BUILD-10779: Always restore from cache key with fallback-to-default-branch

[bwalsh434]

What Changed?

Prioritizes the cache keys in order of:

• direct hit with the specific branch as first attempt
• cache key from fallback branch if set OR cache key from default branch if fallback-to-default-branch: 'true'
• restore key from specific branch
• restore key from fallback branch if set OR restore key from default branch if fallback-to-default-branch: 'true'

Also changes:

• The go module test case ("Cache Go modules with fallback branch") for testing the priority above in a case where we set both the fallback-branch and fallback-to-default-branch: 'true'
• The README to reflect that the default value of fallback-to-default-branch is 'false'

Test Plan

• ✅ Test that the ordering above is what is actually used with the newly modified "Cache Go modules with fallback branch" test, which explicitly sets both the fallback-branch and fallback-to-default-branch: true
  • Success! See job run here

[sonar-review-alpha]

Summary

This PR restructures cache key lookup priority to prioritize exact-match fallbacks over prefix-based restore keys. The key change: when fallback-to-default-branch: true or a fallback-branch is configured, the action now tries an exact-match key on the fallback branch before falling back to prefix-based restore keys on any branch. This fixes a scenario where a feature branch could inadvertently use a stale prefix-match from its own branch instead of a fresh exact-match from the default branch.

The default value of fallback-to-default-branch is also changed from true to false, requiring explicit opt-in to default branch fallback behavior.

What reviewers should know

Code changes to review:

• scripts/prepare-keys.sh: The key logic reorganization. Look for where FALLBACK_EXACT_KEY is injected first, then branch-specific keys, then fallback keys—this ordering is now explicit in the code rather than incidental.
• README.md cache search order: The numbered list (items 1-4) documents the new priority. Compare against the old version to understand what shifted.
• Test workflow: The test now explicitly enables both fallback-branch and fallback-to-default-branch, making it a good validation that both work together correctly.

Gotchas:

• The default value flip (fallback-to-default-branch: false) is a breaking change. Workflows that relied on implicit fallback to default branch must now opt in explicitly.
• The test uses github.run_id as the cache key, so it will never hit the cache—this is intentional, allowing the test to verify the key search order without cache interactions.

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[sonar-review-alpha]

Conclusion: Clean, minimal fix. The logic is correct in both the INPUT_RESTORE_KEYS-provided and not-provided paths, and the new line 84 handles the empty/non-empty RESTORE_KEYS transition correctly via ${RESTORE_KEYS:+${RESTORE_KEYS}$'\n'} — no leading newline when RESTORE_KEYS is empty, proper separator when it isn't.

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[sonar-review-alpha]

Conclusion: Clean, well-reasoned fix. One duplication worth addressing before merge.

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[sonar-review-alpha]

Conclusion: The new commit only updates the test workflow; the logic duplication in scripts/prepare-keys.sh flagged in the previous review is still present.

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[sonar-review-alpha]

The previous logic duplication issue in scripts/prepare-keys.sh has not been fixed — the two INPUT_RESTORE_KEYS loops with inconsistent append idioms remain as flagged.

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[sonar-review-alpha]

Conclusion: The new commit only adjusts README line wrapping — the logic duplication in scripts/prepare-keys.sh flagged in the previous review remains unfixed.

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[sonar-review-alpha]

LGTM! ✅

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[sonar-review-alpha]

LGTM! ✅

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[sonarqubecloud]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[sonar-review-alpha]

LGTM! ✅

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[sonarqube-cloud-us]

SonarQube reviewer guide

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud