Title: AttributeError: 'NoneType' object has no attribute 'cancel' with Calibre
Date: 2020-10-08 14:00

As I was trying to run [Calibre](https://calibre-ebook.com/) 4.99.4 today, I got the
following error:

```python
$ calibre 
Traceback (most recent call last):
  File "/usr/bin/calibre", line 20, in <module>
    sys.exit(calibre())
  File "/usr/lib/calibre/calibre/gui_launch.py", line 73, in calibre
    main(args)
  File "/usr/lib/calibre/calibre/gui2/main.py", line 543, in main
    listener = create_listener()
  File "/usr/lib/calibre/calibre/gui2/main.py", line 514, in create_listener
    return Listener(address=gui_socket_address())
  File "/usr/lib/calibre/calibre/utils/ipc/server.py", line 110, in __init__
    self._listener._unlink.cancel()
AttributeError: 'NoneType' object has no attribute 'cancel'
zsh: exit 1     calibre
$
```

This is fixed by [7b6416ac652]( https://github.com/kovidgoyal/calibre/commit/7b6416ac6522fc40f24f6baf3ca552b17a8b91d6)
in May 2020, with a shitty commit message: "`...`".

The root cause being that Python 3.8.3 is now [making abstract socket
namespaces playing nice with multiprocess]( https://bugs.python.org/issue39850), 
obsoleting Calibre's dirty hack to work around this issue.

The proper way to get the
crash fixed is to upgrade to Calibre `4.15.0`, the stupid one is to downgrade
your Python version, and the gross one is to backport this patch:

```diff
+++ src/calibre/utils/ipc/server.py
@@ -107,7 +107,8 @@ def __init__(self, *args, **kwargs):
            Listener.__init__(self, *args, **kwargs)
            # multiprocessing tries to call unlink even on abstract
            # named sockets, prevent it from doing so.
-           self._listener._unlink.cancel()
+           if self._listener._unlink is not None:
+               self._listener._unlink.cancel()
            # Prevent child processes from inheriting this socket
            # If we dont do this child processes not created by calibre, will
            # inherit this socket, preventing the calibre GUI from being restarted.
```