Traceback when adding/removing files during library scanning

[genodeftest]

When changing a library while it is being scanned, Exaile throws a Runtime Error

Steps to Reproduce (for bugs)

1. Have a huge library (so rescanning takes a while)
2. Trigger (re-)scanning the library while it is being watched, e.g. by adding it to Exaile
3. While Exaile is still scanning, add or delete files
4. Have a look at Exaile's output

Expected Behavior

No tracebacks.

Current Behavior

I'm getting this traceback:

Traceback (most recent call last):
  File "/path/to/exaile/xl/collection.py", line 491, in on_location_changed
    if track in self.__library.collection:
  File "/path/to/exaile/xl/trax/trackdb.py", line 65, in next
    return self.iter.next()[1]._track
RuntimeError: dictionary changed size during iteration

This looks like a threading issue to me, but I'm not sure about that.

I could not spot any misbehaviour, which would be quite hard due to the library size.

Possible Solution

No clue yet.

Environment

• Operating System and version: Fedora 26, python2-2.7.14-2.fc26.x86_64
• Exaile Version: latest git master (af00c46)