Existing cache cannot be restored

[Yuri05]

I have the GHA-workflow below, which installs R and some R packages on windows-latest and should cache the R packages directory. The workflow is always triggered manually (workflow_dispatch) from the main branch of the repo (which is also the only branch in this repo).

Workflow yaml

name: Test Cache R

on:
  workflow_dispatch:

permissions:
  contents: read
  actions: write

env:
  GITHUB_PAT: ${{ secrets.GITHUB_TOKEN }}

jobs:
  evaluation:
    runs-on: windows-latest
    env:
      R_LIBS_USER: ${{ github.workspace }}\RLibrary
    steps:
    - name: Checkout repository
      uses: actions/checkout@v4
    
    - name: Restore setup cache
      id: cache-setup
      uses: actions/cache@v4
      with:
        key: Ronly-Main-123456789
        path: |
          ${{ env.R_LIBS_USER }}

    - name: Setup R
      uses: r-lib/actions/setup-r@v2
      with:
        r-version: '4.3.1'
        use-public-rspm: true
        rtools-version: 'none'

    - id: install-cran-packages
      name: Install cran packages (NO CACHE FOUND)
      if: steps.cache-setup.outputs.cache-hit != 'true'
      run: |
        install.packages(c("remotes", "pak"))
      shell: Rscript {0}

    - name: Debug cached paths before save
      if: steps.cache-setup.outputs.cache-hit != 'true'
      run: |
        echo "Listing R library root: $R_LIBS_USER"
        if [ -d "$R_LIBS_USER" ]; then
          ls -l "$R_LIBS_USER" 
        else
          echo "R library directory missing"
        fi
      shell: bash

The first workflow run creates the cache as expected.

Screens: created cache

All following workflow runs (started again manually from the same main branch of the repo) however cannot restore from cache. But the cache exists - which results in the error message when trying to save the cache again in the "Post restore" workflow step:

E.g. here is the result from the 2nd workflow run: (The error message is misleading; in fact the cache for the given combination of {branch, key, version} exists - compare https://github.com/actions/cache/issues/1665)

Attached are the log files from 3rd workflow run (created with the GHA debug diagnostics turned on): logs_47695047952.zip

[Yuri05]

Update: I have figured out what the problem was. The environment variable R_LIBS_USER, which was used in the cache path to restore, was overwritten by the R installation. This meant that the cache saved the wrong path. Each time I tried to restore the cache, the original R_LIBS_USER path was requested, but the corresponding path was missing from the cache, which is why there was no match.

• While I think this is correct behavior for the cache, an explicit error message would be extremely helpful. For example,

The cache with the key 'MyKey-1234' was found; 
however, the cache cannot be restored because some of the required paths are missing:
 - Path1
 - Path2
 - ...

• It would also be great if this behavior was clearly described in the documentation. I could not find any mention of the fact that the cache is not found not only if there is no matching key/restore-key, but also if a path is missing. https://docs.github.com/en/actions/reference/workflows-and-actions/dependency-caching#cache-hits-and-misses https://github.com/actions/cache?tab=readme-ov-file#cache-scopes