[swift-build-dev] Replace PkgConfig.swift with call to pkg-config binary

[Ankit Aggarwal]

Hey Cory,

Daniel and I talked a bit about this issue and we think it is best to drop
the current implementation of the pkg-config parser and always shell out.
In future, we would want to move towards a model where the pkg-config is
also expressed as dependencies in llbuild (using the llbuild APIs). This
would give us a lot of free things like caching, dependency management etc.
Moving towards that model will be a lot of work so right now lets just
implement the shelling out without any persistent cache. We can think about
the temporary cache if it seriously impacts the build performance.

On Thu, Nov 30, 2017 at 3:42 AM, Cory Benfield via swift-build-dev <
swift-build-dev at swift.org> wrote:

> Hey all,
>
> Over the past few weeks I’ve encountered some subtle issues with SwiftPM
> and the system pkg-config tool disagreeing over what compiler and linker
> flags should be provided on a given Linux system, as shown in SR-6317 (
> https://bugs.swift.org/browse/SR-6317). After some investigation on my
> part, I’d like to propose that we should replace SwiftPM’s hand-rolled
> pkgconfig parsing logic (in PkgConfig.swift) with a tool that simply shells
> out to the system pkg-config binary, if available.
>
> In general, on Linux, any system capable of compiling a SwiftPM project is
> going to have pkg-config installed. It’s an extremely common dependency in
> build toolchains, and the likelihood is that if your project wants to link
> against anything more esoteric than libc and the various Swift shared
> libraries pkg-config will have been installed anyway. If for some reason
> pkg-config is not present on the $PATH on such a system, that is likely a
> signal that pkg-config is not a good tool to use to populate the linker and
> compile flags *anyway*.
>
> Given that pkg-config is so widely deployed in Linux, it seems senseless
> to carry a reimplementation of that tool in SwiftPM. This is doubly true
> given that the pkg-config binary has a number of compile-time
> configurations applied by the system builder that give it special knowledge
> of the system’s default linker and include search paths. SwiftPM’s lack of
> that knowledge is what led to SR-6317, and rather than attempt to add
> complexity to SwiftPM’s implementation we would probably be better served
> by simply deferring to the implementation on the system that knows what to
> do best.
>
> My proposal is to replace the current implementation with one that shells
> out to pkg-config. If pkg-config cannot be found, then we simply fall back
> to the rest of the user configuration, exactly as if there were no
> pkg-config support at all.
>
> What do people think?
>
> Cory
>
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>
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