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

[Jasper St. Pierre]

I don't necessarily like this. The absence of a serial can have
radical meanings on behavior. Being able to pass 0 to mean "no serial"
anywhere we currently rely on a serial seems like poor design to me,
and can easily be done by mistake.

In cases where we have two behaviors for serial-aware and
non-serial-aware operations, I would rather have two different client
requests.

On Wed, Sep 30, 2015 at 12:47 PM, Daniel Stone <daniel at fooishbar.org> wrote:
> Hi,
>
> On 30 September 2015 at 16:59, Derek Foreman <derekf at osg.samsung.com> wrote:
>> Having an invalid serial number is quite useful - for example, we can
>> have protocol requests that optionally take a serial number or 0
>> instead of having two similar requests.
>>
>> Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
>> ---
>>
>> Well, let's see where this goes ;)
>>
>> I've seen a few times now when having an invalid serial number would be
>> helpful, is it too late to do this?  Are we going to break anything?
>>
>> Anything currently doing math on serial numbers can be wrong by one if one
>> of the serial numbers has wrapped around - this probably doesn't matter?
>>
>> I do wonder if we should have a wl_serial_compare() as part of wayland
>> but the semantics are tricky (a big serial and a small serial may actually
>> have happened quite close to eachother if the small happened after a
>> wrap around).
>>
>> I think the usual comparisons of interest are equality and "is A less than B
>> within (arbitrary distance to compensate for wrap-around)".  I think strcmp()
>> like semantics won't necessarily be good here because we don't usually need
>> something to tell us "b happened before a", more "these almost certainly
>> happened in the order specified in the function call".
>>
>> Where do we go from here?
>
> From the IRC discussion way back when:
> 20:27 < krh> if we do need to make a new serial, I'd much prefer to
> have it be 1 << 31
> 20:27 < krh> and then just restrict normal serial numbers space to 0 -
> (1<<31 - 1)
> 20:28 < krh> so that serial number increment is just serial = (serial
> + 1) & 0x7fffffff
>
> I have no relevant opinion on the matter, but I guess that 0x80000000
> (or 0xffffffff) is a much more obviously invalid serial number than 0,
> which could also just be the victim of something uninitialised.
>
> A helper would be nice, though like you say I think it has to be
> directional/multi-return (older inc. wraparound? identical?
> newer/error? invalid?). It'd probably be best to do that inside an
> active user and only then shuffle it into Wayland.
>
> So, with the change to a different serial, and a note in documentation
> somewhere that any extension relying on the invalid-serial behaviour
> must itself explicitly document that:
> Acked-by: Daniel Stone <daniels at collabora.com>
>
> Cheers,
> Daniel
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-- 
  Jasper