add-apt-repository should return exit code and not adding wrong repositories

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Confirmed. The exit code is not !=0 when an error occurs and even though you actually get an error message like
Error reading https://launchpad.net/api/beta/~bxlsjs/+archive/sds: HTTP Error 404: Not Found
is given, the file
/etc/apt/sources.list.d/bxlsjs-sds-lucid.list
is created anyway.

$ apt-cache policy python-software-properties
python-software-properties:
  Installed: 0.75.5
  Candidate: 0.75.5
  Version table:
 *** 0.75.5 0
        600 http://archive.ubuntu.com lucid/main Packages
        100 /var/lib/dpkg/status

Resetting to Medium importance (from WishList) -- this is really a bug

I'll take care about the apt-query command the next time.
Thanks.

In the current state it can't be used in a reliable way by other scripts.

The reason of the bug is that :
1. In softwareproperties/SoftwareProperties.py, a thread is created

  def check_and_add_key_for_whitelisted_channels(self, srcline):
[...]
    if parsed_uri.netloc == 'ppa.launchpad.net':
      worker = AddPPASigningKeyThread(parsed_uri.path)
      worker.start()

2. When failing in method add_ppa_signing_key of AddPPASigningKeyThread class, it prints an error and return False :
[...]
        except URLError, e:
            print "Error reading %s: %s" % (lp_url, e)
            return False

3. But the return code generated by the thread is nowhere managed. In fact, two lines are inconditionnaly added in source file :
  def add_source_from_line(self, line):
[...]
    self.check_and_add_key_for_whitelisted_channels(deb_line)
    self.sourceslist.list.append(new_deb_entry)
    self.sourceslist.list.append(new_debsrc_entry)

Hope it helps...