regression in meson build, AC_PATH_PROG lost

[Pavel Hrdina]

On Thu, Nov 12, 2020 at 10:40:02PM +0100, Olaf Hering wrote:
> autoconf allows to specify the path to a runtime binary that is not required during build via an environment variable:
> AC_PATH_PROG([PARTED], [parted], [], [$LIBVIRT_SBIN_PATH])
> 
> meson lacks such essential feature. As a result the package build environment needs to have more packages than necessary installed. This excessive list also pushes libvirt closer to the end of the build dependency chain.
> 
> Was this considered while the buildsystem was switched to meson?

Hi,

I did not considered this specific use-case where the runtime executable
dependency is not available during build time. Mainly because we have
a lot of other executables that are runtime dependencies but are check
by meson to figure out if some libvirt feature should be enabled or
disabled.

> Since meson does not support environment variables it seems the only way to address this is to introduce an option in meson_options.txt for each runtime executable.
> 
> Will such change be accepted?

This would be one way how to address runtime executables, currently we
have a lot of them. These executables are checked in order to
enable/disable some libvirt feature but is used only in runtime:

    parted, bhyve, bhyvectl, bhyveload, mount, umount, mkfs, pvcreate,
    vgcreate, lvcreate, pvremove, vgremove, lvremove, lvchange, vgchange,
    vgscan, pvs, vgs, lvs, (collie|dog), vstorage, vstorage-mount, zfs,
    zpool, numad

This one is even more broken as if we don't find it during build time it
is set to empty string and never checked in our code:

    showmount

These are checked during build time but if not present they are set to
known absolute path:

    qemu-bridge-helper, qemu-pr-helper, slirp-helper, dbus-daemon

And we have a list of optional_programs that are checked during build
time and if not present they are set to the name of the executable and
resolved during runtime from $PATH.

The last executable, pkcheck, is not used during build and in runtime.
We only check its presence to enable/disable polkit support. We should
be able to simply drop this check and figure out the presence of polkit
using DBus only as we do for machined or logind and other DBus services
that we use.

All of this was copied from autoconf except for the fact that it is no
longer possible to use ENV variables.

I think we need to unify the process how we handle runtime executable
dependencies and possibly add meson options for them.

Pavel
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