[FEATURE] Don't make registry calls when content has completed fetching

[sondavidb]

Description

SOCI doesn't use any sort of credential rotation system, so in systems such as Kubernetes, we can encounter issues where a user can attempt to run tasks after the credential has expired. stargz gets around this by only calling the registry if the fetched size is under the expected layer size: containerd/stargz-snapshotter#1584. We can do the same here.

Ultimately, this is a workaround, and we need a better credentials mechanism, but this should be a simple fix that should help for now.

Describe the solution you'd like

No response

Describe any alternative solutions/features you've considered

No response

Any additional context or information about the feature request

There's a separate issue here which is that we don't know exactly when the background fetcher has concluded fetching all of its data. I think if we can implement that, this fix can be a bit cleaner.

[austinvazquez]

Does this mean we are not checking the cache before going to the registry? If so, what is the point of the cache?

[sondavidb]

Does this mean we are not checking the cache before going to the registry? If so, what is the point of the cache?

AFAIK, before this change, the cache still did its function, but the call order meant that we still made a registry call regardless. The cache is to stop us from fetching data we don't already have, and it seems to do its job