[question] How to separate generator files by requirements and tool requirements

[prince-chrismc]

In https://github.com/conan-io/conan-center-index/pull/13612

We need to do some hackery in the build script to avoid both the dependencies and build requirements (which overlap) PkgConfigDeps being in the same folder.

@jwillikers implemented two different solutions

• one with PkgConfigDeps.build_context_suffix and patching the Meson build script to use the new name.
• one with manually copying the files to a seperate folder

Ideally there would be a generator folder for each for the advance case where we need to seperate them.

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I'll get an error if I don't set the build_context_suffix for the wayland package because this library requires the wayland-client library from the wayland package as a regular requires while simultaneously requiring the wayland-scanner tool from the wayland package as a tool_requires.

If we were able to place the build-context pkg-config files in a separate directory from the others, this would avoid naming conflicts and the necessary patching to find the tool_requires dependencies. Meson would be able to handle this gracefully just by setting the standard project option build.pkg_config_path to a value including the directory for the build-context pkg-config files.

Originally posted by @jwillikers in https://github.com/conan-io/conan-center-index/pull/13612#discussion_r999896514

[memsharded]

I'd like to understand one thing: is the wayland that is used via a tool_require some executable that must be used in the build, or is it a library to link with?

Because in that PR I see:

if self._has_build_profile and self.options.get_safe("with_wayland"):
            self.tool_requires("wayland/1.21.0")
            self.tool_requires("wayland-protocols/1.27")

But the with_wayland option is not removed from package_id(), so it means that it is relevant to create a different binary, not just a tool that is necessary? I don't know much about wayland...

[jwillikers]

@memsharded It's used as both a regular requires and a tool_requires in this case. It provides both a library and a tool known as wayland-scanner that generates source code from XML files.

[jwillikers]

It's worth noting that several projects locate the wayland-scanner utility through its pkg-config file, including the wayland package itself.

There's also an important problem with the current build_context_suffix. It doesn't automatically provide the necessary pkg-config files for dependencies in the build context, so it's actually mixing pkg-config files from the build and host contexts which is concerning.

To illustrate, the wayland-scanner component requires libxml2::libxml2 and expat::expat. The associated pkg-config names are listed in the pkg-config file generated for wayland-scanner. The build context version of wayland-scanner still contains the pkg-config names for expat and libxml2 in the host context.

When attempting to use a separate build context directory for the pkg-config files in the build context, I think it's necessary to include all the required pkg-config files there so that pkgconf doesn't fail to find the dependency when the dependency is required in the build context and set as required in Meson.

[jwillikers]

Update: It looks like updates for the Flex recipe are facing a similar conundrum, but regarding supplied CMake modules: conan-io/conan-center-index#14013

[memsharded]

This couldn't be prioritized into 1.X, but I am moving it to 2.X, because it will get more attention there.

I think this is doable by introducing a PkgConfigDeps.build_prefix, lets give it a try.

[memsharded]

Implemented in https://github.com/conan-io/conan/pull/15813 for next 2.2