Fix Nix pkg-config support

[drawbu]

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Following https://github.com/SFML/SFML/pull/2835

https://github.com/NixOS/nixpkgs/pull/312161#discussion_r1629625142

[ChrisThrasher]

Why not match what SFML does? SFML retains ${exec_prefix}/. I'm only loosely familiar with this situation but isn't Nix weird about treating paths in a way that no other OS does, hence these kinds of issues?

[drawbu]

Hello ! Sorry, that pr was in my draft for a long time. I apologize for not providing a lot of context around it...

I actually tested with both libdir=@CMAKE_INSTALL_FULL_LIBDIR@ and libdir=${exec_prefix}/@SFML_RELATIVE_INSTALL_LIBDIR@ (like in the SFML pr)

Both worked fine (on my end) when I tested, but do you prefer the second option?

[ChrisThrasher]

I prefer the option which is most in line with SFML and requires the least possible changes to CSFML.

[drawbu]

From what I understand, CMAKE_INSTALL_LIBDIR is not really precise in content, so using CMAKE_INSTALL_FULL_LIBDIR and CMAKE_INSTALL_RELATIVE_LIBDIR are prefered.

And for the nix issue, it appears that CMAKE_INSTALL_LIBDIR is set to be the full path, so appending it to ${exec_prefix}/ create weird path

[drawbu]

Okay I'll change that :)

[texus]

The code shouldn't blindly be based on what SFML is using, because that implementation is also partially broken (https://github.com/SFML/SFML/pull/2917).

~~I would expect that ${exec_prefix}/@CSFML_PKGCONFIG_INSTALL_DIR@ would be the correct path when CSFML_PKGCONFIG_INSTALL_DIR is a relative path, but that is probably wrong again when CSFML_PKGCONFIG_INSTALL_DIR is set to an absolute path.~~ Nevermind, libdir needs to be the path to the libraries and not to the pkg-files.

[texus]

I can confirm that this PR solves the issue of libdir being wrong when an absolute path is set in CMAKE_INSTALL_LIBDIR. However I'm not certain if it is the best solution, and I have found an unrelated issue with the pkgconfig generation during testing.

The following is generated inside the .pc files when calling cmake with -DCMAKE_INSTALL_LIBDIR=/home/texus/some_dir:

• Without this PR: libdir=${exec_prefix}//home/texus/some_dir

• With this PR: libdir=${exec_prefix}/../../home/texus/some_dir

• When using libdir=@CMAKE_INSTALL_FULL_LIBDIR@ instead of libdir=${exec_prefix}/@CSFML_RELATIVE_INSTALL_LIBDIR@: libdir=/home/texus/some_dir

So I actually like the CMAKE_INSTALL_FULL_LIBDIR option better: it doesn't require an additional variable. The file(RELATIVE_PATH ...) call in cmake/Config.cmake isn't needed with this method.

---

There is also an unrelated issue: when the prefix that is passed during installation is different from when configuring, the prefix variable in the .pc file will still point to the old location. So when running the following commands, the paths in the .pc files will point to /usr/local instead of to the whatever folder. This is because in the second command the CMakeLists.txt does not seem to be executed again, so configure_file is never called with the new paths.

cmake --build build
cmake --install build --prefix whatever

The same issue also exists when using CMAKE_INSTALL_FULL_LIBDIR instead of relying on the prefix variable. This is even documented in the CMAKE_INSTALL_FULL_<dir> documentation:

they do not work with the cmake --install command's --prefix option

Edit: This seems to be an issue with only hacky solutions (see https://discourse.cmake.org/t/how-to-generate-pc-pkg-config-file-supporting-prefix-of-the-cmake-install/4109 and https://gitlab.kitware.com/cmake/cmake/-/issues/24887), so maybe we can just ignore it and assume that people who want pkg-config files will set CMAKE_INSTALL_PREFIX in a normal way instead of setting a different prefix at install time.

[drawbu]

@ChrisThrasher what's your opinion regarding this @CMAKE_INSTALL_FULL_LIBDIR@ vs ${exec_prefix}/@CSFML_RELATIVE_INSTALL_LIBDIR@ situation ?