();
+}
 
-    json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL);
+static void monitor_backend_init(MonitorQMP *mon, Chardev *chr)
+{
     if (mon->common.use_io_thread) {
         /*
          * Make sure the old iowatch is gone.  It's possible when
@@ -532,7 +548,29 @@ 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_list_append(&mon->common);
+                                 monitor_qmp_change, &mon->common, NULL, true);
+        if (!monitor_in_list(&mon->common)) {
+            monitor_list_append(&mon->common);
+        }
     }
 }
+
+void monitor_init_qmp(Chardev *chr, bool pretty, Error **errp)
+{
+    MonitorQMP *mon = g_new0(MonitorQMP, 1);
+
+    if (!qemu_chr_fe_init(&mon->common.chr, chr, errp)) {
+        g_free(mon);
+        return;
+    }
+    qemu_chr_fe_set_echo(&mon->common.chr, true);
+     /* Note: we run QMP monitor in I/O thread when @chr supports that */
+    monitor_data_init(&mon->common, true, false,
+                      qemu_chr_has_feature(chr, QEMU_CHAR_FEATURE_GCONTEXT));
+
+    mon->pretty = pretty;
+    qemu_mutex_init(&mon->qmp_queue_lock);
+    mon->qmp_requests = g_queue_new();
+    json_message_parser_init(&mon->parser, handle_qmp_command, mon, NULL);
+    monitor_backend_init(mon, chr);
+}
-- 
2.25.1