Cache miss for existing keys when merging PRs

[hammzj]

I have a workflow that is saving a cached file from the head branch to the base branch on a pull request merge. The file keys exists on the head branch, but when it attempts to find it using a restore-keys basic path, it is failing to hit. I see the key exists in my GitHub cache for the head branch, so not sure why it's showing as missing.

Here is an example workflow:

name: Cache file on base branch
on:
  pull_request:
    types: [closed]

env:
  node-version: 20.18.x

jobs:
  cache_file:
    # this job will only run if the PR has been merged
    if: github.event.pull_request.merged == true
    runs-on: ubuntu-latest
    steps:
      - run: |
          echo PR #${{ github.event.number }} has been merged

      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ env.node-version }}

      - name: Restore file from head branch

        uses: actions/cache/restore@v4
        with:
          fail-on-cache-miss: true
          path: main.json
          key:  file-${{ github.head_ref }}-${{ github.run_id }}-${{ github.run_attempt }}
          restore-keys: |
            file-${{ github.head_ref }}-
 
      - name: Save file to base ref
        uses: actions/cache/save@v4
        with:
          path: main.json
          key: file-${{ github.base_ref }}-${{ github.run_id }}-${{ github.run_attempt }}

[AlexanderRichert-NOAA]

I seem to be running into the same issue. As I tried with a few different actions/cache version, I ended up in a situation where it wouldn't save the cache because it already exists, but there are no caches listed on the caches page for my repo.

[hammzj]

Ah, I see this in GitHub's documentation:

Caching dependencies to speed up workflows

When a cache is created by a workflow run triggered on a pull request, the cache is created for the merge ref (refs/pull/.../merge). Because of this, the cache will have a limited scope and can only be restored by re-runs of the pull request. It cannot be restored by the base branch or other pull requests targeting that base branch.

I've decided to use a 3rd party action which allows this to work correctly, but by downloading artifacts from other workflows instead of accessing cache: https://github.com/dawidd6/action-download-artifact

This looks like the path others took as well, but this is a serious limitation of the cache feature. I understand not being able to access child or sibling branches, but it's a glaring issue if a base branch cannot access the cache of the source merged into it.

[AlexanderRichert-NOAA]

For me this is all on the same branch, so maybe I'm running into a separate issue. (edit: apparently the issue is that I was using different file paths between my restore and save steps)