perf(sqlite-native): reuse read cache for partial writes

[NathanFlurry]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[NathanFlurry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• chore(engine): publish engine bases in ci #4649
• chore: lockfile lefthook #4647
• fix(rivetkit): surface native sqlite kv errors to callers #4645
• fix(rivetkit): prevent sleep races during disconnect and db work #4644
• fix(rivetkit): isolate engine envoys and propagate startup failures #4643
• test(rivetkit): remove wasm sqlite-only fixtures and coverage #4642
• chore(sqlite-vfs): remove wasm sqlite packages and workspace wiring #4641
• chore(rivetkit): remove wasm sqlite runtime #4614
• fix(sqlite-native): restore native startup kv preload #4639
• fix(sqlite-native): delete metadata before chunk range #4638
• fix(sqlite-native): keep truncate cache coherent #4637
• fix(sqlite-vfs): use delete range for truncate cleanup #4636
• fix(sqlite-native): restore kv error hook #4635
• perf(sqlite-native): avoid cloning cached read chunks #4634
• perf(sqlite-native): gate kv operation labels #4633
• perf(sqlite-native): reuse read cache for partial writes #4632 👈 (View in Graphite)
• perf(sqlite-native): remove delete file existence probe #4631
• fix(sqlite-native): gate native read cache behind env var #4630
• fix(pegboard): isolate runner config dc lookup failures #4625 : 1 other dependent PR (#4626 )
• docs(pegboard): note runner config upsert split write risk #4624
• fix(pegboard): unpack global runner config keys correctly #4623
• fix(pegboard-gateway): enforce tunnel message state #4622
• fix(pegboard-runner): clear terminal tunnel routes #4621
• fix(pegboard): skip protocol version keys in runner pool backfill #4620
• fix(pegboard): create missing normal runner configs #4619
• fix(epoxy): reuse cached optimistic value #4618
• main

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[claude]

Code Review

PR: perf(sqlite-native): reuse read cache for partial writes

Clean, targeted optimization that avoids redundant KV fetches for partial-write chunks already in the read cache. One compilation bug needs fixing before merge.

---

Bug: state.read_cache.get() called on Option (line 596)

read_cache is typed as Option<HashMap<Vec<u8>, Vec<u8>>> (line 290). The new code calls .get(key) directly on the Option:

// WRONG - Option<HashMap<...>> has no .get(key) method
state.read_cache.get(chunk_key.as_slice()).cloned()

This is a compilation error. The existing read path at lines 431-435 correctly unwraps the Option first:

if let Some(read_cache) = state.read_cache.as_ref() {
    if let Some(cached) = read_cache.get(key.as_slice()) {
        ...
    }
}

Fix - use as_ref().and_then() to match:

let cached_chunk = if needs_existing && ctx.read_cache_enabled {
    let state = get_file_state(file.state);
    state.read_cache.as_ref().and_then(|cache| cache.get(chunk_key.as_slice()).cloned())
} else {
    None
};

---

Everything else looks good

• Logic: The three-way priority (cached chunk -> fetched existing chunk -> write-only) is correctly implemented in the or_else chain at lines 635-644.
• Correctness: Checking ctx.read_cache_enabled before accessing the cache is consistent with the rest of the VFS.
• Scope: Only adds a cached_chunk field to the local WritePlan struct and a cache lookup in the planning loop. No impact on the write path itself.
• No stale-cache risk: The read cache is populated on reads and updated on writes, so a cached chunk used here reflects the current on-disk state.

Request changes - fix the state.read_cache.get() call to state.read_cache.as_ref().and_then(...) to match the pattern used in the existing read path.