Fix #178 -- Close connections when exeting threads

[codingjoe]

Django uses separate connections for each thread be default.
Open connections are only closed by default when the
request_finished signal is called, which does not happen in our
case.

[codecov-io]

Codecov Report

Merging #182 into master will increase coverage by 0.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   86.56%   86.71%   +0.15%     
==========================================
  Files          25       25              
  Lines         253      256       +3     
==========================================
+ Hits          219      222       +3     
  Misses         34       34

Impacted Files	Coverage Δ
health_check/views.py	100% <100%> (ø)	⬆️

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[courcelm]

This fix works if CONN_MAX_AGE = 0 in Django database settings. Otherwise, the connections are still pilling up.

[codingjoe]

@courcelm that's a good start I guess. I really don't have any idea how to close a connection forcefully before the connection timeout is reached.

[codingjoe]

Released in 3.4.2

[courcelm]

@codingjoe I would suggest to use close() instead of close_if_unusable_or_obsolete(). This way it will not cause any problem to anyone with CONN_MAX_AGE > 0.

It seems db health check is not re-using existing connection and makes a new one every time from what I can see. If we want to re-use connection, maybe you should exclude this db check from the parralel execution.

[codingjoe]

Ok @courcelm would you mind creating a Pull-Request for that?