[Bug 1640274] Re: DistUpgradeViewNonInteractive produces error on conffile-prompt from dpkg

[Mario]

a Workaround but... adding this:

Dpkg::Options { "--force-confold"; }

to /etc/apt/apt.conf works. It will still show the errors but will
upgrade just fine

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https://bugs.launchpad.net/bugs/1640274

Title:
  DistUpgradeViewNonInteractive produces error on conffile-prompt  from
  dpkg

Status in ubuntu-release-upgrader package in Ubuntu:
  Confirmed

Bug description:
  When running $do-release-upgrade -f DistUpgradeViewNonInteractive -p on an x64 Ubuntu 14.04 host a conf file prompt for /etc/issue results in:
  "ERROR error ''str' does not support the buffer interface' when trying to write to the conffile"

  System info:
  lsb_release -rd
  Description:	Ubuntu 14.04.5 LTS
  Release:	14.04

  apt-cache policy ubuntu-release-upgrader-core 
  ubuntu-release-upgrader-core:
    Installed: 1:0.220.8
    Candidate: 1:16.04.17
    Version table:
       1:16.04.17 0
          500 http://us.archive.ubuntu.com/ubuntu/ xenial-updates/main amd64 Packages
       1:16.04.12 0
          500 http://us.archive.ubuntu.com/ubuntu/ xenial/main amd64 Packages
   *** 1:0.220.8 0
          100 /var/lib/dpkg/status


  Partial output of main.log:

  2016-11-07 12:53:36,005 DEBUG disabling apt cron job (0o755)
  2016-11-07 13:01:06,388 DEBUG plugins for condition 'StartUpgrade' are '[]'
  2016-11-07 13:01:06,388 DEBUG plugins for condition 'xenialStartUpgrade' are '[]'
  2016-11-07 13:01:06,388 DEBUG plugins for condition 'from_trustyStartUpgrade' are '[]'
  2016-11-07 13:01:06,389 DEBUG quirks: running StartUpgrade
  2016-11-07 13:01:06,389 DEBUG skipping 'README' (no '.')
  2016-11-07 13:01:06,390 DEBUG killing update-notifier
  2016-11-07 13:01:06,403 DEBUG killing kblueplugd kbluetooth4
  2016-11-07 13:01:06,409 DEBUG killing gnome-screensaver
  2016-11-07 13:01:06,417 DEBUG setup poke timer for the scrensaver
  2016-11-07 13:01:06,421 DEBUG failed to determine user upgrading
  2016-11-07 13:01:06,421 ERROR failed to inhibit gnome-session idle
  2016-11-07 13:01:06,421 DEBUG apt btrfs snapshots supported: False
  2016-11-07 13:01:06,421 INFO cache.commit()
  2016-11-07 13:01:06,421 DEBUG failed to SystemUnLock() (E:Not locked) 
  2016-11-07 13:01:06,669 DEBUG doing a pty.fork()
  2016-11-07 13:01:06,671 DEBUG pid is: 1729
  2016-11-07 13:01:26,498 WARNING got a conffile-prompt from dpkg for file: '/etc/issue'
  2016-11-07 13:01:31,499 ERROR error ''str' does not support the buffer interface' when trying to write to the conffile
  2016-11-07 13:41:26,327 WARNING no activity 2400 seconds (Configuring base-files (amd64)) - sending ctrl-c
  2016-11-07 13:41:26,371 ERROR not handled exception:
  Traceback (most recent call last):
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/xenial", line 8, in <module>
      sys.exit(main())
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeMain.py", line 242, in main
      if app.run():
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeController.py", line 1876, in run
      return self.fullUpgrade()
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeController.py", line 1841, in fullUpgrade
      if not self.doDistUpgrade():
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeController.py", line 1182, in doDistUpgrade
      res = self.cache.commit(fprogress,iprogress)
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeCache.py", line 267, in commit
      apt.Cache.commit(self, fprogress, iprogress)
    File "/usr/lib/python3/dist-packages/apt/cache.py", line 501, in commit
      res = self.install_archives(pm, install_progress)
    File "/usr/lib/python3/dist-packages/apt/cache.py", line 465, in install_archives
      res = install_progress.run(pm)
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeView.py", line 234, in run
      res = os.WEXITSTATUS(self.wait_child())
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeView.py", line 200, in wait_child
      self.update_interface()
    File "/tmp/ubuntu-release-upgrader-b1xls5f6/DistUpgrade/DistUpgradeViewNonInteractive.py", line 221, in update_interface
      os.write(self.master_fd,chr(3))
  TypeError: 'str' does not support the buffer interface

  2016-11-07 13:41:26,371 DEBUG confirmRestart() called
  2016-11-07 13:41:26,372 DEBUG enabling apt cron job

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