[runtime/buffer] avoid write-locking the Append buffer during blob writes

[roberto-bayardo]

The append write buffer is currently write-locked while we write data to underlying blob, which prevents any concurrent reads or buffered writes. This PR introduces a separate IO lock for the underlying blob so that buffer reads and appends can be performed concurrently with blob writing.

Impact on existing benchmarks are neutral, however we don't have any benchmarks that perform concurrent reads & writes, which is where this change would shine.

Resolves: #2673

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Status	Name	Latest Commit	Updated (UTC)
✅ Deployment successful! View logs	commonware-mcp	12d03ae	Jan 05 2026, 07:16 PM

[Copilot]

Pull request overview

This PR improves concurrency in the buffer pool by introducing a separate I/O lock (blob_io) for the underlying blob. The key change allows readers to access the buffer pool cache while writes to the blob are in progress, rather than blocking all reads during blob I/O operations.

Key changes:

• Introduces AsyncMutex wrapper for async mutual exclusion primitives
• Adds blob_io RwLock to coordinate reads/writes to the underlying blob separately from the buffer pool lock
• Implements try_read_cached fast path to check cache without acquiring I/O locks

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
runtime/src/utils/mod.rs	Adds AsyncMutex wrapper around async_lock::Mutex for consistency with existing synchronization primitives
runtime/src/utils/buffer/pool.rs	Implements try_read_cached method to enable lock-free cache lookups
runtime/src/utils/buffer/append.rs	Refactors blob I/O to use separate blob_io lock, allowing concurrent cache reads during blob writes

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View logs

[danlaine]

Suggested change

	let mut guard = self.buffer.write().await;
	let mut buf_guard = self.buffer.write().await;

[roberto-bayardo]

done

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.83%. Comparing base (6deffb6) to head (12d03ae).
⚠️ Report is 10 commits behind head on main.

@@            Coverage Diff             @@
##             main    #2679      +/-   ##
==========================================
+ Coverage   92.63%   92.83%   +0.20%     
==========================================
  Files         357      361       +4     
  Lines      103102   106277    +3175     
==========================================
+ Hits        95505    98659    +3154     
- Misses       7597     7618      +21     

Files with missing lines	Coverage Δ
runtime/src/utils/buffer/append.rs	99.63% <100.00%> (+0.05%)	⬆️
runtime/src/utils/buffer/pool.rs	96.05% <100.00%> (+0.31%)	⬆️
storage/src/journal/contiguous/fixed.rs	97.39% <100.00%> (-0.01%)	⬇️

... and 103 files with indirect coverage changes

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