Add SyncV2 Causal Negotiation

[ChrisPenner]

Overview

• Based on Add SyncV2 protocol #5513 , merge that first.

UCM will now use the new causal-dependencies endpoint to determine which dependencies of a new root it already has, then will tell Share to skip certain dependencies in its stream.

This massively speeds up incremental pulls on a given branch.

Implementation notes

Before initiating a download, ask Share to enumerate relevant dependencies (currently the causal spine and all contained library causal hashes). Then pass the list of dependencies we've already got along to the download endpoint.

Note: this now clears the temp_entity tables before starting a pull.v2, we still have code to check temp entities on every insert, so clearing the tables speeds that up. There's not much reason to keep large amounts of code sitting around in there when the user isn't actively trying to pull.v1 something.

Interesting/controversial decisions

Nope

Test coverage

I've done a bunch of testing on staging, but would be good to dogfood it a bit with the Unison team or early adopters.

Loose ends

• Track the remote causal for branches we've pulled so we can just use that as a known causal.

[aryairani]

The temp entity table should be empty except if a past sync failed and was never completed, right?

[ChrisPenner]

Yup; but it's also never otherwise cleared, so if you accidentally clone @unison/base/main instead of lib.install @unison/base or something like that it'll leave junk in there basically forever.