[Bug 1825655] Re: do-release-upgrade fails with "Your python3 install is corrupted" if /usr/bin/python points to /etc/alternatives/python

[Brian Murray]

Hello Dmitrii, or anyone else affected,

Accepted ubuntu-release-upgrader into eoan-proposed. The package will
build now and be available at https://launchpad.net/ubuntu/+source
/ubuntu-release-upgrader/1:19.10.15.2 in a few hours, and then in the
-proposed repository.

Please help us by testing this new package.  See
https://wiki.ubuntu.com/Testing/EnableProposed for documentation on how
to enable and use -proposed.  Your feedback will aid us getting this
update out to other Ubuntu users.

If this package fixes the bug for you, please add a comment to this bug,
mentioning the version of the package you tested and change the tag from
verification-needed-eoan to verification-done-eoan. If it does not fix
the bug for you, please add a comment stating that, and change the tag
to verification-failed-eoan. In either case, without details of your
testing we will not be able to proceed.

Further information regarding the verification process can be found at
https://wiki.ubuntu.com/QATeam/PerformingSRUVerification .  Thank you in
advance for helping!

N.B. The updated package will be released to -updates after the bug(s)
fixed by this package have been verified and the package has been in
-proposed for a minimum of 7 days.

** Changed in: ubuntu-release-upgrader (Ubuntu Eoan)
       Status: New => Fix Committed

** Tags added: verification-needed verification-needed-eoan

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https://bugs.launchpad.net/bugs/1825655

Title:
  do-release-upgrade fails with "Your python3 install is corrupted" if
  /usr/bin/python points to /etc/alternatives/python

Status in ubuntu-release-upgrader package in Ubuntu:
  Fix Released
Status in ubuntu-release-upgrader source package in Eoan:
  Fix Committed

Bug description:
  do-release-upgrade from 18.04 to 18.10 fails (likewise, when I
  upgraded with a workaround the same happened for 18.10 -> 19.04):

  "
  Reading cache

  Checking package manager

  Can not upgrade

  Your python3 install is corrupted. Please fix the '/usr/bin/python3'
  symlink.
  === Command terminated with exit status 1 (Sat Apr 20 17:19:49 2019) ===
  "

  There is nothing wrong with /usr/bin/python3

  ➜  ~ ls -al /usr/bin/python3
  lrwxrwxrwx 1 root root 9 окт 25 14:11 /usr/bin/python3 -> python3.6

  I also have python2 installed for which Debian alternatives are
  configured which causes the problem as far as I can see:

  ➜  ~ update-alternatives --display python
  python - auto mode
    link best version is /usr/bin/python2.7
    link currently points to /usr/bin/python2.7
    link python is /usr/bin/python
  /usr/bin/python2.7 - priority 100

  https://git.launchpad.net/ubuntu/+source/ubuntu-release-
  upgrader/tree/DistUpgrade/DistUpgradeController.py?h=ubuntu/bionic-
  updates#n361

  (Pdb) l
   11  	        debian_defaults = '/usr/share/%s/debian_defaults' % dirname
   12  	        if os.path.exists(debian_defaults):
   13  	            config = SafeConfigParser()
   14  	            with open(debian_defaults) as f:
   15  	                config.readfp(f)
   16  ->	            try:
   17  	                expected_default = config.get('DEFAULT', 'default-version')
   18  	            except NoOptionError:
   19  	                logging.debug("no default version for %s found in '%s'" %
   20  	                              (binary, config))
   21  	                return False
  (Pdb) n
  > <ipython-input-13-93c4105e7796>(17)_pythonSymlinkCheck()
  -> expected_default = config.get('DEFAULT', 'default-version')
  (Pdb) n
  > <ipython-input-13-93c4105e7796>(22)_pythonSymlinkCheck()
  -> try:
  (Pdb) expected_default
  'python2.7'

  > <ipython-input-13-93c4105e7796>(23)_pythonSymlinkCheck()
  -> fs_default_version = os.readlink('/usr/bin/%s' % binary)
  (Pdb) n
  > <ipython-input-13-93c4105e7796>(27)_pythonSymlinkCheck()
  -> if not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default))                     and not (binary == 'python' and fs_default_version in ('python2', '/usr/bin/python2')):
  (Pdb) fs_default_version
  '/etc/alternatives/python'
  (Pdb) os.path.join('/usr/bin', expected_default)
  '/usr/bin/python2.7'

  (Pdb) not fs_default_version in (expected_default, os.path.join('/usr/bin', expected_default))
  True

  (Pdb) print("%s symlink points to: '%s', but expected is '%s' or '%s'" % (binary, fs_default_version, expected_default, os.path.join('/usr/bin', expected_default)))
  python symlink points to: '/etc/alternatives/python', but expected is 'python2.7' or '/usr/bin/python2.7'

  A workaround is:

  sudo rm /usr/bin/python
  sudo ln -s /usr/bin/python2.7 /usr/bin/python
  do-release-upgrade

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