yum-minimal: use DNF tools on host

The latest Debian bullseye release doesn't provide yum any more, only
DNF.  This breaks the minimal builds that are using on-host yum tools
to start the chroot.  Probe for yumdownloader, and if it's not there,
use DNF.

Note this requires "dnf download" which may not be packaged.  See
I21cfbd3935e48be4b92591ea36c7eed301230753 for a sample work-around
that installs this plugin in the nodepool-builder container.

Change-Id: Ia7f1e4d115cc67c378d865d91af94a07b8cdc6cc

bindep.txt

@@ -19,11 +19,12 @@ gnupg2 [!platform:redhat !platform:ubuntu-trusty !platform:suse]
 # ubuntu
 apt-transport-https [platform:dpkg]
 inetutils-ping [platform:dpkg]
-python-lzma [platform:dpkg !platform:ubuntu-focal]
+python-lzma [platform:dpkg !platform:ubuntu-focal !platform:debian-bullseye]
 qemu-utils [platform:dpkg]
 rpm [platform:dpkg]
 uuid-runtime [platform:dpkg]
-yum-utils [platform:dpkg !platform:ubuntu-focal]
+yum-utils [platform:dpkg !platform:ubuntu-focal !platform:debian-bullseye]
+dnf [platform:debian-bullseye]
 debootstrap [platform:dpkg]
 procps [platform:dpkg]
 

diskimage_builder/elements/yum-minimal/root.d/08-yum-chroot

@@ -46,6 +46,16 @@ trap "$EACTION" EXIT
 YUM_CACHE=$DIB_IMAGE_CACHE/yum
 mkdir -p $YUM_CACHE
 
+# Debian Bullseye and beyond only has DNF locally
+HOST_YUM_DOWNLOADER="yumdownloader"
+HOST_YUM="yum"
+if ! command -v yumdownloader &> /dev/null
+then
+    HOST_YUM_DOWNLOADER="dnf download"
+    HOST_YUM="dnf"
+fi
+
+
 # Note, on Debian/Ubuntu, %_dbpath is set in the RPM macros as
 # ${HOME}/.rpmdb/ -- this makes sense as RPM isn't the system
 # packager.  This path is relative to the "--root" argument
@@ -55,7 +65,7 @@ _RPM="rpm --dbpath=/var/lib/rpm"
 # chroot, which are needed to bootstrap yum/dnf
 #
 # note this runs outside the chroot, where we're assuming the platform
-# has yum/yumdownloader
+# has yum/yumdownloader/dnf download
 function _install_repos {
     local packages
     local rc
@@ -130,7 +140,7 @@ function _install_repos {
     # can clean it up nicely
     local temp_tmp
     temp_tmp=$(mktemp -d)
-    TMPDIR=${temp_tmp} yumdownloader --verbose \
+    TMPDIR=${temp_tmp} ${HOST_YUM_DOWNLOADER} --verbose \
         --releasever=${DIB_RELEASE/-*/} \
         --setopt=reposdir=$repo \
         --setopt=cachedir=$temp_tmp \
@@ -237,7 +247,7 @@ function _install_pkg_manager {
             sudo ln -s $TARGET_ROOT/etc/dnf/vars $TARGET_ROOT/etc/yum/vars
         fi
 
-        sudo -E yum -y \
+        sudo -E ${HOST_YUM} -y \
             --disableexcludes=all \
             --setopt=cachedir=$YUM_CACHE/$ARCH/$DIB_RELEASE \
             --setopt=reposdir=$TARGET_ROOT/etc/yum.repos.d \

releasenotes/notes/yum-minimal-dnf-63305c349bcccdd4.yaml

@@ -0,0 +1,6 @@
+---
+upgrade:
+  - |
+    This includes updates to allow `yum-minimal` elements to work on
+    host platforms that only supply DNF (e.g. Debian Bullseye).  Note
+    the `dnf download` plugin is required on these platforms.