[RFC PATCH inputproto] Add XIGetSupportedVersion request

[Daniel Martin]

On Thu, Jan 24, 2013 at 11:38:55AM +1000, Peter Hutterer wrote:
> XIQueryVersion sends the client-supported version to the server. The server
> then uses that version to adjust the behaviour of XI2 for this client.
> Current examples include:
> * XIQueryPointer will not set the button mask for touch events if the client
>   is XI 2.2-aware
> * RawEvents are sent to XI 2.1 clients if a grab is active
> * XIAllowEvents will accept different values from XI 2.2 clients
> 
> This behaviour is an issue for libraries supporting XI2. A library that
> calls XIQueryVersion before the client will lock in behaviour that the
> client may not request. A library that calls XIQueryVersion after the client
> may trigger BadValue errors if the libraries requested version differs from
> the client's requested version.
> 
> This request adds a side-effect free version of XIQueryVersion. It returns
> the server version and the already-requested client version (if any). A
> library may use this request to query the server for the XI2 version
> withouth locking in any behaviour for the client.
> 
> Signed-off-by: Peter Hutterer <peter.hutterer at who-t.net>
> ---
> I was tempted to call this XIQueryServerVersion but IMO that is too close to
> XIQueryVersion.
> 
> Returning the client version is to determine what the client actually
> expects in behavior. Not 100% sure if needed, but if it is needed this opens
> another question: do we need a XIQueryVersionCalled event? If the library
> calls XIGetSupportedVersion before the client calls XIQueryVersion, it
> won't have the information. Unless it keeps calling it until it gets a
> number back, which doesn't seem useful either.

Do we really need an additional request? Can't we reuse XIQueryVersion
for this?
- XIQueryVersion has enough padding at the end for additional fields
  (i.e. supported_server_{major,minor}_version)
- and/or XIQueryVersion could just return the supported server version,
  when called with client_major = client_minor = 0.

The proposed XIQueryVersionCalled event might be more important. But,
how long should the library wait for it, i.e. what if:
- the client never calls XIQueryVersion, cause it doesn't use XI on its
  own or
- the client postpones XIQueryVersion until it opens a sub-window
  (Hopefully, every client initializes it at program start - but, in
  theory it's possible.)


Cheers,
    Daniel Martin