[PATCH wayland] server: Make serial number 0 mean invalid

[Bill Spitzak]

On 10/08/2015 01:00 PM, Daniel Stone wrote:
> Hi,
>
> On 8 October 2015 at 08:27, Jonas Ådahl <jadahl at gmail.com> wrote:
>> On Mon, Oct 05, 2015 at 12:04:49PM -0500, Derek Foreman wrote:
>>> There are cases in weston where it would be quite nice to have a
>>> sentinel value to use instead of having to have a bool for "this serial
>>> number is legit" too.
>>
>> Even though probably unlikely, for clients unaware of a possible 0 == no
>> serial, this would mean that they would suddenly start to be killed when
>> when they before worked just fine.

Clients cannot be killed when they pass bad serial numbers. They don't 
really know what events the compositor accepts for a present request, so 
passing the wrong one cannot kill it.

I suppose a compositor could record the serial number of every event 
sent to the client so it could make sure the request only contained 
actual ones, but I don't think anybody is going to implement a 
compositor that way. I expect compositors will check to see if the event 
is the last mouse-down or mouse-up sent to the client and not remember 
any other history.

>> Is it really a big deal to have to multiple requests that do things
>> differently?
>
> Let's try to solve this empirically, then - which optional-serial
> requests do we have apart from present/needs-attention here, and what
> does/would the difference look like semantically?

I agree that if there really is different behavior then there should be 
two requests.

But, do you really, really, really believe that any actual usable 
compositor is going to treat the no-event present differently than a 
serial for an event that should not raise the window (such as a mouse 
enter event)?

If you think so, please describe carefully exactly why. Since hostile 
clients can call the no-event request, stopping them is not a reason. 
Non-hostile clients will avoid sending nonsense events to the present 
request.

I don't think this will happen, therefore the need for two calls here is 
false.