Summary

Github actions/cache@v3 does not restore symlink properly on Windows by default. See for example restoring Intel MKL below:

11/28/2022  10:54 PM    <DIR>          .
11/28/2022  11:20 PM    <DIR>          ..
11/28/2022  10:54 PM    <DIR>          2022.0.3
11/28/2022  11:20 PM    <SYMLINKD>     latest [\??\C:\Program Files (x86)\Intel\oneAPI\mkl\2022.0.3]

Context

This occurred during an action to cache Intel MKL. The minimal example is reproduced in the following repository with the following script

Digging previous issues, I could find one possible solution by adding the following step:

    steps:
    - name: Use GNU Tar from Msys
      if: ${{ runner.os == 'Windows' }}
      run: |
        echo "C:\msys64\usr\bin" >> $env:GITHUB_PATH

However this is hardly convenient and was time consuming to first identify the problem and then to find it.

Differences

Logs are there: https://github.com/FabienPean/test-actions-cache/actions/runs/3586177236/jobs/6035270661

Caching difference

C:\Windows\System32\tar.exe --posix -z -cf cache.tgz --exclude cache.tgz -P -C D:/a/test-actions-cache/test-actions-cache --files-from manifest.txt

vs

C:\msys64\usr\bin\tar.exe --posix --use-compress-program "zstd -T0" -cf cache.tzst --exclude cache.tzst -P -C D:/a/test-actions-cache/test-actions-cache --files-from manifest.txt --force-local

Restoring difference

C:\Windows\System32\tar.exe -z -xf D:/a/_temp/79e339ad-c303-4c67-aa2c-d0533f9a6395/cache.tgz -P -C D:/a/test-actions-cache/test-actions-cache

vs

C:\msys64\usr\bin\tar.exe --use-compress-program "zstd -d" -xf D:/a/_temp/e365a544-fe05-44e5-86cd-035de603e3e4/cache.tzst -P -C D:/a/test-actions-cache/test-actions-cache --force-local

Desired solution

By order of preference:

1. Work by default
2. Input to the action
3. Add documentation