[Bug 1883980] Re: Upgrade corrupted due to "sh: 1: pager: not found"

[Paul Smith]

Also if anyone has any ideas about how to fix my system, other than
reinstalling from scratch, I'm definitely interested...

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Title:
  Upgrade corrupted due to "sh: 1: pager: not found"

Status in dpkg package in Ubuntu:
  New

Bug description:
  Description:    Ubuntu 20.04 LTS
  Release:        20.04

  dpkg:
    Installed: 1.19.7ubuntu3
    Candidate: 1.19.7ubuntu3
    Version table:
   *** 1.19.7ubuntu3 500
          500 http://us.archive.ubuntu.com/ubuntu focal/main amd64 Packages
          100 /var/lib/dpkg/status


  This past weekend I performed an upgrade of Ubuntu 18.04 LTS to Ubuntu
  20.04 LTS.  I fully updated my 18.04 system and rebooted, then I ran
  do-release-upgrade -d to start the upgrade.

  During the upgrade process for the mime-support package it asked me
  about changes to /etc/mime.types file:

  Configuration file '/etc/mime.types'
   ==> Modified (by you or by a script) since installation.
   ==> Package distributor has shipped an updated version.
     What would you like to do about it ?  Your options are:
      Y or I  : install the package maintainer's version
      N or O  : keep your currently-installed version
        D     : show the differences between the versions
        Z     : start a shell to examine the situation
   The default action is to keep your current version.
  *** mime.types (Y/I/N/O/D/Z) [default=N] ?

  I wanted to see the difference so I typed "d" here.  Then I got this
  error:

  *** mime.types (Y/I/N/O/D/Z) [default=N] ? d
  sh: 1: pager: not found
  diff: standard output: Broken pipe
  dpkg: error processing package mime-support (--configure):
   conffile difference visualizer subprocess returned error exit status 127

  After this, many, MANY packages were failing because mime-support was
  not configured:

  dpkg: dependency problems prevent configuration of python2.7:
   python2.7 depends on mime-support; however:
    Package mime-support is not configured yet.

  dpkg: error processing package python2.7 (--configure):
   dependency problems - leaving unconfigured

  etc. etc.  Eventually it just bailed:

  dpkg: too many errors, stopping
  Errors were encountered while processing:
   mime-support
   python2.7
     ...
  Processing was halted because there were too many errors.

  
  Now my system is in a weird state where it is upgraded and mostly works but I'm missing a bunch of icons, I can't seem to properly select different "Appearance" values from the Settings dialog, etc.  I've tried to run "apt dist-upgrade" and "dpkg --configure -a" and neither of those has any impact: they both had no work to do.

  I think that breaking the entire install is an overreaction to not
  being able to run a pager to display a diff: dpkg should handle this
  error much better and simply say that diffs are not available at this
  time, but not fail the package configuration!

  Of course it would be good to know why "pager" was not available at
  this point in the upgrade, as well.

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