Re: [Qemu-devel] [PATCH v3 35/38] console: make screendump asynchronous

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Marc-André Lureau (marcandre.lureau@redhat.com) wrote:
> Make screendump asynchronous to provide correct screendumps.
> 
> HMP doesn't have async support, so it has to remain synchronous and
> potentially incorrect to avoid potential races.
> 
> Fixes:
> https://bugzilla.redhat.com/show_bug.cgi?id=1230527
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

Hi,
  Looking at this and the previous pair of patches, I think we'd be
better if we defined that async commands took a callback function
pointer and data that they call.

  Here we've got 'graphic_hw_update_done' that explicitly walks qmp
lists; but I think it's better just to pass graphic_hw_update() the
completion data together with a callback function and it calls that when
it's done.

(Also isn't it a little bit of a race, calling graphic_hw_update() and
*then* adding the entry to the list? Can't it end up calling the _done()
before we've added it to the list).

Also, do we need a list of outstanding screendumps, or should we just
have a list of async commands.

It wouldn't seem hard to use the async-QMP command from the HMP
and then just provide a way to tell whether it had finished.

Dave

> ---
>  qapi/ui.json         |  3 +-
>  include/ui/console.h |  3 ++
>  hmp.c                |  2 +-
>  ui/console.c         | 99 ++++++++++++++++++++++++++++++++++++++++----
>  4 files changed, 96 insertions(+), 11 deletions(-)
> 
> diff --git a/qapi/ui.json b/qapi/ui.json
> index 5d01ad4304..4d2c326fb9 100644
> --- a/qapi/ui.json
> +++ b/qapi/ui.json
> @@ -96,7 +96,8 @@
>  #
>  ##
>  { 'command': 'screendump',
> -  'data': {'filename': 'str', '*device': 'str', '*head': 'int'} }
> +  'data': {'filename': 'str', '*device': 'str', '*head': 'int'},
> +  'async': true }
>  
>  ##
>  # == Spice
> diff --git a/include/ui/console.h b/include/ui/console.h
> index fc21621656..0c02190963 100644
> --- a/include/ui/console.h
> +++ b/include/ui/console.h
> @@ -74,6 +74,9 @@ struct MouseTransformInfo {
>  };
>  
>  void hmp_mouse_set(Monitor *mon, const QDict *qdict);
> +void hmp_screendump_sync(const char *filename,
> +                         bool has_device, const char *device,
> +                         bool has_head, int64_t head, Error **errp);
>  
>  /* keysym is a unicode code except for special keys (see QEMU_KEY_xxx
>     constants) */
> diff --git a/hmp.c b/hmp.c
> index 679467d85a..da9008fe63 100644
> --- a/hmp.c
> +++ b/hmp.c
> @@ -2144,7 +2144,7 @@ void hmp_screendump(Monitor *mon, const QDict *qdict)
>      int64_t head = qdict_get_try_int(qdict, "head", 0);
>      Error *err = NULL;
>  
> -    qmp_screendump(filename, id != NULL, id, id != NULL, head, &err);
> +    hmp_screendump_sync(filename, id != NULL, id, id != NULL, head, &err);
>      hmp_handle_error(mon, &err);
>  }
>  
> diff --git a/ui/console.c b/ui/console.c
> index 29234605a7..da51861191 100644
> --- a/ui/console.c
> +++ b/ui/console.c
> @@ -32,6 +32,7 @@
>  #include "chardev/char-fe.h"
>  #include "trace.h"
>  #include "exec/memory.h"
> +#include "monitor/monitor.h"
>  
>  #define DEFAULT_BACKSCROLL 512
>  #define CONSOLE_CURSOR_PERIOD 500
> @@ -116,6 +117,12 @@ typedef enum {
>      TEXT_CONSOLE_FIXED_SIZE
>  } console_type_t;
>  
> +struct qmp_screendump {
> +    gchar *filename;
> +    QmpReturn *ret;
> +    QLIST_ENTRY(qmp_screendump) link;
> +};
> +
>  struct QemuConsole {
>      Object parent;
>  
> @@ -165,6 +172,8 @@ struct QemuConsole {
>      QEMUFIFO out_fifo;
>      uint8_t out_fifo_buf[16];
>      QEMUTimer *kbd_timer;
> +
> +    QLIST_HEAD(, qmp_screendump) qmp_screendumps;
>  };
>  
>  struct DisplayState {
> @@ -190,6 +199,8 @@ static void dpy_refresh(DisplayState *s);
>  static DisplayState *get_alloc_displaystate(void);
>  static void text_console_update_cursor_timer(void);
>  static void text_console_update_cursor(void *opaque);
> +static void ppm_save(const char *filename, DisplaySurface *ds,
> +                     Error **errp);
>  
>  static void gui_update(void *opaque)
>  {
> @@ -256,8 +267,42 @@ static void gui_setup_refresh(DisplayState *ds)
>      ds->have_text = have_text;
>  }
>  
> +static void qmp_screendump_finish(QemuConsole *con, const char *filename,
> +                                  QmpReturn *ret)
> +{
> +    Error *err = NULL;
> +    DisplaySurface *surface;
> +    Monitor *prev_mon = cur_mon;
> +
> +    if (qmp_return_is_cancelled(ret)) {
> +        return;
> +    }
> +
> +    cur_mon = qmp_return_get_monitor(ret);
> +    surface = qemu_console_surface(con);
> +
> +    /* FIXME: async save with coroutine? it would have to copy or lock
> +     * the surface. */
> +    ppm_save(filename, surface, &err);
> +
> +    if (err) {
> +        qmp_return_error(ret, err);
> +    } else {
> +        qmp_screendump_return(ret);
> +    }
> +    cur_mon = prev_mon;
> +}
> +
>  void graphic_hw_update_done(QemuConsole *con)
>  {
> +    struct qmp_screendump *dump, *next;
> +
> +    QLIST_FOREACH_SAFE(dump, &con->qmp_screendumps, link, next) {
> +        qmp_screendump_finish(con, dump->filename, dump->ret);
> +        g_free(dump->filename);
> +        QLIST_REMOVE(dump, link);
> +        g_free(dump);
> +    }
>  }
>  
>  bool graphic_hw_update(QemuConsole *con)
> @@ -358,35 +403,70 @@ write_err:
>      goto out;
>  }
>  
> -void qmp_screendump(const char *filename, bool has_device, const char *device,
> -                    bool has_head, int64_t head, Error **errp)
> +
> +static QemuConsole *get_console(bool has_device, const char *device,
> +                                bool has_head, int64_t head, Error **errp)
>  {
> -    QemuConsole *con;
> -    DisplaySurface *surface;
> +    QemuConsole *con = NULL;
>  
>      if (has_device) {
>          con = qemu_console_lookup_by_device_name(device, has_head ? head : 0,
>                                                   errp);
> -        if (!con) {
> -            return;
> -        }
>      } else {
>          if (has_head) {
>              error_setg(errp, "'head' must be specified together with 'device'");
> -            return;
> +            return NULL;
>          }
>          con = qemu_console_lookup_by_index(0);
>          if (!con) {
>              error_setg(errp, "There is no console to take a screendump from");
> -            return;
>          }
>      }
>  
> +    return con;
> +}
> +
> +void hmp_screendump_sync(const char *filename,
> +                         bool has_device, const char *device,
> +                         bool has_head, int64_t head, Error **errp)
> +{
> +    DisplaySurface *surface;
> +    QemuConsole *con = get_console(has_device, device, has_head, head, errp);
> +
> +    if (!con) {
> +        return;
> +    }
> +    /* This may not complete the drawing with Spice, you may have
> +     * glitches or outdated dumps, use qmp instead! */
>      graphic_hw_update(con);
>      surface = qemu_console_surface(con);
>      ppm_save(filename, surface, errp);
>  }
>  
> +void qmp_screendump(const char *filename,
> +                    bool has_device, const char *device,
> +                    bool has_head, int64_t head,
> +                    QmpReturn *qret)
> +{
> +    Error *err = NULL;
> +    bool async;
> +    QemuConsole *con = get_console(has_device, device, has_head, head, &err);
> +
> +    if (!con) {
> +        qmp_return_error(qret, err);
> +        return;
> +    }
> +    async = graphic_hw_update(con);
> +    if (async) {
> +        struct qmp_screendump *dump = g_new(struct qmp_screendump, 1);
> +        dump->filename = g_strdup(filename);
> +        dump->ret = qret;
> +        QLIST_INSERT_HEAD(&con->qmp_screendumps, dump, link);
> +    } else {
> +        qmp_screendump_finish(con, filename, qret);
> +    }
> +}
> +
>  void graphic_hw_text_update(QemuConsole *con, console_ch_t *chardata)
>  {
>      if (!con) {
> @@ -1280,6 +1360,7 @@ static QemuConsole *new_console(DisplayState *ds, console_type_t console_type,
>      obj = object_new(TYPE_QEMU_CONSOLE);
>      s = QEMU_CONSOLE(obj);
>      s->head = head;
> +    QLIST_INIT(&s->qmp_screendumps);
>      object_property_add_link(obj, "device", TYPE_DEVICE,
>                               (Object **)&s->device,
>                               object_property_allow_set_link,
> -- 
> 2.17.0.rc1.1.g4c4f2b46a3
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK