Building RPMs from source with mkosi

If you want to build an RPM from source and install it within a mkosi image, you can do that with mkosi itself without using mock. The steps required are as follows:

Install BuildRequires dependencies in the build overlay
Install dynamic BuildRequires dependencies in the build overlay
Build the RPM with rpmbuild
Install the built rpms in the image

In the following examples, we’ll use mkosi itself and its Fedora RPM spec as an example.

To keep things snappy, we execute the first 3 steps in a prepare script so that they’re cached on subsequent runs of mkosi if the Incremental= setting is enabled.

First, we need access to the upstream sources and the RPM spec and related files. These can be mounted into the current working directory when running mkosi scripts by using the BuildSources= setting. For example, in mkosi.local.conf, we could have the following settings:

[Build]
BuildSources=../mkosi:mkosi
             ../fedora/mkosi:mkosi/rpm
BuildSourcesEphemeral=yes

Which instructs mkosi to mount the local version of the mkosi upstream repository at ../mkosi to mkosi in the current working directory when running mkosi. The Fedora RPM spec is mounted at mkosi/rpm.

We enable the BuildSourcesEphemeral= option as rpmbuild will write quite a few files to the source directory as part of building the rpm which we don’t want to remain there after the build finishes.

We use rpmspec and rpmbuild, but these do not really support running from outside of the image that the RPM is being built in, so we have to make sure they’re available inside the image by adding the following to mkosi.conf:

[Content]
Packages=rpm-build
# If you don't want rpm-build in the final image.
RemovePackages=rpm-build

The prepare script mkosi.prepare then looks as follows:

#!/bin/sh
set -e

if [ "$1" = "final" ]; then
    exit 0
fi

mkosi-chroot \
    env --chdir=mkosi \
    rpmspec \
    --query \
    --buildrequires \
    --define "_topdir /var/tmp" \
    --define "_sourcedir $PWD/mkosi/rpm" \
    rpm/mkosi.spec |
        sort --unique |
        tee /tmp/buildrequires |
        xargs --delimiter '\n' mkosi-install

until mkosi-chroot \
    env --chdir=mkosi \
    rpmbuild \
    -bd \
    --noprep \
    --build-in-place \
    --define "_topdir /var/tmp" \
    --define "_sourcedir $PWD/mkosi/rpm" \
    --define "_build_name_fmt %%{NAME}-%%{VERSION}-%%{RELEASE}.%%{ARCH}.rpm" \
    rpm/mkosi.spec
do
    EXIT_STATUS=$?
    if [ $EXIT_STATUS -ne 11 ]; then
        exit $EXIT_STATUS
    fi

    mkosi-chroot \
        rpm \
        --query \
        --package \
        --requires \
        /var/tmp/SRPMS/mkosi-*.buildreqs.nosrc.rpm |
            grep --invert-match '^rpmlib(' |
            sort --unique >/tmp/dynamic-buildrequires

    sort /tmp/buildrequires /tmp/dynamic-buildrequires |
        uniq --unique |
        tee --append /tmp/buildrequires |
        xargs --delimiter '\n' mkosi-install
done

To install non-dynamic dependencies, we use rpmspec. What’s important is to set _sourcedir to the directory containing the RPM sources for the RPM spec that we want to build. We run rpmspec inside the image to make sure all the RPM macros have their expected values and then run mkosi-install outside the image to install the required dependencies. mkosi-install will invoke the package manager that’s being used to build the image to install the given packages.

We always set _topdir to /var/tmp to avoid polluting the image with rpmbuild artifacts.

After installing non-dynamic BuildRequires dependencies, we have to install the dynamic BuildRequires dependencies by running rpmbuild -bd until it succeeds or fails with an exit code that’s not 11. After each run of rpmbuild -bd that exits with exit code 11, there will be an SRPM in the SRPMS subdirectory of the rpm working directory (_topdir) of which the BuildRequires dependencies have to be installed. We retrieve the list of BuildRequires dependencies with rpm this time (because we’re operating on a package instead of a spec), remove all rpmlib style dependencies which can’t be installed and store them in a temporary file after filtering duplicates. Because the BuildRequires dependencies from the SRPM will also contain the non-dynamic BuildRequires dependencies, we have to filter those out as well.

Now we have an image and build overlay with all the necessary dependencies installed to be able to build the RPM.

Next is the build script. We suffix the build script with .chroot so that mkosi runs it entirely inside the image. In the build script, we invoke rpmbuild -bb --build-in-place to have rpmbuild build the RPM in place from the upstream sources. Because --build-in-place configures _builddir to the current working directory, we change directory to the upstream sources before invoking rpmbuild. Again, _sourcedir has to point to the RPM spec sources. We also have to override _rpmdir to point to the mkosi output directory (stored in $OUTPUTDIR). The build script mkosi.build.chroot then looks as follows:

#!/bin/sh
set -e

env --chdir=mkosi \
    rpmbuild \
    -bb \
    --noprep \
    --build-in-place \
    $([ "$WITH_TESTS" = "0" ] && echo --nocheck) \
    --define "_topdir /var/tmp" \
    --define "_sourcedir $PWD/mkosi/rpm" \
    --define "_rpmdir $OUTPUTDIR" \
    ${BUILDDIR:+--define} \
    ${BUILDDIR:+"_vpath_builddir $BUILDDIR"} \
    --define "_build_name_fmt %%{NAME}-%%{VERSION}-%%{RELEASE}.%%{ARCH}.rpm" \
    --define "_binary_payload w.ufdio" \
    --define "debug_package %{nil}" \
    --define "__brp_strip %{nil}" \
    --define "__brp_compress %{nil}" \
    --define "__brp_mangle_shebangs %{nil}" \
    --define "__brp_strip_comment_note %{nil}" \
    --define "__brp_strip_static_archive %{nil}" \
    rpm/mkosi.spec

The _vpath_builddir directory will be used to store out-of-tree build artifacts for build systems that support out-of-tree builds (CMake, Meson) so we set it to mkosi’s out-of-tree build directory in $BUILDDIR if one is provided. This will make subsequent RPM builds much faster as CMake or Meson will be able to do an incremental build.

Setting _binary_payload to w.ufdio disables compression to speed up the build. We also disable debug package generation using debug_package and various rpm build root policy scripts to speed up the build. Note that the build root policy macros we use here are CentOS/Fedora specific.

After the build script finishes, the produced rpms will be located in $OUTPUTDIR. We can now install them from the mkosi.postinst post-installation script:

#!/bin/sh
set -e

rpm --install "$OUTPUTDIR"/*mkosi*.rpm