[FFmpeg-devel] [PATCH v2 0/3] hwcontext_vaapi: dlopen libva-x11 and libva-drm

[Emil Velikov]

On Wed, 27 Jul 2022 at 20:51, Emil Velikov <emil.l.velikov at gmail.com> wrote:
>
> On Thu, 21 Jul 2022 at 21:47, Mark Thompson <sw at jkqxz.net> wrote:
> >
> > On 20/07/2022 17:41, Emil Velikov wrote:
> > > On Tue, 19 Jul 2022 at 19:16, Nicolas George <george at nsup.org> wrote:
> > >>
> > >> Emil Velikov (12022-07-19):
> > >>> As you may know the libva* set of libraries share an internal ABI
> > >>> between them. In a resent libva commit, the va_fool API was removed.
> > >>>
> > >>> Thus if one is to mix different versions of libva.so and libva-x11.so
> > >>> they will get an error, leading to a crash of the whole stack.
> > >>>
> > >>> The simple solution is
> > >>
> > >> ... a configure check.
> > >>
> > >> If the person who installs replaces a library with another, it is their
> > >> responsibility to check they are compatible.
> > >>
> > >
> > > While I wholeheartedly agree, it's not so easy to enforce compile time
> > > decisions at runtime. In the past, I have debugged and reported issues
> > > where Linux distributions do not enforce the above.
> > >
> > > We do have the typical Linux distribution model (where we have dozens
> > > upon distros) and other distribution models. IMHO checking each
> > > instance and combination doesn't scale. We could bring awareness to
> > > the issue in say distribution/workflow X, which sadly may come as
> > > finger-pointing and thus alienating.
> > >
> > > Hope that makes sense and the team is willing to consider the extra 90
> > > lines worth of code.
> >
> > The argument "libfoo can be broken in some particular configuration, so lets use dlopen() to make errors happen later" seems like it applies to every library.  Why is this case so special?  Who are the users running into this specific problem and who are stuck with broken versions they can't update?
> >
> It's a long story, hope I don't bore you to death :-P
>
> Even though I've been itching to hack on ffmpeg for a while, the bug
> that allowed me to do that is
> https://github.com/ValveSoftware/steam-for-linux/issues/8673
>
> As a background, steam as well as some of the programs/games shipped
> use libraries provided by ffmpeg. In addition, steam ships with a
> steam runtime, which is effectively a partial chroot of an old Ubuntu.
> For various compatibility reasons, one cannot simply update it, so the
> startup scripting will try and promote a set of the host libraries (if
> newer) so that they're used instead of the bundled Ubuntu ones.
>
> What happens in the libva case is that distributions can provide only
> libva.so and omit libva-x11.so. Which due to the internal ABI break
> (removal of the va_fool API), means that steam and likely some games
> will simply crash out.
>
> Now let me try and draw an analogy to another set of libraries which
> also share internal ABI - libdrm.so, libdrm_nouveau.so,
> libdrm_amdgpu.so libdrm_intel.so, etc. To the best of my knowledge
> there was no breakage in there be that internal or public ABI.
>
> In addition, while distribution may allow you to install only some
> (say libdrm.so without libdrm_intel.so), a pair of those is pulled by
> the respective GL and Vulkan drivers. For example: the amdgpu GL
> driver (amdgpu_dri.so) and radv Vulkand driver (libvulkan_radeon.so)
> depend on libdrm_amdgpu.so and libdrm.so. Hence, in practical terms
> users cannot hit a similar issue... unless they very very deliberately
> try to do so.
>
> So while one solution is to go around telling users and distributions
> that they're "doing it wrong", IMHO a more pragmatic solution is to
> include this brief workaround in ffmpeg. At least in the short to mid
> term.
> As mentioned in the cover letter (sorry again for sending the series
> multiple times), I have some plans for a proper long term, which would
> reside in libva. Alas as you have experienced yourself, the libva
> maintainers can be rather busy, so we're looking at least a couple of
> months until a new libva release is out and further X months, until it
> ripples down to end-users.
>

Mark, humble ping?

Can you kindly let me know if the above argument seems reasonable and
more importantly if it even makes sense. I am more than happy to
provide more details and elaborate, if anything is unclear.

Thanks
Emil