Avoid manipulating uninitialized instances on monitoring thread.

[davidbau]

This change addresses the race that causes #404 and also mentioned in #323, which is a race that happens with some regularity (but intermittently). The problem is that the monitoring thread is manipulating instances that have been inserted into cls._instances (at the end of __new__) before they have finished their __init__.

This change doesn't change when instances are added to _instances (which might be another change to consider). This change verifies that any retrieved instances have been initialized before manipulating them in the monitoring thread.

The bug is difficult to reproduce, but this fix seems to prevent the bug from occurring for me.

• I have visited the source website, and in particular
  read the known issues
• I have searched through the issue tracker for duplicates
• I have mentioned version numbers, operating system and
  environment, where applicable
• If applicable, I have mentioned the relevant/related issue(s)

[codecov-io]

Codecov Report

Merging #493 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #493      +/-   ##
=========================================
- Coverage   99.24%   99.2%   -0.04%     
=========================================
  Files           7       7              
  Lines         658     631      -27     
  Branches      118     120       +2     
=========================================
- Hits          653     626      -27     
+ Misses          3       2       -1     
- Partials        2       3       +1

[casperdcl]

ah, nice, I like this...

[lrq3000]

All good for me! Thanks a lot! 👍

[lrq3000]

@casperdcl I think the codacy report bugged out, we can probably safely merge!

[MaticConradi]

Any updates?