[Bug 1837254] Autopkgtest regression report (livecd-rootfs/2.408.52)

[Ubuntu SRU Bot]

All autopkgtests for the newly accepted livecd-rootfs (2.408.52) for xenial have finished running.
The following regressions have been reported in tests triggered by the package:

ubuntu-image/1.7+16.04ubuntu1 (amd64)


Please visit the excuses page listed below and investigate the failures, proceeding afterwards as per the StableReleaseUpdates policy regarding autopkgtest regressions [1].

https://people.canonical.com/~ubuntu-archive/proposed-
migration/xenial/update_excuses.html#livecd-rootfs

[1] https://wiki.ubuntu.com/StableReleaseUpdates#Autopkgtest_Regressions

Thank you!

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https://bugs.launchpad.net/bugs/1837254

Title:
  ubuntu-cpc parallel builds produce unused files

Status in livecd-rootfs package in Ubuntu:
  Fix Released
Status in livecd-rootfs source package in Xenial:
  Fix Committed
Status in livecd-rootfs source package in Bionic:
  Fix Committed
Status in livecd-rootfs source package in Disco:
  Fix Committed

Bug description:
  [Impact]

  In parallel builds where a list of image targets are provided, the build
  may produce binaries that are not part of the named set of targets but
  are created by series dependencies.  These implicitly created binaries
  may be generated by multiple builds but are unused as our convention for
  the ubuntu-cpc project is to only consume binaries from the explicitly
  named image targets; this avoids overwriting the same object by multiple
  parallel builds.  The problem is that as we build out parallel jobs for
  a number of image targets (multiplied by each supported architecture)
  we're wasting a lot of resources.

  [Test Case]

   * Specify an image target within the ubuntu-cpc project for a build

   * Observe artifacts in addition to the ones for the named image
  target are produced and downloaded

  [Regression Potential]

   * An artifact from an explicitly named artifact is missing and builds
  will begin to remove desired output.  Testing is being done to address
  this regression.

  [Other Info]

   * This has been running in production in Eoan for a few weeks and
  we're satisfied with the results.  We have seen overall time reduced
  by 20-50% on some builds.

  [Original Description]

  In parallel builds where a list of image targets are provided, the build
  may produce binaries that are not part of the named set of targets but
  are created by series dependencies.  These implicitly created binaries
  may be generated by multiple builds but are unused as our convention for
  the ubuntu-cpc project is to only consume binaries from the explicitly
  named image targets; this avoids overwriting the same object by multiple
  parallel builds.  The problem is that as we build out parallel jobs for
  a number of image targets (multiplied by each supported architecture)
  we're wasting a lot of resources.

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