Prevent blank revisions from being read from the backend

[rosstimothy]

Overwrites any empty resource revisions with a placeholder value to prevent any blank revisions from being provided to users.

[espadolini]

Why do we need the blank revision handling for the memory backend?

[rosstimothy]

Why do we need the blank revision handling for the memory backend?

We either have to insert the place holder into the event stream so that resources in the cache can never have an empty string or add blank revision handling to the memory backend.

[espadolini]

We either have to insert the place holder into the event stream so that resources in the cache can never have an empty string or add blank revision handling to the memory backend.

It's the job of the backend driver to never produce an empty revision string, if that means changing the event stream code to make it so that the empty revision string never appears even when "it should" then that's what we should do, IMO.

[rosstimothy]

We either have to insert the place holder into the event stream so that resources in the cache can never have an empty string or add blank revision handling to the memory backend.

It's the job of the backend driver to never produce an empty revision string, if that means changing the event stream code to make it so that the empty revision string never appears even when "it should" then that's what we should do, IMO.

What's the downside to having the cache just work in the event the upstream backend has a bug?

[fspmarshall]

R.E. the memory backend handling blank revisions; I think it should handle them correctly, but if it observes them while in mirror mode it should emit a warning.

[espadolini]

LGTM other than the memory backend

[espadolini]

Considering the situation with the cache applyFns I think we should reconsider the special handling of the blank revision string in the memory backend in mirror mode: an item of a cached type that's stored in dynamodbbk or litebk that has a blank revision because it was last modified before revisions were introduced will be yielded from the cache with its correct revision until it's overwritten, resulting in very confusing behavior if it's conditionally updated. If we instead return it from the cache with a blank string revision we will never accidentally condupdate it.