Comment 9 for bug 1813464

The dist-upgrader tarball contains a copy of invoke-rc.d which it patches so that errors are not fatal and the release upgrade can proceed. Here's part of the patch as an example:

  1 --- /usr/sbin/invoke-rc.d 2018-11-21 15:15:24.000000000 -0800
  2 +++ DistUpgrade/imported/invoke-rc.d 2019-04-12 09:48:59.230957528 -0700
  3 @@ -346,6 +346,10 @@ verifyrclink () {
  4 shift
  5 done
  6 if test x${doexit} != x && test x${RETRY} = x; then
  7 + if [ -n "$RELEASE_UPGRADE_IN_PROGRESS" ]; then
  8 + printerror "release upgrade in progress, error is not fatal"
  9 + exit 0
 10 + fi
 11 exit ${doexit}
 12 fi
 13 return 0

The failure in this bug can be tested quickly in a chroot by installing the packages sasl2-bin and ubuntu-release-upgrader-core.