Re: [PATCH 2/2] build: use dependency() instead of find_library()

[Bruce Richardson <bruce.richardson@intel.com>]

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.

> 
> 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. 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.

> 
> 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

>