monitor_cleanup() is one of the last things main() calls before it
returns.  In the following patch, monitor_cleanup() will release the
monitor_lock during flushing. There may be pending commands to insert
new monitors, which would modify the mon_list during iteration, and
the clean-up could thus miss those new insertions.

Add a monitor_destroyed global to check if monitor_cleanup() has been
already called. In this case, don't insert the new monitor in the
list, but free it instead.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
---
 monitor.c | 19 +++++++++++++++++--
 1 file changed, 17 insertions(+), 2 deletions(-)

diff --git a/monitor.c b/monitor.c
index 25cf4223e8..f0256bdec5 100644
--- a/monitor.c
+++ b/monitor.c
@@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
 /* QMP checker flags */
 #define QMP_ACCEPT_UNKNOWNS 1
 
-/* Protects mon_list, monitor_qapi_event_state.  */
+/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
 static QemuMutex monitor_lock;
 static GHashTable *monitor_qapi_event_state;
 static QTAILQ_HEAD(mon_list, Monitor) mon_list;
+static bool monitor_destroyed;
 
 /* Protects mon_fdsets */
 static QemuMutex mon_fdsets_lock;
@@ -4538,8 +4539,21 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
 static void monitor_list_append(Monitor *mon)
 {
     qemu_mutex_lock(&monitor_lock);
-    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
+    /*
+     * This prevents inserting new monitors during monitor_cleanup().
+     * A cleaner solution would involve the main thread telling other
+     * threads to terminate, waiting for their termination.
+     */
+    if (!monitor_destroyed) {
+        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
+        mon = NULL;
+    }
     qemu_mutex_unlock(&monitor_lock);
+
+    if (mon) {
+        monitor_data_destroy(mon);
+        g_free(mon);
+    }
 }
 
 static void monitor_qmp_setup_handlers_bh(void *opaque)
@@ -4635,6 +4649,7 @@ void monitor_cleanup(void)
 
     /* Flush output buffers and destroy monitors */
     qemu_mutex_lock(&monitor_lock);
+    monitor_destroyed = true;
     QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
         QTAILQ_REMOVE(&mon_list, mon, entry);
         monitor_flush(mon);
-- 
2.20.0.rc1

Marc-André Lureau <marcandre.lureau@redhat.com> writes:

> monitor_cleanup() is one of the last things main() calls before it
> returns.  In the following patch, monitor_cleanup() will release the
> monitor_lock during flushing. There may be pending commands to insert
> new monitors, which would modify the mon_list during iteration, and
> the clean-up could thus miss those new insertions.
>
> Add a monitor_destroyed global to check if monitor_cleanup() has been
> already called. In this case, don't insert the new monitor in the
> list, but free it instead.
>
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  monitor.c | 19 +++++++++++++++++--
>  1 file changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index 25cf4223e8..f0256bdec5 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -263,10 +263,11 @@ typedef struct QMPRequest QMPRequest;
>  /* QMP checker flags */
>  #define QMP_ACCEPT_UNKNOWNS 1
>  
> -/* Protects mon_list, monitor_qapi_event_state.  */
> +/* Protects mon_list, monitor_qapi_event_state, monitor_destroyed.  */
>  static QemuMutex monitor_lock;
>  static GHashTable *monitor_qapi_event_state;
>  static QTAILQ_HEAD(mon_list, Monitor) mon_list;
> +static bool monitor_destroyed;
>  
>  /* Protects mon_fdsets */
>  static QemuMutex mon_fdsets_lock;
> @@ -4538,8 +4539,21 @@ void error_vprintf_unless_qmp(const char *fmt, va_list ap)
>  static void monitor_list_append(Monitor *mon)
>  {
>      qemu_mutex_lock(&monitor_lock);
> -    QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +    /*
> +     * This prevents inserting new monitors during monitor_cleanup().
> +     * A cleaner solution would involve the main thread telling other
> +     * threads to terminate, waiting for their termination.

Let's add this sentence to the commit message as well.

> +     */
> +    if (!monitor_destroyed) {
> +        QTAILQ_INSERT_HEAD(&mon_list, mon, entry);
> +        mon = NULL;
> +    }
>      qemu_mutex_unlock(&monitor_lock);
> +
> +    if (mon) {
> +        monitor_data_destroy(mon);
> +        g_free(mon);
> +    }
>  }
>  
>  static void monitor_qmp_setup_handlers_bh(void *opaque)
> @@ -4635,6 +4649,7 @@ void monitor_cleanup(void)
>  
>      /* Flush output buffers and destroy monitors */
>      qemu_mutex_lock(&monitor_lock);
> +    monitor_destroyed = true;
>      QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
>          QTAILQ_REMOVE(&mon_list, mon, entry);
>          monitor_flush(mon);

Reviewed-by: Markus Armbruster <armbru@redhat.com>