Re: [dpdk-dev] [dpdk-stable] [PATCH 2/2] build: use dependency() instead of find_library()

[Luca Boccassi <bluca@debian.org>]

On Mon, 2019-01-07 at 14:28 +0000, Bruce Richardson wrote:
> On Thu, Jan 03, 2019 at 06:57:25PM +0100, Luca Boccassi wrote:
> > Whenever possible (if the library ships a pkg-config file) use
> > meson's
> > dependency() function to look for it, as it will automatically add
> > it
> > to the Requires.private list if needed, to allow for static builds
> > to
> > succeed for reverse dependencies of DPDK. Otherwise the recursive
> > dependencies are not parsed, and users doing static builds have to
> > resolve them manually by themselves.
> > When using this API avoid additional checks that are superfluos and
> > take extra time, and avoid adding the linker flag manually which
> > causes
> > it to be duplicated.
> > 
> > An internal checker has been added to Meson 0.42 to detect libpcap,
> > which ships a custom tool rather than a pkg-config file, so bump
> > the
> > minimum Meson version from 0.41 to 0.42.
> 
> If we are going to bump the version, I think we should bump it
> further to
> e.g. 0.46 or 0.47 unless there is anyone who still wants an earlier
> version. That should get rid of a number of the meson version
> warnings we
> see on each run.

The distros situation, as far as I can see:

Debian 10 0.49
Ubuntu 18.04 LTS 0.45
Ubuntu 18.10 0.47
Fedora 27 0.43
Fedora 28 0.45
SUSE Leap 15 0.46
FreeBSD 10 0.46
CentOS 7 0.47

So by bumping to 0.47, required to fix the bug below, we'd leave behind
a fair few distros.

Now for me it's fine to go to 0.47 - but I think the CC to stable
should then be removed.

> > 
> > For libbsd, which is checked in a top level file and used to be
> > added
> > to the global linker flags array, add it to the ext_deps array of
> > all top level meson files (app, test, lib, examples, drivers). The
> > most correct change would be to let each individual
> > library/driver/app
> > depend on it individually if they use symbols from it, but it would
> > diverge from the legacy Makefile's behaviour and make life a bit
> > more
> > difficult for contributors.
> 
> It shouldn't be necessary to add libbsd as a dependency for
> everything. I
> think just adding it as a dependency of EAL should work fine. 

Won't that mean that the shared libraries other than EAL will have
undefined references?
Now, in practice it would be fine because I'm pretty sure none of them
can and would actually be used without EAL, so when linking executables
everything will be fine, but for example the Debian build tools will at
the very least print warnings if a shared library links

> However, in
> conjunction with meson version checks, I believe this was done this
> way
> originally because of a meson bug which caused recursive dependencies
> for
> things like this to get duplicated many times in the build.ninja
> file.
> 
> https://github.com/mesonbuild/meson/issues/2150
> 
> If we take the approach of adding bsd explicitly using dependency
> object
> our minimum version needs to have the fix for this bug included.

Ah that's not nice. Just verified, and it happens with dependency() as
well as find_library(). It was fixed in 0.47.1.

> > 
> > Fixes: a25a650be5f0 ("build: add infrastructure for meson and ninja
> > builds")
> > Cc: stable@dpdk.org
> > 
> > Signed-off-by: Luca Boccassi <bluca@debian.org>
> > ---
> > 
> > Bruce, dependency() by default tries pkg-config first, then cmake,
> > then
> > the internal project-specific finders (like pcap). If you think
> > it's
> > worth it I can add fallbacks in case a system, for whatever reason,
> > does not install a pc file despite the upstream project providing
> > one.
> > It would add more clutter and more verbosity, but it would not
> > cause
> > other issues.
> 
> I'd prefer to keep it without that for now. If the lack of .pc files
> causes
> issues we can revisit.
> 
> /Bruce

Ok.

-- 
Kind regards,
Luca Boccassi