Cache is no longer updated when set with save as `false`

[Naros]

Our CI pipeline utilizes a combination of actions/cache and this action to manage the restoration and saving of various aspects of our workspace, utilizing the .ccache folder. Since we only need to have one save/restore, we use actions/cache for this, but rely on using this action to expedite the build of our project with ccache that is provided, like this:

    # Installs ccache
    - name: Install ccache
      uses: hendrikmuhs/ccache-action@v1.2
      with:
        key: ${{ inputs.cache-name }}|${{ github.ref_name }}|${{ github.sha }}
        restore: false
        save: false
        append-timestamp: false
        # Explicitly disable job statistics summary
        job-summary: ""

Unfortunately, in the latest 1.2.18, this action treats the cache as read-only, no longer updating the cache files across builds, thus making the entire pipeline experience 100% cache misses. While I understand the intent behind treating the cache as read-only with save set to false, this approach is overly restrictive and prevents combining with other actions.

Therefore, I would suggest that the cache should be treated as read-only; then, there should be a separate toggle to convey this, rather than trying to infer it from the fact that save is set to false. Otherwise unfortunately our pipelines are stuck at 1.2.17 :(

[hendrikmuhs]

Thanks for the report.

So #301 turns out to be a regression for you.
@planetmarshall any input?

Therefore, I would suggest that the cache should be treated as read-only; then, there should be a separate toggle to convey this, rather than trying to infer it from the fact that save is set to false. Otherwise unfortunately our pipelines are stuck at 1.2.17 :(

I tend to agree. I wonder if #301 should be reverted. The PR says:

If we aren't saving the cache we can get a small performance improvement by not writing new cache files during a build.

I wonder if that improvement is measurable. @planetmarshall do you have any numbers?

Don't get me wrong: I am fine with another setting if useful. If the "performance improvement" however isn't measurable, I would rather keep things simple and revert to the old state.

[Naros]

I would imagine it would look similar to what I see with my Java repositories and actions/cache. Our caches are ~ 1.7 GB in size and take around 10-15 seconds to download and approximately 10-15 seconds to upload if the cache is updated. But if the cache is unchanged, that step takes 1-2s.

So I can imagine in a volatile repository, that 10-15s can easily add up.

[hendrikmuhs]

#301 prevents updating the cache locally, if save is set to false. So the question is: what do we save from not locally updating the cache? We for sure avoid some runtime in ccache and some local file access. However, is this worth adding a special setting, or not?

[Naros]

I'm not familiar with the code, but I would say there's little to no savings here, @hendrikmuhs, outside of what you've mentioned. When you have a large project, the overhead at runtime you mentioned is negligible compared to dealing with builds that take minutes or hours to complete, because the cache always misses.

[planetmarshall]

Rethinking this I can think of other scenarios where being able to write the cache even if it isn't being saved is useful - such as multi-stage compiler builds.

I'm fine with reverting this, and I'll submit a PR for a separate option. I still think being able to avoid cache writes where they aren't necessary is useful - as on CI systems with large projects, small costs can add up.

[hendrikmuhs]

Thanks @planetmarshall that make sense. As said, I am fine with a separate option as you have a valid use case.

[Naros]

Thanks @planetmarshall for understanding 👍