[waffle] [PATCH] wayland: Wrap wl_proxy_marshal_constructor_versioned v2

[Emil Velikov]

On 14 April 2016 at 09:23, Michel Dänzer <michel at daenzer.net> wrote:
> From: Michel Dänzer <michel.daenzer at amd.com>
>
> Fixes build failure due to wl_proxy_marshal_constructor_versioned being
> unresolved when building against current wayland.
>
> This API was introduced in wayland 1.9.91 by commit 557032e3 ("Track
> protocol object versions inside wl_proxy."). The waffle code doesn't
> reference wl_proxy_marshal_constructor_versioned directly but
> indirectly via wayland-scanner.
>
> v2:
> * Add paragraph about how wl_proxy_marshal_constructor_versioned was
>   introduced. (Emil Velikov)
> * Only resolve wl_proxy_marshal_constructor_versioned with wayland >=
>   1.9.91.
>
> Signed-off-by: Michel Dänzer <michel.daenzer at amd.com>
> ---
>  src/waffle/wayland/wayland_wrapper.c | 5 +++++
>  src/waffle/wayland/wayland_wrapper.h | 8 ++++++++
>  2 files changed, 13 insertions(+)
>
> diff --git a/src/waffle/wayland/wayland_wrapper.c b/src/waffle/wayland/wayland_wrapper.c
> index 6ffd5a9..fb66f9a 100644
> --- a/src/waffle/wayland/wayland_wrapper.c
> +++ b/src/waffle/wayland/wayland_wrapper.c
> @@ -106,6 +106,11 @@ wayland_wrapper_init(void)
>      RETRIEVE_WL_CLIENT_SYMBOL(wl_proxy_add_listener);
>      RETRIEVE_WL_CLIENT_SYMBOL(wl_proxy_marshal);
>      RETRIEVE_WL_CLIENT_SYMBOL(wl_proxy_marshal_constructor);
> +#if WAYLAND_VERSION_MAJOR == 1 && \
> +    (WAYLAND_VERSION_MINOR > 9 || \
> +     (WAYLAND_VERSION_MINOR == 9 && WAYLAND_VERSION_MICRO >= 91))
> +    RETRIEVE_WL_CLIENT_SYMBOL(wl_proxy_marshal_constructor_versioned);
> +#endif
>  #undef RETRIEVE_WL_CLIENT_SYMBOL
>
I am slightly worried about this approach. It adds a so called 'hidden
dependency' and with it a possibility of things going horribly wrong.
It is something that we try to avoid with mesa as the deps version at
build time != run-time one. Or in other words, one might build against
wayland 1.9 and things will go crazy as you run wayland 1.10, or vice
versa.

Obviously that's not perfect, although unavoidable. Why ? As distros
do not know about the requirement (i.e. it's not mandated at configure
time) they won't rebuild and things won't work. At the same time if
they do rebuild (again without the explicit requirement), things will
break if one needs to revert to older (yet still in version range as
per the deps list) wayland.

TL;DR: The situation is quite sensitive and fragile. The only robust
solutions that I can think of are: a) non-fatal (only for newer
symbols) dlsym or b) bumping the req. version at configure time.

-Emil