BUILD-10699: Adds fallback-to-default-branch option for setting restore-keys

[bwalsh434]

What Changed?

• Adds the fallback-to-default-branch option, which is enabled by default, for setting the restore-keys value to the same exact cache path, but on the repo's default branch
  • If a repo consuming the action explicitly sets the restore-keys: node-linux- and fallback-to-default-branch: true, then the final branch-restore-keys passed to runs-on/cache would be:

feature/my-branch/node-linux-        ← user's restore key (branch-scoped), tried first
refs/heads/main/node-linux-     

• Updates README to reflect this new option

First Test

• Using BUILD-10699: test fallback-to-default-branch cache behaviour sonarlint-core#1918 to test this:
  • Test 1, first run (cold cache) shows the cache was restored from the default branch on cold cache
  • Test 1, second run (warm cache) shows the cache was still restored from the specific branch on the second run (direct cache hit on warm cache)

[Edit 19/03 1:16pm CET] Tested Successfully using latest commit:

• ✅ cache successfully restored from master branch cache on first run in new PR - link
  • Cache restored from key: refs/heads/master/maven-Linux-Build-b50b0c779ede66991f6393f3b8e901d98462992773465df71c125f902ada551f
• ✅ cache successfully restored from feature branch cache on second run in new PR - link
  • Cache restored from key: BUILD-10699-test-fallback-to-default-branch-v2/maven-Linux-Build-b50b0c779ede66991f6393f3b8e901d98462992773465df71c125f902ada551f

[sonar-review-alpha]

Summary

Adds a new fallback-to-default-branch input (enabled by default) that automatically provides a fallback restore key pointing to the repository's default branch cache on the S3 backend. The feature works with or without explicit restore-keys — if none are provided, it falls back to the exact cache key on the default branch. A safeguard prevents attempting fallback when already on the default branch itself.

What reviewers should know

Where to start: Check scripts/prepare-keys.sh for the core logic changes. The script now decouples branch-specific restore keys from default branch fallback, applying the latter conditionally based on the new toggle.

Key decisions to review:

• When no restore-keys are provided and fallback is enabled, the script now uses the primary INPUT_KEY for the default branch fallback (exact match), extending the feature beyond prefix matching
• The script skips default branch fallback if the current branch matches the fallback branch, avoiding unnecessary restore key duplication
• Default of true makes this a backward-compatible quality-of-life improvement

Test coverage: The author links to real-world testing in sonarlint-core showing the feature works on both cold and warm caches. Worth verifying the behavior matches the README examples.

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[sonar-review-alpha]

Conclusion: Solid implementation. The feature logic is correct, fits the existing patterns, and the on-default-branch guard is a nice touch. One dead-code path to clean up, but it doesn't affect runtime behavior.

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[sonar-review-alpha]

LGTM! ✅

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[julien-carsique-sonarsource]

There's a bug to resolve (dead code).

Clarify the validation and setup of default values at the top, or first use. Not on every call (i.e. "${INPUT_RESTORE_KEYS:-}").
Use the simplified syntax (i.e. in [[ ]], there is no need for double quotes on the left side of the condition).

[sonar-review-alpha]

The dead-code branch flagged in the previous review is still present — the if [[ -n "$RESTORE_KEYS" ]] guard inside the no-restore-keys path is unreachable and the simplification to a direct assignment was not applied.

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[sonar-review-alpha]

Conclusion: The PR needs one fix before merge — the dead-code branch flagged in the previous review is still there.

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[sonar-review-alpha]

Conclusion: The previously flagged dead code is still present, and the new commit introduced a new bash correctness issue.

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[sonar-review-alpha]

Conclusion: The dead-code simplification is clean. One previous review comment remains unaddressed.

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[sonar-review-alpha]

Conclusion: The feature logic is sound and the new fallback-to-default-branch behaviour is correctly implemented. However, the previously requested fix (quoting $INPUT_RESTORE_KEYS in the [[ -n ]] tests) has not been applied — it still appears unquoted at both lines 46 and 79.

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[sonar-review-alpha]

Conclusion: The new fallback-to-default-branch feature works correctly for the common cases, but there is a semantic conflict between the two related inputs that will silently misbehave for users who have both configured.

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[sonar-review-alpha]

Conclusion: The fallback-branch / fallback-to-default-branch interaction is now correctly handled. The quoting inconsistency from the previous review round is still present.

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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

[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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