Add Incremental Snapshot support to RPC

[brooksprumo]

Problem

There's no way to get incremental snapshot information from RPC.

Summary of Changes

• Add new RPC method, getHighestSnapshotSlot that returns a SnapshotSlotInfo, which contains both the highest full snapshot slot, and the highest incremental snapshot slot based on the full snapshot.
• Deprecate old RPC method, getSnapshotSlot
• Update API docs

Fixes #19579

[mvines]

Is there a reason to have three methods here? It seems like just getSnapshotSlots would be sufficient?

Aside: I don't love the name getSnapshotSlots. To me that implies it'll return multiple slots that snapshots are available for, rather than a full/incremental snapshot slot pair

[brooksprumo]

@mvines

Is there a reason to have three methods here? It seems like just getSnapshotSlots would be sufficient?

Good point. I this will likely come down to desired usage. I can see two use cases:

1. A node already has a full snapshot, and wants to get the highest incremental snapshot for their full snapshot. This would only be useful as long as the slot hasn't crossed a full snapshot interval.
2. A node just wants to get the latest and is OK downloading a new full snapshot if necessary.

Use-case 1 may not be all that valuable though. If a node A has a full snapshot at slot 800,000, and node B has a full snapshot at slot 810,000, then node A won't be able to use an incremental snapshot from node B anyway. So getting both the highest full snapshot and highest incremental snapshot would be the right way to go.

OK, I'm on board with just having a single RPC function!

Aside: I don't love the name getSnapshotSlots. To me that implies it'll return multiple slots that snapshots are available for, rather than a full/incremental snapshot slot pair

Yeah, the name sucks. How about one of these:

• getSnapshotInfo
• getSnapshotSlot2
• getSnapshotSlotPair

I'm in favor of getSnapshotInfo.

[mvines]

How about: getHighestSnapshotSlot(full: Number, incremental: Number | undefined)

[mvines]

Oh, also lets turn the response from getHighestSnapshotSlot into a struct with these fields. If getSnapshotSlot returned an object instead of a simple number, we'd be able to extend it in a backwards-compatible way and not be stuck with creating a new method!

[codecov]

Codecov Report

Merging #19559 (0e458ed) into master (82a6bbe) will increase coverage by 0.0%.
The diff coverage is 37.5%.

@@           Coverage Diff           @@
##           master   #19559   +/-   ##
=======================================
  Coverage    82.6%    82.7%           
=======================================
  Files         461      461           
  Lines      131212   131249   +37     
=======================================
+ Hits       108483   108557   +74     
+ Misses      22729    22692   -37