Summary:
Add read-scoped block buffers for scan reads. Introduce an experimental read-scoped block buffer provider API, configured through ReadOptions::read_scoped_block_buffer_provider, so supported block-based table iterator scans and MultiScan data-block reads can use caller-provided read-scoped storage for final data-block contents.
When configured, supported provider-backed scan data-block reads bypass the data-block cache while preserving normal index/filter block-cache behavior. Known-uncompressed reads can attach provider cleanup to provider-backed read buffers without copying. Compressed reads decompress directly into provider-backed output, while maybe-compressed reads that turn out to be uncompressed copy once into provider-backed final contents. mmap reads ignore the provider.
Extend AlignedBuffer and RandomAccessFileReader direct-I/O paths to support external aligned allocations, then use that support for read-scoped iterator, async I/O, and MultiRead scratch buffers. Centralize read-scoped I/O policy, keep coalesced async reads safe when blocks are released before completion, and validate provider lease contracts.
Add focused coverage for read-scoped ownership, compressed and uncompressed blocks, direct I/O, data-block cache bypass behavior, invalid provider leases, async release handling, and stress-test provider invariants. Add public API release notes for read-scoped block buffers.
Bonus change: Fixed a flaky test in ReserveThread
## Testing
- CI
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14806
Reviewed By: anand1976
Differential Revision: D106999951
Pulled By: xingbowang
fbshipit-source-id: b1f23d4bab6318b6373ba2ca99a5c4d6a842dc5a
f39e9a94f0