[Storage] PR 3627 feedback

[danlaine]

Follow-up on review feedback for PR #3627, which replaces the Current QMDB's layered bitmap with Arc<SharedBitmap<RwLock<BitMap>>>. This PR addresses two small inefficiencies and a coverage gap surfaced in review, without changing the PR's design.

Changes

1. Cache terminal Arc<SharedBitmap> on BitmapBatchLayer (batch.rs)

Add shared: Arc<SharedBitmap<N>> to BitmapBatchLayer. Collapses:

• BitmapBatch::shared() from an O(chain-depth) walk to a 2-arm match.
• BitmapBatch::pruned_chunks() from an O(chain-depth) walk to self.shared().pruned_chunks().

Cost: 8 bytes per layer + one Arc::clone at construction. The invariant (cached shared equals the chain terminal) is trivial by construction at both sites (compute_current_layer, trim_committed) — each sets the field via Arc::clone(parent.shared()), and BitmapBatch::Base is already the terminal.

Both callers (trim_committed on every new_batch; pruned_chunks once per build_chunk_overlay) are on the per-batch hot path. The walk was redundant after PR #3627's restructuring and is now gone.

2. Add test_current_live_batch_child_after_prune (mod.rs)

Covers the specific scenario: a live parent batch whose new_batch() is called after the DB has advanced its pruning boundary. The internal trim_committed must observe the advanced boundary on the shared bitmap, and the resulting child must merkleize and apply correctly.

Not covered by the existing test_current_live_batch_safe_across_prune, which tests applying a pre-prune child rather than extending a live parent post-prune.

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[roberto-bayardo]

re: benchmark, I was going to remove it from this PR in favor of the standalone #3633

...just in case we come up with a non-locking based solution to this issue.

So could you move the benchmark suggestions there?

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[danlaine]

re: benchmark, I was going to remove it from this PR in favor of the standalone #3633

...just in case we come up with a non-locking based solution to this issue.

So could you move the benchmark suggestions there?

Removed benchmark changes from here

[roberto-bayardo]

apparently I neglected to merge this when I approved it and now there are conflicts :-(

[codecov]

Codecov Report

❌ Patch coverage is 98.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 95.87%. Comparing base (0fc4391) to head (f6947ad).
⚠️ Report is 14 commits behind head on locked-bitmap.

Files with missing lines	Patch %	Lines
storage/src/qmdb/current/batch.rs	96.87%	2 Missing ⚠️

@@              Coverage Diff               @@
##           locked-bitmap    #3635   +/-   ##
==============================================
  Coverage          95.87%   95.87%           
==============================================
  Files                440      440           
  Lines             172039   172131   +92     
  Branches            3998     3999    +1     
==============================================
+ Hits              164941   165030   +89     
- Misses              5831     5835    +4     
+ Partials            1267     1266    -1     

Files with missing lines	Coverage Δ
storage/src/qmdb/current/mod.rs	99.08% <100.00%> (+0.01%)	⬆️
storage/src/qmdb/current/batch.rs	91.31% <96.87%> (+0.42%)	⬆️

... and 3 files with indirect coverage changes

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