[PATCH wayland 2/3] scanner: Allow adding a prefix to exported symbols

[Derek Foreman]

On 2017-08-18 07:41 AM, Pekka Paalanen wrote:
> On Fri, 28 Jul 2017 14:06:23 +0100
> Emil Velikov <emil.l.velikov at gmail.com> wrote:
> 
>> On 27 July 2017 at 14:01, Pekka Paalanen <ppaalanen at gmail.com> wrote:
>>> On Wed, 26 Jul 2017 16:09:32 +0100
>>> Emil Velikov <emil.l.velikov at gmail.com> wrote:
>>>   
>>>> On 25 July 2017 at 10:24, Pekka Paalanen <ppaalanen at gmail.com> wrote:
>>>>> On Tue, 25 Jul 2017 15:25:58 +0800
>>>>> Jonas Ådahl <jadahl at gmail.com> wrote:
>>>>>   
>>>>>> On Mon, Jul 24, 2017 at 02:16:04PM +0300, Pekka Paalanen wrote:
>>>>>>> On Mon,  3 Jul 2017 17:16:45 +0800
>>>>>>> Jonas Ådahl <jadahl at gmail.com> wrote:
>>>>>>>   
>>>>>>>> Two different protocols may use interfaces with identical names.
>>>>>>>> Implementing support for both those protocols would result in symbol
>>>>>>>> clashes, as wayland-scanner generates symbols from the interface names.
>>>>>>>>
>>>>>>>> Make it possible to avoiding these clashes by adding a way to add a
>>>>>>>> prefix to the symbols generated by wayland-scanner. Implementations
>>>>>>>> (servers and clients) can then use these prefix:ed symbols to implement
>>>>>>>> different objects with the same name.
>>>>>>>>
>>>>>>>> Signed-off-by: Jonas Ådahl <jadahl at gmail.com>
>>>>>>>> ---
>>>>>>>>
>>>>>>>> Something like this would be needed if a compositor/client wants to implement
>>>>>>>> xdg-shell unstable v5 alongside xdg-shell stable, unless we want to rename all
>>>>>>>> our xdg-shell interfaces. Implementing xdg-shell unstable v6 alongside
>>>>>>>> xdg-shell stable does not have this issue.
>>>>>>>>
>>>>>>>> See issue raised here:
>>>>>>>> https://lists.freedesktop.org/archives/wayland-devel/2017-June/034380.html
>>>>>>>>
>>>>>>>>
>>>>>>>> Jonas
>>>>>>>>
>>>>>>>>
>>>>>>>>   src/scanner.c | 94 ++++++++++++++++++++++++++++++++++++++++++++++-------------
>>>>>>>>   1 file changed, 73 insertions(+), 21 deletions(-)
>>>>>>>
>>>>>>>
>>>>>>> Hi,
>>>>>>>
>>>>>>> while this seems to change the ABI symbol names, it does not change the
>>>>>>> names in the documentation, and it does not change the names of
>>>>>>> #defines of enums, or the inline functions. That means that this is not
>>>>>>> enough to fulfill the purpose: being able to use two similarly named
>>>>>>> but different protocols by adding a prefix.
>>>>>>
>>>>>> The idea I had was rather that one would avoid changing any names on
>>>>>> non-symbols. It'd still be possible to implement both by doing it in
>>>>>> separate C files. I can see the point in adding the prefix on everything
>>>>>> though, so I'll provide a patch for that.
>>>>>>   
>>>>>
>>>>> Hi,
>>>>>
>>>>> recapping the discussion from IRC, we pretty much agreed that prefixing
>>>>> is not a nice solution. Jonas found out that we cannot actually prefix
>>>>> everything, because there usually are references to other protocol
>>>>> things (like you would never want to prefix wl_surface). So it becomes
>>>>> very hard to prefix things appropriately.
> 
> ...
> 
>> Tl:Dr; I think that everything should be prefixed. See below for details.
>>
>>   - ABI symbols - to prevent symbol collision
>>   - inline/other API - to minimise ambiguity, confusion, plus you can
>> have multiple implementations in the same translation unit*
>>
>>
>> *Some projects may want to support xdg-shell vX and vX+1 in the same file.
>>   - The inline API from vX may work with vY or vise-versa. Yet it's
>> easier to pass the wrong object to the inline function.
>>   - One can (and will have) cases, where API foo() is available for
>> only one version.
>>   - If the function signature (of the inline API) differs the compiler
>> will barf at you - say vX+1 adds extra argument to function bar()
> 
> Yes, I agree, which is why I was not too fond of this patch originally.
> This patch does not prefix API, but only ABI.
> 
> The difficulty is in what interfaces to prefix as a whole (ABI and API)
> and what not. As an example, one would never want to prefix wl_surface,
> yet extension interfaces use wl_surface as a message argument.
> 
> Jonas, I think we can discriminate between interfaces (protocol object
> types) defined in the XML file being processed vs. external interfaces
> from elsewhere (like wl_surface). Would that be good enough for
> prefixing not just #defines and inline functions but the C types as
> well?
> 
> The case where it would not work if one wants to prefix things from
> both one.xml and two.xml, and two.xml used interfaces defined in
> one.xml. Is that an acceptable limitation?
> 
> As a stretch I'd rather avoid for now, there could be an option to
> prefix all external interfaces with another prefix, but it would not be
> able to make a difference between things from wayland.xml and one.xml,
> for instance.

I think I'd like to avoid that too...

> Where are we at with the name collision problem in general now?

Bit of a necropost, but was any kind of consensus in reach here? :)

Emil's nominated this for inclusion in an upcoming release, and it would 
certainly get us out of the xdg-shell-v5 vs xdg-shell-stable collisions 
we face now (if we intend to have weston support xdg-shell-stable 
without dropping v5 at some point).

Does this have a strong use case outside of just dealing with the v5 vs 
stable thing?  We could probably solve that with a sed script (in 
wayland-protocols?)

Thanks,
Derek

> 
> Thanks,
> pq
> 
> 
> 
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