Re: [Qemu-devel] [PATCH v4 3/7] monitor: flush qmp responses when CLOSED

[Markus Armbruster]

Peter Xu <address@hidden> writes:

> Previously we clean up the queues when we got CLOSED event.  It was used
> to make sure we won't send leftover replies/events of a old client to a
> new client which makes perfect sense. However this will also drop the
> replies/events even if the output port of the previous chardev backend
> is still open, which can lead to missing of the last replies/events.
> Now this patch does an extra operation to flush the response queue
> before cleaning up.
>
> In most cases, a QMP session will be based on a bidirectional channel (a
> TCP port, for example, we read/write to the same socket handle), so in
> port and out port of the backend chardev are fundamentally the same
> port. In these cases, it does not really matter much on whether we'll
> flush the response queue since flushing will fail anyway.  However there
> can be cases where in & out ports of the QMP monitor's backend chardev
> are separated.  Here is an example:
>
>   cat $QMP_COMMANDS | qemu -qmp stdio ... | filter_commands
>
> In this case, the backend is fd-typed, and it is connected to stdio
> where in port is stdin and out port is stdout.  Now if we drop all the
> events on the response queue then filter_command process might miss some
> events that it might expect.  The thing is that, when stdin closes,
> stdout might still be there alive!
>
> In practice, I encountered SHUTDOWN event missing when running test with
> iotest 087 with Out-Of-Band enabled.  Here is one of the ways that this
> can happen (after "quit" command is executed and QEMU quits the main
> loop):
>
> 1. [main thread] QEMU queues a SHUTDOWN event into response queue.
>
> 2. "cat" terminates (to distinguish it from the animal, I quote it).
>
> 3. [monitor iothread] QEMU's monitor iothread reads EOF from stdin.
>
> 4. [monitor iothread] QEMU's monitor iothread calls the CLOSED event
>    hook for the monitor, which will destroy the response queue of the
>    monitor, then the SHUTDOWN event is dropped.
>
> 5. [main thread] QEMU's main thread cleans up the monitors in
>    monitor_cleanup().  When trying to flush pending responses, it sees
>    nothing.  SHUTDOWN is lost forever.
>
> Note that before the monitor iothread was introduced, step [4]/[5] could
> never happen since the main loop was the only place to detect the EOF
> event of stdin and run the CLOSED event hooks.  Now things can happen in
> parallel in the iothread.
>
> Without this patch, iotest 087 will have ~10% chance to miss the
> SHUTDOWN event and fail when with Out-Of-Band enabled (the output is
> manually touched up to suite line width requirement):

I wouldn't wrap lines when quoting a diff.

>
> --- /home/peterx/git/qemu/tests/qemu-iotests/087.out
> +++ /home/peterx/git/qemu/bin/tests/qemu-iotests/087.out.bad
> @@ -8,7 +8,6 @@
>  {"return": {}}
>  {"error": {"class": "GenericError", "desc": "'node-name' must be
>   specified for the root node"}}
>  {"return": {}}
> -{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP},
> - "event": "SHUTDOWN", "data": {"guest": false}}
>
>  === Duplicate ID ===
> @@ -53,7 +52,6 @@
>  {"return": {}}
>  {"return": {}}
>  {"return": {}}
>
> -{"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP},
> - "event": "SHUTDOWN", "data": {"guest": false}}

Please indent the quoted diff a bit, so make it more obviously not part
of the patch.  In fact, git-am chokes on it for me.

>
> This patch fixes the problem.
>
> Fixes: 6d2d563f8c ("qmp: cleanup qmp queues properly", 2018-03-27)
> Suggested-by: Markus Armbruster <address@hidden>
> Signed-off-by: Peter Xu <address@hidden>
>
> Signed-off-by: Peter Xu <address@hidden>
> ---
>  monitor.c | 33 ++++++++++++++++++++++++++++++---
>  1 file changed, 30 insertions(+), 3 deletions(-)
>
> diff --git a/monitor.c b/monitor.c
> index d4a463f707..c9a02ee40c 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -512,6 +512,27 @@ struct QMPResponse {
>  };
>  typedef struct QMPResponse QMPResponse;
>  
> +static QObject *monitor_qmp_response_pop_one(Monitor *mon)
> +{
> +    QObject *data;
> +
> +    qemu_mutex_lock(&mon->qmp.qmp_queue_lock);
> +    data = g_queue_pop_head(mon->qmp.qmp_responses);
> +    qemu_mutex_unlock(&mon->qmp.qmp_queue_lock);
> +
> +    return data;
> +}
> +
> +static void monitor_qmp_response_flush(Monitor *mon)
> +{
> +    QObject *data;
> +
> +    while ((data = monitor_qmp_response_pop_one(mon))) {
> +        monitor_json_emitter_raw(mon, data);
> +        qobject_unref(data);
> +    }
> +}
> +
>  /*
>   * Pop a QMPResponse from any monitor's response queue into @response.
>   * Return false if all the queues are empty; else true.
> @@ -523,9 +544,7 @@ static bool monitor_qmp_response_pop_any(QMPResponse 
> *response)
>  
>      qemu_mutex_lock(&monitor_lock);
>      QTAILQ_FOREACH(mon, &mon_list, entry) {
> -        qemu_mutex_lock(&mon->qmp.qmp_queue_lock);
> -        data = g_queue_pop_head(mon->qmp.qmp_responses);
> -        qemu_mutex_unlock(&mon->qmp.qmp_queue_lock);
> +        data = monitor_qmp_response_pop_one(mon);
>          if (data) {
>              response->mon = mon;
>              response->data = data;
> @@ -4364,6 +4383,14 @@ static void monitor_qmp_event(void *opaque, int event)
>          mon_refcount++;
>          break;
>      case CHR_EVENT_CLOSED:
> +        /*
> +         * Note: this is only useful when the output of the chardev
> +         * backend is still open.  For example, when the backend is
> +         * stdio, it's possible that stdout is still open when stdin
> +         * is closed.  After all, CHR_EVENT_CLOSED event is currently
> +         * only bound to stdin.
> +         */

I'm not sure I get the last sentence.  What do you mean by "bound to
stdin"?

> +        monitor_qmp_response_flush(mon);
>          monitor_qmp_cleanup_queues(mon);
>          json_message_parser_destroy(&mon->qmp.parser);
>          json_message_parser_init(&mon->qmp.parser, handle_qmp_command);