refactor(in-process): flagd-evaluator PoC

[aepfli]

…icate logic

Replaces custom evaluation implementation with direct dependency on flagd-evaluator crate, eliminating ~1000 lines of duplicate code.

Changes:

• Remove custom operators (fractional, semver, datalogic-rs integration)
• Remove custom flag parsing and model conversion logic
• Remove custom FlagStore, use evaluator's storage directly
• Refactor InProcessResolver and FileResolver to call evaluator functions
• Add proper reason/error code mapping to OpenFeature types
• Add flag metadata conversion (primitives only)
• Set permissive validation mode for spec compliance
• Add initial sync validation for both gRPC and file connectors

Benefits:

• Single source of truth across all providers (Python, Java, Rust)
• Zero FFI overhead (direct Rust-to-Rust calls)
• Perfect spec alignment with flagd evaluation engine
• Simpler codebase (-70% code in in-process module)

All 36 tests passing.

Refs open-feature/flagd#1842

🤖 Generated with Claude Code

This PR

• adds this new feature

Related Issues

Fixes #1234523

Notes

Follow-up Tasks

How to test

[gemini-code-assist]

Summary of Changes

Hello @aepfli, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly refactors the in-process evaluation module by integrating the flagd-evaluator crate. This change streamlines the codebase, removes redundant custom implementations for flag evaluation, parsing, and storage, and ensures a single source of truth for evaluation logic across various providers. The refactor aims to improve spec alignment, reduce maintenance overhead, and enhance performance by leveraging direct Rust-to-Rust calls.

Highlights

• Dependency Update: Replaced custom evaluation dependencies (datalogic-rs, murmurhash3, semver) with a direct dependency on the flagd-evaluator crate.
• Codebase Simplification: Eliminated approximately 1000 lines of custom evaluation logic, including custom operators, flag parsing, model conversion, and the custom FlagStore implementation.
• Evaluation Engine Integration: Refactored InProcessResolver and FileResolver to directly utilize functions from the flagd-evaluator crate for all flag evaluations.
• OpenFeature Compliance: Added proper mapping for reason and error codes from the flagd-evaluator to OpenFeature types, and included flag metadata conversion (for primitive types).
• Validation Mode: Set the flagd-evaluator to permissive validation mode to ensure spec compliance.
• Initial Sync Validation: Implemented initial synchronization validation for both gRPC and file connectors to ensure flag configurations are loaded correctly at startup.

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[gemini-code-assist]

Code Review

This pull request is a significant and valuable refactoring. Replacing the custom evaluation logic with the flagd-evaluator crate is a great move that reduces the codebase size, eliminates duplicated logic from other flagd providers, and improves maintainability. The changes are well-contained within the in-process resolver module.

I've identified a few areas for improvement, primarily concerning code duplication between the new FileResolver and InProcessResolver, and a correctness issue in how the evaluation context is constructed. Addressing these will make the new implementation even more robust and maintainable.