Merge remote-tracking branch 'remotes/armbru/tags/pull-monitor-2019-1…

…1-19' into staging

Monitor patches for 2019-11-19

# gpg: Signature made Tue 19 Nov 2019 08:50:57 GMT
# gpg:                using RSA key 354BC8B3D7EB2A6B68674E5F3870B400EB918653
# gpg:                issuer "armbru@redhat.com"
# gpg: Good signature from "Markus Armbruster <armbru@redhat.com>" [full]
# gpg:                 aka "Markus Armbruster <armbru@pond.sub.org>" [full]
# Primary key fingerprint: 354B C8B3 D7EB 2A6B 6867  4E5F 3870 B400 EB91 8653

* remotes/armbru/tags/pull-monitor-2019-11-19:
  monitor/qmp: resume monitor when clearing its queue

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

monitor/qmp.c

@@ -75,10 +75,35 @@ static void monitor_qmp_cleanup_req_queue_locked(MonitorQMP *mon)
     }
 }
 
-static void monitor_qmp_cleanup_queues(MonitorQMP *mon)
+static void monitor_qmp_cleanup_queue_and_resume(MonitorQMP *mon)
 {
     qemu_mutex_lock(&mon->qmp_queue_lock);
+
+    /*
+     * Same condition as in monitor_qmp_bh_dispatcher(), but before
+     * removing an element from the queue (hence no `- 1`).
+     * Also, the queue should not be empty either, otherwise the
+     * monitor hasn't been suspended yet (or was already resumed).
+     */
+    bool need_resume = (!qmp_oob_enabled(mon) ||
+        mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX)
+        && !g_queue_is_empty(mon->qmp_requests);
+
     monitor_qmp_cleanup_req_queue_locked(mon);
+
+    if (need_resume) {
+        /*
+         * handle_qmp_command() suspended the monitor because the
+         * request queue filled up, to be resumed when the queue has
+         * space again.  We just emptied it; resume the monitor.
+         *
+         * Without this, the monitor would remain suspended forever
+         * when we get here while the monitor is suspended.  An
+         * unfortunately timed CHR_EVENT_CLOSED can do the trick.
+         */
+        monitor_resume(&mon->common);
+    }
+
     qemu_mutex_unlock(&mon->qmp_queue_lock);
 }
 
@@ -263,9 +288,10 @@ static void handle_qmp_command(void *opaque, QObject *req, Error *err)
 
     /*
      * Suspend the monitor when we can't queue more requests after
-     * this one.  Dequeuing in monitor_qmp_bh_dispatcher() will resume
-     * it.  Note that when OOB is disabled, we queue at most one
-     * command, for backward compatibility.
+     * this one.  Dequeuing in monitor_qmp_bh_dispatcher() or
+     * monitor_qmp_cleanup_queue_and_resume() will resume it.
+     * Note that when OOB is disabled, we queue at most one command,
+     * for backward compatibility.
      */
     if (!qmp_oob_enabled(mon) ||
         mon->qmp_requests->length == QMP_REQ_QUEUE_LEN_MAX - 1) {
@@ -332,7 +358,7 @@ static void monitor_qmp_event(void *opaque, int event)
          * stdio, it's possible that stdout is still open when stdin
          * is closed.
          */
-        monitor_qmp_cleanup_queues(mon);
+        monitor_qmp_cleanup_queue_and_resume(mon);
         json_message_parser_destroy(&mon->parser);
         json_message_parser_init(&mon->parser, handle_qmp_command,
                                  mon, NULL);