Fix flakiness during import of a cache with empty layers removed

[jgiannuzzi]

This PR aims at fixing issues #1980 and #2198. It does so by ensuring that all results get returned by cache.remotecache.v1.(*cacheResultStorage).LoadWithParents, and not only partial results, as can be the case when 2 or more vertexes point to the same result.

See #2198 (comment) for more information about my understanding of the issue.

[jgiannuzzi]

Tests are running fine on my repo: https://github.com/jgiannuzzi/buildkit/actions/runs/1261157927

I guess this is a transient error. Could a maintainer please re-run all jobs? 🙏

[crazy-max]

@jgiannuzzi Done

[jgiannuzzi]

Thanks @crazy-max!

[tonistiigi]

Thanks for taking the time to figure this out.

[tonistiigi]

@jgiannuzzi Now that you are familiar with this code, what is your thinking about removing the empty layer blob removal and always exporting empty layer blobs. It messes up the history array in image config and therefore reproducible builds and I don't have any better solution to fix that.

I was also looking at

buildkit/solver/exporter.go

Line 109 in deb1440

res, err := cm.results.Load(ctx, res)

if it would need a similar change but I think it doesn't atm as the cachekey of that result is not being used. The whole Load vs LoadWithParents thing could use some cleanup though. Initially, there was only Load but it wasn't enough for all use cases.