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Overlay-base cache restore misses on cache backends that only prefix-match restoreKeys

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Assessment

Difficulty
1/5
Estimated time
Under an hour
Newbie friendliness
90/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
typescript
Domain
ci-cd

Research direction

Start at src/overlay/caching.ts around lines 290-302, where downloadOverlayBaseDatabaseFromCache calls actionsCache.restoreCache with the key prefix as the only argument; compare with the upload path just above it that writes codeql-overlay-base-database-<hash>-<sha>-<run_id>. The fix is passing the same prefix as the third argument (restoreKeys), then run the existing overlay caching tests in the TypeScript suite to confirm no current GitHub-cache behavior regresses. Done means PR runs on prefix-only backends can restore while GitHub-hosted behavior matches today.

Written by the indexing model from the issue text.

Description

Summary

downloadOverlayBaseDatabaseFromCache restores the overlay-base database by passing the key prefix as the primary key, with no restoreKeys:

https://github.com/github/codeql-action/blob/d208ec71e39ae46397cbf6569bef8439b9aab974/src/overlay/caching.ts#L290-L302

actionsCache.restoreCache(
  [dbLocation],
  cacheRestoreKeyPrefix,
  undefined,
  { segmentTimeoutInMs: 3000 },
)

This depends on the primary key matching by prefix. GitHub's cache service does that, but some Actions-cache-compatible backends only prefix-match restoreKeys. One example is the transparent cache on Blacksmith runners. On those backends, PR runs can never restore the overlay-base database, even though default-branch runs upload it successfully. Every PR run then falls back to a full analysis.

What we see

Default-branch run, on the same runner type:

Setting overlay database mode to overlay-base with caching because we are analyzing the default branch.
Uploading overlay-base database to Actions cache with key codeql-overlay-base-database-1-<hash>-python-2.27.1-<sha>-<run_id>-1
Successfully uploaded overlay-base database from /home/runner/_work/_temp/codeql_databases

Pull request run:

Setting overlay database mode to overlay with caching because we are analyzing a pull request.
Looking in Actions cache for overlay-base database with restore key codeql-overlay-base-database-1-<hash>-python-2.27.1-
No overlay-base database found in Actions cache
No overlay-base database found in cache, reverting overlay database mode to none.
Minimal reproduction of the backend difference

A cache entry saved as probe-<run_id>-full, then restored in a later job on the same branch with actions/cache/restore@v4:

Restore GitHub-hosted runner Blacksmith runner
key: probe-<run_id>- (prefix as the primary key, like this action) hit miss
key: probe-<run_id>-nomatch, restore-keys: probe-<run_id>- hit hit
Suggested fix

Also pass the prefix as a restore key:

actionsCache.restoreCache(
  [dbLocation],
  cacheRestoreKeyPrefix,
  [cacheRestoreKeyPrefix],
  { segmentTimeoutInMs: 3000 },
)

On GitHub's cache this matches exactly the same entries as today, since the prefix is tried first either way. On backends that only prefix-match restore keys, overlay analysis starts working.

Environment
  • github/codeql-action/init and analyze @v4, CodeQL bundle 2.27.1
  • build-mode: none, languages python and javascript-typescript
  • Third-party hosted runners (Blacksmith) whose actions/cache calls go to their own cache backend
Dominant language
TypeScript
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Avg merge
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Merged PRs (30d)
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