[Bug 1652641] Update Released

[Łukasz Zemczak]

The verification of the Stable Release Update for apt-xapian-index has
completed successfully and the package has now been released to
-updates.  Subsequently, the Ubuntu Stable Release Updates Team is being
unsubscribed and will not receive messages about this bug report.  In
the event that you encounter a regression using the package from
-updates please report a new bug using ubuntu-bug and tag the bug report
regression-update so we can easily find any regressions.

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https://bugs.launchpad.net/bugs/1652641

Title:
  crash in indexer.py, AttributeError: 'NoneType' object has no
  attribute 'unlink'

Status in apt-xapian-index package in Ubuntu:
  Fix Released
Status in apt-xapian-index source package in Xenial:
  Triaged
Status in apt-xapian-index source package in Zesty:
  Fix Released

Bug description:
  [Impact]

  When the apt-xapian-index is already up to date, and under certain
  conditions, update-apt-xapian-index throws an exception.

  The index /var/lib/apt-xapian-index is up to date
  Exception ignored in: <bound method ServerProgress.__del__ of
  <axi.indexer.ServerProgress object at 0x7f04a5ae4c18>>
  Traceback (most recent call last):
    File "/usr/lib/python3/dist-packages/axi/indexer.py", line 306, in __del__
  AttributeError: 'NoneType' object has no attribute 'unlink'

  This happens more frequently when it is run with nice and ionice,
  which is the case in the cron.weekly/apt-xapian-index cron job.

  This is not severe, since the exception is thrown on exit, and depends
  on the way python3 cleans up the environment. Anyway, this leaves a
  socket file behind, and creates noise in cron logs (see comment #2).

  The patch keeps a reference to the os.unlink function in the
  ServerProgress instance, such that it has the same lifespan.

  [Test Case]

  * `sudo nice -n 19 ionice -c 3 update-apt-xapian-index --quiet`
  * may need to run 4 times or more
  * no exception is thrown

  [Regression Potential]

  * If the os module is unloaded anyway and self.os_unlink becomes
  unreachable, we get the same exception again.

  [Other Info]

  * There is probably a more "pythonic" way to fix this. For instance,
  avoiding __del__. That requires a larger change.

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