[Spice-devel] Issues with Clipboard Grab/Release Callbacks

[Marc-André Lureau]

Hi

On Mon, Jun 20, 2022 at 6:09 PM Nick Couchman <vnick at apache.org> wrote:

> Hello, everyone,
> I've posted here, before, with some questions during my journey of trying
> to add support for the SPICE protocol to the Guacamole project. The good
> news is that I've actually made measurable progress in the implementation -
> I can now connect to a SPICE server, and the basics work (mouse and
> keyboard - mostly anyway), so I'm moving on to the extra stuff - clipboard,
> audio, file transfer, etc.
>
> In working on the clipboard integration, I'm currently running into an
> issue with a couple of the callback functions, specifically the clipboard
> grab/release functions, where the number of arguments seems to be
> mismatched. According to the documentation, these callbacks should be
> called with the following arguments:
>
> gboolean
> user_function (SpiceMainChannel *main,
>                guint             selection,
>                gpointer          types,
>                guint             ntypes,
>                gpointer          user_data)
>
>
types is actually guint32*, but that's not the problem here.


> void
> user_function (SpiceMainChannel *main,
>                guint             selection,
>                gpointer          user_data)
>
> I've implemented the callbacks with those arguments, as follows:
>
> void guac_spice_clipboard_selection_grab_handler(SpiceMainChannel channel,
>         guint selection, gpointer types, guint ntypes, guac_client* client)
>
> void guac_spice_clipboard_selection_release_handler(SpiceMainChannel
> channel,
>         guint selection, guac_client* client)
>

It looks correct. Can you point to your code connecting the signals with
the handlers?


>
> and registered them appropriately. However, if I use them as implemented
> above, when the callbacks are triggered, the application segfaults when I
> try to access the "guac_client* client" data structure. I used GDB to try
> to help track this down, and I noticed that the value of "client" was 0x1,
> which looks less like a pointer to a memory location and more like the
> number 1.
>
> So, I decided to add another argument to the callback functions, just
> before the client argument:
>
> void guac_spice_clipboard_selection_grab_handler(SpiceMainChannel channel,
>         guint selection, gpointer types, guint ntypes, guint extra,
> guac_client* client)
>
> void guac_spice_clipboard_selection_release_handler(SpiceMainChannel
> channel,
>         guint selection, guint extra, guac_client* client)
>
> Strangely, this works - the client data structure can be referenced, there
> are no segfaults (yet), etc. So, I decided to print the values that are
> being passed for all of these parameters, and I get the following:
>
> guacd[100252]: DEBUG: Notifying client of clipboard grab in the guest.
> guacd[100252]: DEBUG: Arg: channel: 0x00000000
> guacd[100252]: DEBUG: Arg: selection: 1275303536
> guacd[100252]: DEBUG: Arg: types: 0x00000001
> guacd[100252]: DEBUG: Arg: ntypes: 1276022924
> guacd[100252]: DEBUG: Arg: extra: 1
>
>
Weird


> I printed them in the format I thought they should be in based on what the
> arguments are supposed to be - I probably should have just done all hex.
> But, it seems like maybe the "extra" parameter being passed is in front of
> the channel, since the channel is showing up as all zeros?
>
> I was trying to find the code where the callbacks are actually called -
> I'm guessing, since this is clipboard integration, it'll be in the vdagent
> code somewhere - but I was having trouble tracking that down.
>
>
The callbacks are not directly called, since those a GObject signals. The
main channel will call g_signal_emit().



> If anyone has any ideas, I'd appreciate the insight into this - I'm
> puzzled by this apparent mismatch in the number of arguments. Also, if it
> matters, I'm running CentOS 8 Stream, using Xspice to provide a test SPICE
> server, and running spice-vdagentd/spice-vdagent within my X session. Yes,
> I know Xspice is unmaintained, but I just needed something simple and that
> I didn't have to spend a bunch of time building in order to give me a spice
> server to point at, and, so far, this has been pretty reliable. Also, at
> its core, it appears to use the same spice-qxl X driver that x11spice uses,
> just with a simple Python wrapper script for generating an X config file
> and starting the X server/display. So, I think it's still pretty "safe" for
> attempting to develop this Guacamole integration - if for some reason you
> believe me to be wrong about that, please let me know.
>

Your testing environment shouldn't be a problem, it's really your
client-side code integration with spice-glib that looks broken.

-- 
Marc-André Lureau
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