Unattended do-release-upgrade fails with Python import error

[Cerin]

Trying to do an unattended upgrade of Ubuntu 14.04 to 16.04 with:

    sudo do-release-upgrade -f DistUpgradeViewNonInteractive

fails with the Python traceback:

    Configuration file '/etc/default/rcS'
     ==> 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.
    *** rcS (Y/I/N/O/D/Z) [default=N] ? Error in sys.excepthook:
    Traceback (most recent call last):
      File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
        from apport.fileutils import likely_packaged, get_recent_crashes
      File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
        from apport.report import Report
      File "/usr/lib/python3/dist-packages/apport/report.py", line 15, in <module>
        import xml.dom, xml.dom.minidom
    ImportError: No module named 'xml.dom'

    Original exception was:
    Traceback (most recent call last):
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/xenial", line 8, in <module>
        sys.exit(main())
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeMain.py", line 242, in main
        if app.run():
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeController.py", line 1876, in run
        return self.fullUpgrade()
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeController.py", line 1841, in fullUpgrade
        if not self.doDistUpgrade():
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeController.py", line 1182, in doDistUpgrade
        res = self.cache.commit(fprogress,iprogress)
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeCache.py", line 286, in commit
        apt.Cache.commit(self, fprogress, iprogress)
      File "/usr/lib/python3/dist-packages/apt/cache.py", line 501, in commit
        # which is less than optimal!
      File "/usr/lib/python3/dist-packages/apt/cache.py", line 465, in install_archives

      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeView.py", line 234, in run
        res = os.WEXITSTATUS(self.wait_child())
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeView.py", line 200, in wait_child
        self.update_interface()
      File "/tmp/ubuntu-release-upgrader-8dpb24f4/DistUpgrade/DistUpgradeViewNonInteractive.py", line 221, in update_interface
        os.write(self.master_fd,chr(3))
    TypeError: 'str' does not support the buffer interface

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: ubuntu-release-upgrader-core 1:16.04.21
ProcVersionSignature: Ubuntu 4.4.0-62.83-generic 4.4.40
Uname: Linux 4.4.0-62-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.5
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Sat Feb 4 09:41:55 2017
InstallationDate: Installed on 2017-01-21 (13 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Christian Schmidt (xgione)]

Same Problem here with ntp:

Configuration file '/etc/ntp.conf'
 ==> 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.
*** ntp.conf (Y/I/N/O/D/Z) [default=N] ?

[Erik Hj (erikhj)]

Similar issue:

Configuration file '/etc/default/rcS'
 ==> 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.
*** rcS (Y/I/N/O/D/Z) [default=N] ?
Error in sys.excepthook:
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/apport_python_hook.py", line 63, in apport_excepthook
    from apport.fileutils import likely_packaged, get_recent_crashes
  File "/usr/lib/python3/dist-packages/apport/__init__.py", line 5, in <module>
    from apport.report import Report
  File "/usr/lib/python3/dist-packages/apport/report.py", line 15, in <module>
    import xml.dom, xml.dom.minidom
ImportError: No module named 'xml.dom'

Original exception was:
Traceback (most recent call last):
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/xenial", line 8, in <module>
    sys.exit(main())
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeMain.py", line 242, in main
    if app.run():
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeController.py", line 1876, in run
    return self.fullUpgrade()
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeController.py", line 1841, in fullUpgrade
    if not self.doDistUpgrade():
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeController.py", line 1182, in doDistUpgrade
    res = self.cache.commit(fprogress,iprogress)
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeCache.py", line 286, in commit
    apt.Cache.commit(self, fprogress, iprogress)
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 501, in commit
    # which is less than optimal!
  File "/usr/lib/python3/dist-packages/apt/cache.py", line 465, in install_archives

  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeView.py", line 234, in run
    res = os.WEXITSTATUS(self.wait_child())
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeView.py", line 200, in wait_child
    self.update_interface()
  File "/tmp/ubuntu-release-upgrader-q8tt9j6b/DistUpgrade/DistUpgradeViewNonInteractive.py", line 221, in update_interface
    os.write(self.master_fd,chr(3))
TypeError: 'str' does not support the buffer interface