rpc: Fix short-circuit condition going into big-table

[joncinque]

Problem

While updating the bigtable fallback logic in #7937, we incorrectly short-circuited the check into bigtable. Currently, we always go into bigtable if until is not specified, even if we've hit the limit of transactions to return.

Summary of changes

Only respect found_until if until is provided.

[mergify]

Backports to the beta branch are to be avoided unless absolutely necessary for fixing bugs, security issues, and perf regressions. Changes intended for backport should be structured such that a minimum effective diff can be committed separately from any refactoring, plumbing, cleanup, etc that are not strictly necessary to achieve the goal. Any of the latter should go only into master and ride the normal stabilization schedule. Exceptions include CI/metrics changes, CLI improvements and documentation updates on a case by case basis.

[mergify]

If this PR represents a change to the public RPC API:

1. Make sure it includes a complementary update to rpc-client/ (example)
2. Open a follow-up PR to update the JavaScript client @solana/kit (example)

Thank you for keeping the RPC clients in sync with the server API @joncinque.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 83.2%. Comparing base (813aede) to head (adcda86).
⚠️ Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #12068     +/-   ##
=========================================
- Coverage    83.2%    83.2%   -0.1%     
=========================================
  Files         861      861             
  Lines      322768   322768             
=========================================
- Hits       268743   268740      -3     
- Misses      54025    54028      +3     

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[steviez]

Makes sense and confirmed that found_until is false if the until sig is None:

agave/ledger/src/blockstore.rs

Lines 4002 to 4003 in 81e58b2

	let (lowest_slot, until_excluded_signatures, found_until) = match until {
	None => (first_available_block, HashSet::new(), false),

The change looks fine to fix + BP by keeping it minimal. Afterwards, I'm thinking we might consider using the type system to avoid the ambiguity / the bug. Namely, make this an Option<bool>:

agave/ledger/src/blockstore.rs

Line 125 in 81e58b2

pub found_until: bool,

Then we'd have:

• None => until wasn't set so we didn't look in bstore / don't need to look in bigtable
• Some(false) => until was set but we didn't find it in bstore so go check bigtable
• Some(true) => until was set and we found it in bstore so skip bigtable

[steviez]

Going to merge on your behalf to get CI rolling on the BP