Automatic fallback to pkgconfig if `find_package` fails?

Development
1

Wherever dependencies are installed, CMake should be able to find them. OS package-manager, vcpkg | conan (and friends), explicit submodule, explicit FetchContent | file(DOWNLOAD.

Use-cases: much faster build times in CI, Docker, &etc. Switch between development mode (say, vcpkg) and production (say, OS deps). Use vcpkg on Windows and OS deps everywhere else.

Currently I create a pkg-config aware solution that works pretty well:

find_package(LibArchive QUIET)
if (NOT LibArchive_FOUND)
    find_package(PkgConfig)
    if (PkgConfig_FOUND)
        set(PKG_CONFIG_USE_CMAKE_PREFIX_PATH ON)
        pkg_search_module(LibArchive LibArchive)
    endif (PkgConfig_FOUND)
endif (NOT LibArchive_FOUND)

if (NOT LibArchive_FOUND AND NOT DEFINED LibArchive_QUIET)
    message(FATAL_ERROR "Could NOT find LibArchive")
endif (NOT LibArchive_FOUND AND NOT DEFINED LibArchive_QUIET)

…and sometimes I go pretty hardcore like with: FindCurlCustom.cmake. But it would be preferable to have a simple find_package for all scenarios, maybe with a PREFER keyword to choose which location to prefer it from (maybe a priority list).

What do you think?

2

Part of that picture has been discussed at some length in issue 21687. It covers integrating find_package() and FetchContent. I had to put that work on hold but am planning on returning to it once CMake 3.23 is released.

In big projects, I find that the time taken for dependency/package-related things often dominates the overall configure time. One of the limitations of find_package() is that it is inherently serial. FetchContent_MakeAvailable() or something like it, on the other hand, has the potential to allow fetching dependencies in parallel in the future. This matters when you start having sources downloaded or where a package manager hooks into the process (you may want to see the very long discussion in issue 22619 for more on that side topic).

3

One idea I had concerning this topic is, CMake could offer a feature that allows the user/project to specify a script to be run if find_package fails. This could be a way for package managers to hook into the process, and could also offer projects a way to fall back to FetchContent without manually writing find modules for every possible case where this fallback may be necessary.

4

There’s no good way of doing this. Questions to consider:

  • Given find_package(Foo), what is the .pc name associated with it? (Tricky examples include GTK mapping to gtk+-3.0.pc.)
  • If a component is requested, how does that map to a .pc file?
  • The proposed PREFER is just going to always be a variable that is forwarded from the cache (and possibly supplemented in code), but a static list isn’t something I see being useful to projects that can’t already just say “look here” in some other way. Basically, I don’t see this being any better than _ROOT or CMAKE_PREFIX_PATH which are already available.
  • As for user/project hooks, I think this is solely something for a package manager to do. I would rather just set paths or prefixes up rather than code up some imperative callback to do the same thing.
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