Ensure only one active InstanceConfig can exist

[rmol]

Status

Ready for review

Description of Changes

Fixes #5973.

Adds a unique partial index on InstanceConfig.valid_until where it is null, and in InstanceConfig.get_current, handles the now-possible IntegrityError and queries again to pick up the row added between the previous query and the insertion attempt.

Testing

• git checkout develop
• git checkout origin/there_can_be_only_one -- securedrop/tests/test_db.py
• make test TESTFILES="tests/test_db.py::test_only_one_active_instance_config_can_exist" will fail because it doesn't encounter IntegrityError.
• git checkout there_can_be_only_one
• Run the test again. It should pass.

Deployment

The migration that adds the index could conceivably fail if a site had multiple rows in instance_config with valid_until null, but every production database should have been created with migrations, which would have created a default row in instance_config. Any site that did somehow have multiple active configurations would not be functioning.

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

If you made non-trivial code changes:

• I have written a test plan and validated it for this PR

Choose one of the following:

• I have opened a PR in the docs repo for these changes, or will do so later
• I would appreciate help with the documentation
• These changes do not require documentation

[lgtm-com]

This pull request introduces 1 alert when merging 4bcc5de into 59da5b7 - view on LGTM.com

new alerts:

• 1 for Testing equality to None

[lgtm-com]

This pull request introduces 1 alert when merging ffc8bec into 59da5b7 - view on LGTM.com

new alerts:

• 1 for Testing equality to None

[codecov-commenter]

Codecov Report

Merging #5974 (ffc8bec) into develop (2971b91) will decrease coverage by 0.08%.
The diff coverage is 72.72%.

@@             Coverage Diff             @@
##           develop    #5974      +/-   ##
===========================================
- Coverage    85.32%   85.23%   -0.09%     
===========================================
  Files           53       54       +1     
  Lines         3883     3900      +17     
  Branches       481      481              
===========================================
+ Hits          3313     3324      +11     
- Misses         457      463       +6     
  Partials       113      113              

Impacted Files	Coverage Δ
...81fb88c2_unique_index_for_instanceconfig_valid_.py	66.66% <66.66%> (ø)
securedrop/models.py	91.32% <80.00%> (-0.33%)	⬇️

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[zenmonkeykstop]

Tsst plan passes, and new instance configs can be created via the JI without any issues. Agreed on deployment comments.