Cmake plugin and ninja backend

joseph.wakeling · June 16, 2018, 9:51am

Hello all,

Recently I discovered some build issues in a project that I create a snap package for: when make was used as the cmake backend, there were race conditions that caused the build to fail.

Switching to the ninja backend (i.e. cmake -GNinja), as supplied by the ninja-build deb package, solved the race conditions, but required some workarounds: I had to create a custom override-build script that completely excluded the regular snapcraftctl build. The solution I came up with is here:

… but in short amounts to,

cmake ../src [OTHER_OPTIONS]
ninja
DESTDIR=../install ninja install

The reason this is necessary is because the cmake plugin hardcodes the assumption of a make backend; in fact, it explicitly derives from the make plugin:

github.com

snapcore/snapcraft/blob/e506e76077851c39b22c47426cd3cf64438940ee/snapcraft/plugins/cmake.py#L42


      (list of strings)
      configure flags to pass to the build using the common cmake semantics.
"""
import os
import shutil
import snapcraft.plugins.make
class CMakePlugin(snapcraft.plugins.make.MakePlugin):
    @classmethod
    def schema(cls):
        schema = super().schema()
        schema['properties']['configflags'] = {
            'type': 'array',
            'minitems': 1,
            'uniqueItems': True,
            'items': {
                'type': 'string',

… and invokes that plugin’s make method to actually build:

github.com

snapcore/snapcraft/blob/e506e76077851c39b22c47426cd3cf64438940ee/snapcraft/plugins/cmake.py#L85


    if source_subdir:
        sourcedir = os.path.join(self.sourcedir, source_subdir)
    else:
        sourcedir = self.sourcedir
    env = self._build_environment()
    self.run(['cmake', sourcedir, '-DCMAKE_INSTALL_PREFIX='] +
             self.options.configflags, env=env)
    self.make(env=env)
def _build_environment(self):
    env = os.environ.copy()
    env['CMAKE_PREFIX_PATH'] = '$CMAKE_PREFIX_PATH:{}'.format(
        self.project.stage_dir)
    env['CMAKE_INCLUDE_PATH'] = '$CMAKE_INCLUDE_PATH:' + ':'.join(
        ['{0}/include', '{0}/usr/include', '{0}/include/{1}',
         '{0}/usr/include/{1}']).format(
             self.project.stage_dir, self.project.arch_triplet)
    env['CMAKE_LIBRARY_PATH'] = '$CMAKE_LIBRARY_PATH:' + ':'.join(

This seems worth submitting a feature request on launchpad for (the cmake plugin should probably support multiple backends, probably by using composition with a plugin for that backend rather than inheritance) but I thought I’d raise the concern here first in case anyone has any interesting/relevant comments. Alternatively it might be possible to use more generic cmake commands (cmake --build for example) which invoke the backend under the hood.

I’d also be interested in people’s thoughts on my custom solution and whether it can be improved.

Thanks & best wishes,

  -- Joe