Re: [Qemu-devel] [RFC 5/7] qxl-render: call ppm_save on callback

[Gerd Hoffmann <kraxel@redhat.com>]

On 02/19/12 22:28, Alon Levy wrote:
> This changes the behavior of the monitor command. After the previous
> patch, there is no longer an option of deadlock with virt-manager, but
> ppm_save is called too early, before the update has completed. With this
> patch it is called at the correct moment, but that means there is a race
> between the monitor command completing and the screendump file being saved.
> 
> The only solution is to use an asynchronous monitor command. For a
> previous round of this see:
>  http://lists.gnu.org/archive/html/qemu-devel/2011-10/msg02810.html
> 
> Since that's contentious, I'm suggesting we do something that is almost
> correct and doesn't hang, instead of correct and hangs. The screendump
> user can inotify on the directory and the file if need be. For casual
> monitor usage there is no difference.

Hmm, that is pretty lame.  There are users like autotest which expect
the screen dump being there when the monitor command is finished, that
change will break them.

Unfortunaly there is no easy way out.  I think the options are:

 (1) Keep existing behavior.  That means the screenshot might show old
     screen content.  Not very nice too.  Would work sort-of ok for
     autotest though as autotest does screenshots every second and thus
     the screen content wouldn't be older than a second.

 (2) Async monitor command.  Keeps interface and works nicely.  A bunch
     of QAPI bits tickled into master meanwhile, so we could look at
     this again.  Luiz?  What is the status here?

 (3) Something like this patch + additionally introduce a
     "your-screenshot-is-finished-now" qmp event.  Will break existing
     users too.  But at least they can be adapted without requiring
     some external, nonportable service like inotify ...

cheers,
  Gerd