SnapshotTransaction stream refactor proof-of-concept

[wi11dey]

Could probably also separate out all the postfiltering logic into its own PostFilter class

Birthday present for @rhuffy

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Generate changelog in changelog-dir>`changelog/@unreleased`</changelog-dir

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Type

- [ ] Feature - [ ] Improvement - [ ] Fix - [ ] Break - [ ] Deprecation - [ ] Manual task - [ ] Migration

Description

SnapshotTransaction stream refactor proof-of-concept **Check the box to generate changelog(s)** - [ ] Generate changelog entry

[rhuffy]

As an exercise I think this is fine, but to actually merge this, we'd probably need more than just "streams are easier to read" as a justification, especially if there's additional allocation cost in the hot read path.

[jeremyk-91]

Could probably also separate out all the postfiltering logic into its own PostFilter class

Yep, there's actually an internal project ongoing to slice this into many parts (see #7000 or internal RFCs). I'm curious if there was something that motivated this though?

Birthday present for @rhuffy

🎉

[rhuffy]

I was talking to Will about this refactor in general, and how some of the control flow (specifically the use of a resultsAccumulator) is a bit confusing for the uninitiated.

[wi11dey]

yup, motivation was just a proof-of-concept since raymond and i were discussing the code in the context of db upgrading and that it was difficult to plug in the kvs migrator, and also the code was a bit hard to follow for the uninitiated because of the various future chaining. i suggested streams and either raymond or jakub said it wouldn't be possible to use them async so this was a proof of concept using streams while keeping async behavior

the extra allocation of keysToDeleteOrReload could be removed with a map view -- we were already allocating for the result map, just renamed it to filteredResults. the extra iterations could probably also be removed with a little effort

I think this version is easier to reason about since there's no bouncing back and forth between getWithPostFilteringIterate/getWithPostFilteringInternal, i believe @Output in general should be avoided when possible so that the flow of values matches the code flow, and you need to keep less state in your head while reading through the code

anyway not suggesting we merge this right now, but the Future Spliterator approach could be something to keep in mind/reference if this code is going to be refactored anyway