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

[Jakub Janku]

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?

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.

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