vnc clipboard support

[Gerd Hoffmann]

  Hi folks,

I'm looking for a good way to implement cut+paste support for vnc.

The vnc core protocol has support for text/plain cut+paste, and there
is an extension adding support for other formats.  That'll cover one
part of the problem, exchanging cut+paste data between vnc client and
qemu vnc server.

The tricky part is the second: the guest <=> qemu communication.
I see basically two possible approaches here:

  (1) Have some guest agent (spice does it that way).
      Advantage: more flexible, allows more features.
      Disadvantage: requires agent in the guest.
  
  (2) Send text as key events.
      Advantage: no guest agent needed.
      Disadvantage: is translated by guests keyboard map, so qemu
      needs to know the map for proper char -> key event translation.
      Only works for text/plain and only for chars you can easily
      type, anything needing input methods (emoji 😊 for example)
      isn't going to fly.

I think that (1) is clearly the better way.  Given that the agent
would need to run in user wayland/xorg session context the existing
qemu-guest-agent will not work.  Also linking against some UI library
like gtk3 for clipboard handling is not something we want for the
qemu-guest-agent.  So we need another one, I'll name it
qemu-clipboard-agent for the rest of this mail.  And we need a
communication channel.

I'd tend to model the qemu-clipboard-agent simliar to the
qemu-guest-agent, i.e. have some stream as communication path and run
some stream protocol over it.

Stream options I see are (in order of personal preference):

  (1) New virtio-serial port.  virtio-serial likely is there anyway
      for the qemu-guest-agent ...

  (2) Have qemu-clipboard-agent and qemu-guest-agent share the agent
      channel, i.e. qemu-clipboard-agent will proxy everything through
      qemu-guest-agent (spice does it that way).

Protocol options I see are (not sure yet which to prefer, need to have
a closer look at the candidates):

  (1) Add clipboard commands to QMP and use these.

  (2) Reuse the clipboard bits of the vnc protocol (forward
      VNC_MSG_CLIENT_CUT_TEXT messages to the guest agent)

  (3) Reuse the clipboard bits of the spice-agent protocol.

  (4) Reuse the clipboard bits of the wayland protocol.

Once we have sorted the qemu <-> guest communication path it should be
possible to also hook up other UIs (specifically gtk) without too much
effort.  Which probably makes (2) a rather poor choice.

Comments?
Suggestions?
Other ideas?

take care,
  Gerd