chore(tiering): Optimize serialization

[dranikpg]

• Simplfy TieredStorage::Stash logic by using OpManager::PrepareAndStash helper
• Don't serialize SmallBin to string, return object that can be serialized to destination buffer
• Return string_view[2] from CompactObj::GetRawString to determine size/get string copyless with small strings
• Use Listpack in SerializedMap constructor to avoid intermediate format

[Copilot]

Pull request overview

This PR optimizes tiering serialization by eliminating intermediate allocations and enabling zero-copy operations. The changes simplify the stash logic, improve API ergonomics, and reduce memory overhead during offloading operations.

Key Changes

• SmallBins API refactoring: Changed FilledBin from pair<BinId, string> to a struct with an explicit SerializeBin() method that writes directly to a destination buffer, eliminating intermediate string allocations
• Zero-copy string access: Modified CompactObj::GetRawString() to return array<string_view, 2> instead of StringOrView, enabling copyless size determination and access to small strings split across two memory segments
• Direct listpack serialization: Updated SerializedMap::Serialize() to accept ListpackWrap directly instead of materializing an intermediate vector of string pairs

Reviewed changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/server/tiering/small_bins.h	Redefined FilledBin as a struct with private members; added SerializeBin() method declaration; updated IsPending() to use new structure
src/server/tiering/small_bins.cc	Implemented SerializeBin() to write directly to destination buffer; updated internal state management to use FilledBin struct; added memory reuse optimization for bin entries
src/server/tiering/small_bins_test.cc	Changed test fixture from class to struct; added Serialize() helper method; updated all tests to use new FilledBin API
src/server/tiering/serialized_map.h	Replaced SerializeSize() with EstimateSize(); changed Serialize() signature to accept ListpackWrap instead of span of pairs
src/server/tiering/serialized_map.cc	Implemented EstimateSize() and updated Serialize() to iterate directly over listpack; moved kLenBytes constant to file scope
src/server/tiering/serialized_map_test.cc	Added test suite setup to initialize zmalloc; updated test to create and use ListpackWrap instead of vector of pairs
src/server/tiering/op_manager.h	Added PrepareAndStash() helper method declaration
src/server/tiering/op_manager.cc	Implemented PrepareAndStash() to combine buffer preparation, serialization, and stashing in one call
src/server/tiering/op_manager_test.cc	Refactored Stash() test helper to use new PrepareAndStash() pattern
src/server/tiered_storage.cc	Simplified TryStash() to use PrepareAndStash() with bind_front for both large values and small bins; updated serialization logic to handle two-part string views
src/core/compact_object.h	Changed GetRawString() return type from StringOrView to array<string_view, 2>
src/core/compact_object.cc	Implemented new GetRawString() to return two views; handles both ROBJ_TAG (single view) and SMALL_TAG (two views) cases
src/core/compact_object_test.cc	Updated test to handle new GetRawString() return type by combining both views
src/core/detail/listpack_wrap.h	Added Bytes() method declaration
src/core/detail/listpack_wrap.cc	Implemented Bytes() to return total listpack size via lpBytes()

[romange]

pls respond to copilot comments

[dranikpg]

augment review

[augmentcode]

Sorry, Augment does not have access to the org this pull request's branch is from.

[dranikpg]

@romange PTAL at the updates

[romange]

nit: BytesUsed()

[romange]

bytes_

[romange]

entries_