Add Merge Operators support

[Havret]

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[Copilot]

Pull request overview

This pull request adds comprehensive support for RocksDB merge operators, enabling efficient atomic read-modify-write operations. The implementation introduces a new inheritance hierarchy with RocksDbStoreBase as the common base class, from which both the existing RocksDbStore and new MergeableRocksDbStore derive. This allows stores to perform atomic operations like incrementing counters or modifying lists without requiring separate read-before-write operations.

Key changes include:

• Introduction of MergeableRocksDbStore<TKey, TValue, TOperand> base class for stores that support merge operations
• Implementation of ListMergeOperator<T> with support for atomic add/remove operations on lists
• Refactoring of RocksDbStore into a two-level hierarchy (RocksDbStoreBase → RocksDbStore/MergeableRocksDbStore)
• Upgrade of RocksDB package from version 9.10.0 to 10.4.2

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/RocksDb.Extensions/RocksDbStoreBase.cs	Refactored from RocksDbStore; now serves as abstract base class for both regular and mergeable stores with common CRUD operations
src/RocksDb.Extensions/RocksDbStore.cs	New file wrapping RocksDbStoreBase to maintain backward compatibility; contains the obsolete GetAll() method
src/RocksDb.Extensions/MergeableRocksDbStore.cs	New base class for stores requiring merge operations; exposes Merge() method for atomic updates
src/RocksDb.Extensions/IMergeOperator.cs	Interface defining FullMerge and PartialMerge operations for custom merge logic
src/RocksDb.Extensions/IMergeAccessor.cs	Internal interface extending IRocksDbAccessor with Merge() capability
src/RocksDb.Extensions/MergeAccessor.cs	Implementation of merge operations with serialization of keys and operands
src/RocksDb.Extensions/MergeOperatorConfig.cs	Internal configuration binding IMergeOperator to RocksDB native merge callbacks
src/RocksDb.Extensions/MergeOperators/CollectionOperation.cs	Represents add/remove operations for list merge operator
src/RocksDb.Extensions/MergeOperators/OperationType.cs	Enum defining Add and Remove operation types
src/RocksDb.Extensions/MergeOperators/ListMergeOperator.cs	Built-in merge operator for atomic list modifications with add/remove support
src/RocksDb.Extensions/ListOperationSerializer.cs	Serializes CollectionOperation with optimized formats for primitive vs. complex types
src/RocksDb.Extensions/IRocksDbBuilder.cs	Adds AddMergeableStore() method for registering stores with merge operators
src/RocksDb.Extensions/RocksDbBuilder.cs	Implements merge store registration with serializer creation for operands
src/RocksDb.Extensions/RocksDbContext.cs	Configures merge operators per column family and refactors ColumnFamilyOptions setup
src/RocksDb.Extensions/RocksDbAccessor.cs	Changes field visibility to 'private protected' to enable MergeAccessor inheritance; adds WriteOptions to all write operations
src/RocksDb.Extensions/RocksDbOptions.cs	Adds internal dictionary for merge operator configurations per column family
src/RocksDb.Extensions/RocksDb.Extensions.csproj	Upgrades RocksDB package from 9.10.0.55496 to 10.4.2.63147
test/RocksDb.Extensions.Tests/MergeOperatorTests.cs	Comprehensive test suite covering add, remove, mixed operations, edge cases, and both string and primitive list types
README.md	Extensive documentation on merge operators including examples, use cases, and custom implementation guide

Comments suppressed due to low confidence (2)

src/RocksDb.Extensions/RocksDbStoreBase.cs:14

• The documentation comment states the class is "not intended for direct use by library consumers" and suggests using RocksDbStore or MergeableRocksDbStore instead. However, the class is marked with [EditorBrowsable(EditorBrowsableState.Never)] but is still public abstract.

For consistency with the stated intent, consider making this class internal rather than public, since it's truly an implementation detail. If it must remain public for technical reasons (e.g., inheritance), the current approach with EditorBrowsable is acceptable, but document why it must be public in a comment.
src/RocksDb.Extensions/RocksDbStoreBase.cs:25

• The constructor uses protected internal access modifier. This allows both derived classes (protected) and classes in the same assembly (internal) to call it. Since RocksDbStore and MergeableRocksDbStore are in the same assembly, protected alone would be sufficient.

Consider whether protected internal is intentional (e.g., for test projects or extensibility), or if protected would be more appropriate to limit access scope.

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