add-apt-repository fails if it encounters immutable sources.list.d entry

[Martin Schmitt]

Reason:

We're trying to protect an internal APT source from trivial accidental modification by users (who do have sudo as root).

Steps to reproduce:

echo '# dummy' > /etc/apt/sources.list.d/dummy.list
chattr +i /etc/apt/sources.list.d/dummy.list
add-apt-repository -y ppa:longsleep/golang-backports

Expected behaviour:

New sources.list for the PPA gets created.

Observed behaviour:

Traceback (most recent call last):
  File "/usr/bin/add-apt-repository", line 364, in <module>
    sys.exit(0 if addaptrepo.main() else 1)
  File "/usr/bin/add-apt-repository", line 357, in main
    shortcut.add()
  File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 221, in add
    self.add_source()
  File "/usr/lib/python3/dist-packages/softwareproperties/shortcuthandler.py", line 307, in add_source
    sourceslist.save()
  File "/usr/lib/python3/dist-packages/softwareproperties/extendedsourceslist.py", line 391, in save
    with open(filename, "w") as f:
PermissionError: [Errno 1] Operation not permitted: '/etc/apt/sources.list.d/dummy.list'

[Launchpad Janitor (janitor)]

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