Overlay: Determine which versions of CodeQL are compatible with cached base DBs

[henrymercer]

• Refactor caching logic into overlay/caching.ts
• Add an unused getCompatibleCodeQlVersionsForOverlayBaseDatabases(config, logger) function that looks up overlay base databases for the languages in config and returns a list of the CodeQL versions that were used to generate those overlay base databases. The intention is to use this in a future PR to determine what CodeQL version to set up. Returning a list rather than just the latest means we can filter the list down based on the enabled default version feature flags. This gives us the ability to skip a CodeQL version that we might initially roll out but then decide to rollback.

Risk assessment

For internal use only. Please select the risk level of this change:

• Low risk: Changes are fully under feature flags, or have been fully tested and validated in pre-production environments and are highly observable, or are documentation or test only.

Which use cases does this change impact?

None yet.

How did/will you validate this change?

• Unit tests - I am depending on unit test coverage (i.e. tests in .test.ts files).

If something goes wrong after this change is released, what are the mitigation and rollback strategies?

• Rollback - Change can only be disabled by rolling back the release or releasing a new version with a fix.

How will you know if something goes wrong after this change is released?

• Telemetry - I rely on existing telemetry or have made changes to the telemetry.
  • Alerts - New or existing monitors will trip if something goes wrong with this change.

Are there any special considerations for merging or releasing this change?

• No special considerations - This change can be merged at any time.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Consider adding a changelog entry for this change.
• Confirm the readme and docs have been updated if necessary.

[Copilot]

Pull request overview

Refactors overlay base-database caching into a dedicated module and introduces a helper to discover which CodeQL CLI versions are represented in existing cached overlay base databases (to support future version-selection logic).

Changes:

• Moved overlay base DB cache download/upload and cache-key generation logic into src/overlay/caching.ts and added src/overlay/caching.test.ts.
• Extracted OverlayDatabaseMode into src/overlay/overlay-database-mode.ts and updated imports across the codebase.
• Added getCompatibleCodeQlVersionsForOverlayBaseDatabases(config, logger) to list CodeQL versions inferred from matching cache entries.

Show a summary per file

File	Description
src/testing-utils.ts	Updates OverlayDatabaseMode import path after enum extraction.
src/status-report.ts	Updates overlay cache stats type import to the new caching module.
src/overlay/overlay-database-mode.ts	New enum module for overlay database mode.
src/overlay/index.ts	Removes caching responsibilities; retains overlay file/OID helpers and version constants.
src/overlay/index.test.ts	Removes overlay caching tests that were moved to the new caching test file.
src/overlay/caching.ts	New home for overlay base DB cache logic + compatible-version discovery helper.
src/overlay/caching.test.ts	Adds unit tests for cache key stability, cache restore behavior, and compatible-version discovery.
src/init-action.ts	Switches overlay cache download imports to overlay/caching and enum to overlay-database-mode.
src/init-action-post-helper.ts	Updates OverlayDatabaseMode import path.
src/init-action-post-helper.test.ts	Updates OverlayDatabaseMode import path.
src/database-upload.ts	Updates OverlayDatabaseMode import path.
src/config-utils.ts	Updates OverlayDatabaseMode import path; keeps overlay min-version constants from overlay/index.
src/config-utils.test.ts	Updates OverlayDatabaseMode import path.
src/codeql.ts	Updates OverlayDatabaseMode import path.
src/analyze.ts	Updates OverlayDatabaseMode import path.
src/analyze-action.ts	Switches overlay cache upload import to overlay/caching.
lib/upload-sarif-action-post.js	Generated build output update.
lib/upload-lib.js	Generated build output update.
lib/start-proxy-action-post.js	Generated build output update.
lib/resolve-environment-action.js	Generated build output update.
lib/init-action.js	Generated build output update.
lib/init-action-post.js	Generated build output update.
lib/autobuild-action.js	Generated build output update.
lib/analyze-action.js	Generated build output update.
lib/analyze-action-post.js	Generated build output update.

Copilot's findings

• Files reviewed: 16/28 changed files
• Comments generated: 1

[mbg]

Thanks for working on this! This is much simpler than the previous attempt at addressing this problem.

I have picked "Request changes" here because there are a few comments that I had which we should think about before we merge this.

Also, it might be good to have a separate PR for the first commit. That would make it easier to verify that it's just a refactoring, which I think we can merge straight away. It would then reduce the noise here.