Pin secret_key_base in the test app to stop a flaky Rails 8 CI failure

[exoego]

Problem

The test (ruby:4.0, 8.1.2, coverage) job fails now and then with this error.

ArgumentError:
  `secret_key_base` for test environment must be a type of String`

spec/apps/rails/tmp/local_secret.txt is gitignored, so it does not exist when CI starts. scripts/parallel_rspec starts 4 workers at once, and each spawns a Rails child against the same app root. Rails' generate_local_secret checks for the file, creates it, writes it and then reads it. A child can read the file after another child created it but before it was written. That returns an empty string. Rails 8.0+ rejects an empty string in the secret_key_base= setter, so only the Rails 8.x jobs fail, and only sometimes.

Seen on #448. A local simulation of the Rails code with 4 concurrent processes hit the empty read 15 times in 300 trials.

Fix

Set a fixed config.secret_key_base in the test environment. Rails then never touches tmp/local_secret.txt. The setting works the same way on Rails 6.1 through 8.1.

Verification

Removed the local secret files and ran bundle exec scripts/parallel_rspec on Rails 8.0.2. All 88 examples passed and the secret file was not recreated.

🤖 Generated with Claude Code

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ℹ️ Recent review info

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Run ID: f117fe0d-bde7-45e1-b303-6ca3fe5655d9

📥 Commits

Reviewing files that changed from the base of the PR and between af2b203 and fa40cc1.

📒 Files selected for processing (1)

• spec/apps/rails/config/environments/test.rb

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📝 Walkthrough

Walkthrough

Changes

Rails test configuration

Layer / File(s)	Summary
Fixed test secret configuration spec/apps/rails/config/environments/test.rb	The test environment sets a fixed config.secret_key_base value and documents its purpose for parallel application boots.

Priority: ⬇️ Low

Merge Risk: ⚪ Minimal · up to fa40c

Rails test boots now use a stable secret key, preventing parallel workers from observing an incomplete local secret file. The change is limited to test configuration and has no remaining merge-blocking risk.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly summarizes the main change: pinning secret_key_base in the test app to prevent flaky Rails 8 CI failures.
Description check	✅ Passed	The description directly explains the concurrent file access problem, the fixed config.secret_key_base solution, and the verification results.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.42%. Comparing base (af2b203) to head (fa40cc1).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #449   +/-   ##
=======================================
  Coverage   98.42%   98.42%           
=======================================
  Files          29       29           
  Lines        1144     1144           
  Branches      222      222           
=======================================
  Hits         1126     1126           
  Partials       18       18           

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