[FIX] owlouvainclustering: Fix race conditions

[ales-erjavec]

Issue

There are race conditions in the Louvain clustering widget.

The pca_projection, graph and partitions attributes are accessed and set directly from threads. In particular from the already 'cancelled' jobs. The code does not wait for cancelled jobs to finish, but they do modify the computed partial results. This can lead to undefined final results
on the output.

I can trigger this reliably by:

File: wdbc.tab
Louvain Clustering (both):

• Apply PCA is checked: 20 components
• Euclidean distance
• k neighbors: 10
• Resolution 2.5
• Apply automatically is checked

Python script contents:

import numpy as np
d1, d2 = in_datas

c1, _ = d1.get_column_view("Cluster")
c2, _ = d2.get_column_view("Cluster")
assert np.array_equal(c1, c2)

In one of the Louvain clustering widgets tab to k neighbors spin box and enter 50, then quickly press down key on the keyboard a few times then even quicker enter 10 (you might copy 10 to clipboard and just paste it).

Description of changes

• Refactor the code to eliminate race conditions in the access/setting of pca_projection, graph and partitions.

• Limit the number of executing tasks to one.

Includes

• Code changes
• Tests
• Documentation

[codecov]

Codecov Report

Merging #3322 into master will increase coverage by 0.03%.
The diff coverage is 92.8%.

@@            Coverage Diff            @@
##           master   #3322      +/-   ##
=========================================
+ Coverage   82.06%   82.1%   +0.03%     
=========================================
  Files         351     351              
  Lines       61898   62076     +178     
=========================================
+ Hits        50799   50965     +166     
- Misses      11099   11111      +12