[Spice-devel] [PATCH spice-protocol 3/3] vdagent: introduce VD_AGENT_CAP_CLIPBOARD_GRAB_SERIAL

[Marc-André Lureau]

Hi

On Sun, Mar 24, 2019 at 6:49 PM Jakub Janku <jjanku at redhat.com> wrote:
>
> On Fri, Mar 22, 2019 at 2:57 PM <marcandre.lureau at redhat.com> wrote:
> >
> > From: Marc-André Lureau <marcandre.lureau at redhat.com>
> >
> > When this capability is negoticated by both the client & the agent,
> > the clipboard grab messages have an associated serial counter.
> >
> > The serial is reset to 0 upon client connection.
> >
> > The counter is increment by 1 on each grab message, by both sides.
> >
> > The sender of the message with the highest serial should be the
> > clipboard grab owner, and the current session serial should be
> > updated.
> >
> > If a lower serial than the current session serial is received, the
> > grab should be discarded.
> >
> > Whenever two grabs share the same serial, the one coming from the
> > client should have a higher priority and the client should gain the
> > clipboard ownership.
>
> Why should the client be the one with a higher priority?

No strong reason, there has to be a winner in this case: both side are
taking the grab simultaneously, we have to decide which one actually
holds it in the end.  Other ideas?

>
> If you look at James' case again:
> with you proposed change, the clipboard manager would take over if a
> race occurs, but the clipboard manager usually supports only a limited
> number of targets.
> For example, you copy a graph in Excel, (regrab and race occurs),
> clipboard manager from the client wins and sets the clipboard, now
> you're able to paste the graph only as a non-editable image.

If there is a clipboard manager on the client side, it's the client
desire to do such transformation, isn't it?

>
> So to provide an "uninterrupted" experience, I think that the
> component with keyboard focus should actually get the priority.

You are working against the clipboard manager in this case, which may
do as much harm as good.

> >
> > No special treatement is done for the unlikely case of overflowing the
> > counter. It may temporarily inverse the priority, until both side have
> > overflown and/or synchronized.
> >
> > Note: this mechanism isn't aiming at making "the most recent" (as in
> > time) side gaining the ownership. One side sending subsequent grab
> > messages earlier will likely take the ownership over a side sending a
> > single message simultaneously the other way. It only clears the
> > situation where both side believe that the other is the current
> > clipboard owner, by having a global ordering and priority in case of
> > serial conflict.
> >
> > Signed-off-by: Marc-André Lureau <marcandre.lureau at redhat.com>
> > ---
> >  spice/vd_agent.h | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/spice/vd_agent.h b/spice/vd_agent.h
> > index 862cb5c..9ae3cc7 100644
> > --- a/spice/vd_agent.h
> > +++ b/spice/vd_agent.h
> > @@ -218,6 +218,9 @@ typedef struct SPICE_ATTR_PACKED VDAgentClipboardGrab {
> >  #if 0 /* VD_AGENT_CAP_CLIPBOARD_SELECTION */
> >      uint8_t selection;
> >      uint8_t __reserved[sizeof(uint32_t) - 1 * sizeof(uint8_t)];
> > +#endif
> > +#if 0 /* VD_AGENT_CAP_CLIPBOARD_GRAB_SERIAL */
> > +    uint32_t serial;
> >  #endif
> >      uint32_t types[0];
> >  } VDAgentClipboardGrab;
> > @@ -288,6 +291,7 @@ enum {
> >      VD_AGENT_CAP_FILE_XFER_DETAILED_ERRORS,
> >      VD_AGENT_CAP_GRAPHICS_DEVICE_INFO,
> >      VD_AGENT_CAP_CLIPBOARD_NO_RELEASE_ON_REGRAB,
> > +    VD_AGENT_CAP_CLIPBOARD_GRAB_SERIAL,
> >      VD_AGENT_END_CAP,
> >  };
> >
> > --
> > 2.21.0.4.g36eb1cb9cf
> >
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-- 
Marc-André Lureau