ubuntu-minimal doesn't find focal debootstrap script on CentOS 7

[Eduardo Santos]

When trying to build an image with the ubuntu-minimal element and the DIB_RELEASE="focal" variable on a CentOS 7 host, diskimage-builder exits with the following error:

E: No such script: /usr/share/debootstrap/scripts/focal

The official CentOS 7 repositories do not provide the debootstrap package, which is a dependency for the ubuntu-minimal element. The simplest solution for this problem is to add the EPEL repository to the system, which provides the debootstrap package.

However, this solution is also in itself the source of the above error. The package version of debootstrap from EPEL is outdated and does not include the script for Ubuntu Focal.

Inspecting the source code of debootstrap will reveal that the script for Ubuntu Focal (scripts/focal) is just a symlink to the script for Ubuntu Gutsy (scripts/gutsy).

Therefore, the final solution, although ad hoc, would be to recreate the same symlink on the CentOS 7 host.

This is not a diskimage-builder bug, but I believe this issue should be addressed in the documentation so that users can apply this solution, even if it's not the ideal one, and help reduce their time spent debugging.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/diskimage-builder/+/806308

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/diskimage-builder/+/806308
Committed: https://opendev.org/openstack/diskimage-builder/commit/5779fa85256ebe35896c220714eb755844f564d3
Submitter: "Zuul (22348)"
Branch: master

commit 5779fa85256ebe35896c220714eb755844f564d3
Author: Eduardo Santos <email address hidden>
Date: Fri Aug 27 02:22:08 2021 -0300

    General improvements to the ubuntu-minimal docs

    This change:
    - adds a note regarding an error when building focal ubuntu-minimal
      images on operating systems with older versions of debootstrap
    - adds a reference to where the DIB_RELEASE variable definition can be
      found

    Closes-Bug: #1941831
    Change-Id: Ibc1e04dba0562c4f4909a8cb8af041d9b8ac45c4