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

[Dmitrii Shcherbakov]

Public bug reported:

do-release-upgrade from 18.04 to 18.10 fails:

"
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'

** Affects: ubuntu-release-upgrader (Ubuntu)
     Importance: Undecided
         Status: New

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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:
  New

Bug description:
  do-release-upgrade from 18.04 to 18.10 fails:

  "
  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'

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