Re: [PATCH v2 8/9] vnc: add support for extended desktop resize

[Gerd Hoffmann]

On Tue, Dec 08, 2020 at 06:13:28PM +0000, Daniel P. Berrangé wrote:
> On Tue, Dec 08, 2020 at 12:57:36PM +0100, Gerd Hoffmann wrote:
> > The extended desktop resize encoding adds support for (a) clients
> > sending resize requests to the server, and (b) multihead support.
> > 
> > This patch implements (a).  All resize requests are rejected by qemu.
> > Qemu can't resize the framebuffer on its own, this is in the hands of
> > the guest, so all qemu can do is forward the request to the guest.
> > Should the guest actually resize the framebuffer we can notify the vnc
> > client later with a separate message.
> > 
> > This requires support in the display device.  Works with virtio-gpu.
> > 
> > https://github.com/rfbproto/rfbproto/blob/master/rfbproto.rst#extendeddesktopsize-pseudo-encoding
> 
> The spec says
> 
>    "The server must send an ExtendedDesktopSize rectangle in response 
>     to a FramebufferUpdateRequest with incremental set to zero, assuming
>     the client has requested the ExtendedDesktopSize pseudo-encoding 
>     using the SetEncodings message. This requirement is needed so that
>     the client has a reliable way of fetching the initial screen 
>     configuration, and to determine if the server supports the 
>     SetDesktopSize message.
> 
>     A consequence of this is that a client must not respond to an
>     ExtendedDesktopSize rectangle by sending a FramebufferUpdateRequest
>     with incremental set to zero. Doing so would make the system go into
>     an infinite loop."
> 
> Historically gtk-vnc always sets incremental=0 after a resize message,
> so it should trigger an infinite loop. This doesn't happen with QEMU's
> impl, so I think QEMU isn't correctly following the sec here. IIUC,
> tt needs to force send an ExtendedDesktopSize message every time
> incremental=0, not merely when a resize happens.

There are three cases where qemu sends an ExtendedDesktopSize message:

  (1) during the initial handshake (to cover the feature detection).
  (2) as response to a SetDesktopSize request, and
  (3) when an resize happens.

So, yes, I skimmed the spec a bit to fast and didn't implement it
correctly.  Which also explains why you can't trigger the infinite
loop issue.

> Having said that, I find this part of the spec rather crazy. I dont
> see why clients need more than the first ExtendedDesktopSize message
> in order to detect the feature, rather than after every single
> incremental=0 update request.

Yes, it doesn't make much sense.

> None the less the spec says we should get an infinite loop and with
> my intentional attempt to cause this, QEMU doesn't play ball.

Not fully sure I should consider this a bug or a feature.
This patch doesn't conform to the spec.
But it clearly is more robust ...

take care,
  Gerd