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Re: A message from CMake upstream: announcing dh-cmake

So, to clarify: we've changed VTK to use GNUInstallDirs, which *itself*
sets the proper directories for Debian, as I will explain below.

On Thu, 2018-07-05 at 15:38 +0100, Simon McVittie wrote:
> debhelper's Debian::Debhelper::BuildSystem::cmake already passes
> -DCMAKE_INSTALL_LIBDIR=lib/$DEB_HOST_MULTIARCH (among other options)
> to packages built using cmake, although for some reason it only does
> this when cross-compiling (this seems sufficiently odd that I've
> reported it as a bug).

The CMAKE_INSTALL_LIBDIR variable comes from the GNUInstallDirs module,
which already sets it to lib/$DEB_HOST_MULTIARCH if it detects a Debian
installation and if CMAKE_INSTALL_PREFIX is set to /usr. Though, as you
have pointed out, CMAKE_INSTALL_LIBDIR can be overridden if so desired.

> The most helpful thing that CMake could do here would be to have a
> predictable set of conventional installation paths, similar to the
> --libdir, --bindir etc. in Autotools and Meson, so that debhelper can
> define the same paths for all CMake-built packages and have the right
> things happen 99% of the time, as it already does for Autotools and
> Meson. If I understand correctly, CMake doesn't *necessarily* provide
> anything like this, but individual CMake-using projects can opt-in to
> it
> by using <https://cmake.org/cmake/help/latest/module/GNUInstallDirs.h
> tml>?
> (debhelper does set many of those variables, in the hope that the
> project
> being built uses GNUInstallDirs.)

Yes, we've changed VTK to use GNUInstallDirs for this exact reason.
GNUInstallDirs should be considered the standard set of installation
directories for CMake projects.

> In general I think we prefer upstream build systems to do something
> predictable rather than something clever, because we're usually going
> to be passing options to them anyway

I think we're on the same page here. In fact, another predictable thing
we'd like to see upstream buildsystems do is install their libraries in
the "Libraries" or "Runtime" component (via the install() command's
COMPONENT argument) and install the headers and symlinks in the
"Development" component. Then, Debian maintainers can use dh-cmake with
these standard options and have everything go in the correct packages.
Or, if it's a large project with lots of output packages, it could have
more fine-grained components like "example1-Development", "example2-
Development", etc., and then the Debian packaging can take this into
account. (This is exactly what we plan to do with VTK.)

The whole point of this project is to encourage upstream CMake projects
to follow a set of standards that are Debian-friendly (and distro-
friendly in general), so that they "do something predictable."