Don't call backend.set when backend.get returns None and default value is None

[i2xS]

Hi there!

Preconditions:
Any constance variable
default value: None
current value: None

Reading value through config.VALUE_NAME leads to backend.set('VALUE_NAME', None) call, which updates None to None.

This behaviour led us to database failure on production when 80 simultaneous value reads happen, which eventually led to simultaneous writes and deadlocks. Deadlocks led to service restarts, emitting new connections to database and new deadlocks.

[codecov]

Codecov Report

Merging #444 (bd09021) into master (a16cca3) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   82.11%   82.11%           
=======================================
  Files          15       15           
  Lines         548      548           
  Branches       88       88           
=======================================
  Hits          450      450           
  Misses         65       65           
  Partials       33       33           

Impacted Files	Coverage Δ
constance/base.py	86.95% <100.00%> (ø)

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[jezdez]

Please provide a test case to make sure this won't break again.

[camilonova]

@i2xS please include a test so we can merge this.

[camilonova]

@i2xS should we close this one?

[MichaelDanielTom]

Any reason not to merge this? Seems like this should be the desired behavior, and it's still relevant given this.

[camilonova]

@i2xS @MichaelDanielTom can you provide a test so we can merge this?