cmake subproject dependencies not working

Describe the bug DOSBox Staging is transitioning to meson. We depend on FluidSynth (2.x, a CMake project), which in turn depends on Glib-2.0 (a Meson project), both of which aren’t guaranteed to exist on all our supported platforms - so we’ve written Meson wraps for them.

When both FluidSynth and Glib do not exist on a system, the fluidsynth subproject is not being provided the glib-2.0 dependencies generated from the glib subproject.

Here’s how it plays out sequentially if Glib doesn’t exist:

glib_dep = dependency('glib-2.0', fallback : ['glib'])
gthread_dep = dependency('gthread-2.0', fallback : ['glib'])

All goes well – Meson falls back to the glib subproject wrap and reports the dependencies are found:

Run-time dependency glib-2.0 found: NO (tried pkgconfig and cmake)
Looking for a fallback subproject for the dependency glib-2.0
|Executing subproject glib method meson 
|Project name: glib
|Project version: 2.67.1
... 
Dependency glib-2.0 found: YES 2.67.1 (overridden)
Dependency gthread-2.0 found: YES 2.67.1 (overridden)

Great - so Meson is taking care of that for us. Onto FluidSynth…

Executing subproject fluidsynth method cmake 
||Configuring the build directory with CMake version 3.16.3
||-- Checking for modules 'glib-2.0>=2.6.5;gthread-2.0>=2.6.5'
||--   No package 'glib-2.0' found
||--   Package 'glib-2.0', required by 'gthread-2.0', not found
||  A required package was not found

So despite Meson telling us it has control of glib-2.0 found: YES 2.67.1 and gthread-2.0 found: YES 2.67.1, it’s unable to provide them to the cmake subproject, despite FluidSynth being configured it with these dependencies:

fluidsynth_proj = cmake.subproject('fluidsynth')
fluidsynth_proj.dependency('glib-2.0', fallback: ['glib'])
fluidsynth_proj.dependency('gthread-2.0', fallback: ['glib'])

fluidsynth_dep = fluidsynth_proj.dependency(
  'fluidsynth',
  version : '>=2',
  fallback: ['fluidsynth'],
  dependencies: ['glib-2.0', 'gthread-2.0']
)

To Reproduce

See attached tarball consisting of tiny 19-line meson.build and two wraps to reproduce the issue.

cmake-subproject-deps.tar.gz

Note: To have Meson fallback to using the Glib subproject, you will need to hide your system’s existing Glib library. You can do this temporarily by moving glib’s pkg-config (.pc) file, such as:

cd /usr/lib/x86_64-linux-gnu/pkgconfig && sudo mv glib-2.0.pc glib-2.0.pc-bak

Expected behavior

If Meson says it has a given dependency (via system discovery or subproject fallback), then subsequently using that dependency (such as in other subprojects) should “just work”.

I guess this means that Meson doesn’t know how to provide dependencies to cmake.subprojects?

system parameters

• Native Ubuntu 20.10 installation
• Python 3.8.6
• Meson 0.56.0
• Ninja 1.10.0.git.kitware.jobserver-1

[mensinda]

OK, I managed to get -uninstalled.pc files to work with #8104. However, the project still has some minor issues (somehow the include path for <gmodule.h> is missing + a tricky bug involving generated source files):

project('test', ['c', 'cpp'])
glib_dep = dependency('glib-2.0', fallback : ['glib'])
gthread_dep = dependency('gthread-2.0', fallback : ['glib'])

cmake = import('cmake')

cm_opts = cmake.subproject_options()
cm_opts.add_cmake_defines({'enable-ladspa': false})

fluidsynth_proj = cmake.subproject('fluidsynth', options: cm_opts)

meson build
ninja -C build subprojects/fluidsynth/gentables-install
ninja -C build

[kcgen]

Maybe it would make sense to automatically add the meson-uninstalled directory to the pkg_config_path for at least cmake subprojects. Less manual intervention for the user trying to do the build.

Yes please 😃 Otherwise we’ll be wrapping meson in shell script to do this.

Ideally we could describe the dependencies of a cmake.subproject, something like:

subproj = cmake.subproject(
    'my_subproj',
    dependencies : [ 'glib-2.0', 'gthread-2.0'])

Where meson has previously been told how to resolve those dependencies:

glib_dep = dependency('glib-2.0', fallback : ['glib'])
gthread_dep = dependency('gthread-2.0', fallback : ['glib'])

• If those are available in the system, then carry on.
• If they’re provided via fallback, then add the meson-uninstalled directory for the respective dependency to the respective cmake-subproject’s pkg_config_path