refactor(mesh): move tree path hashing out of mesh into kv-index

[CatherineSue]

Description

Problem

crates/mesh/src/hash.rs exports three tree-node identification helpers — hash_node_path, hash_token_path, and GLOBAL_EVICTION_HASH — that hash a string prefix or token sequence to identify a radix-tree node in the TreeSync wire format. They lived in the mesh crate as leftovers from the v1 cleanup.

The mesh v2 spec (.claude/docs/mesh/mesh-v2-generic-kv.md) says explicitly:

The mesh crate should have ZERO knowledge of trees, workers, policies, or routing.

Tree-prefix hash helpers are by definition tree-specific. The mesh-side TreeSync adapter doesn't even use them — it forwards already-computed node_hash values on TreeDelta without ever computing one. The only caller that actually computes hashes is CacheAwarePolicy in the gateway, which already depends on kv-index for the radix tree types these hashes identify.

Solution

Move the three symbols to kv-index as a new path_hash module, re-exported at the crate root so existing kv_index::hash_node_path(...) / kv_index::hash_token_path(...) call shapes match the prior smg_mesh::hash_node_path(...) shape. No behavior change.

Changes

• crates/kv_index/Cargo.toml — add blake3 workspace dependency.
• crates/kv_index/src/path_hash.rs — new module (moved verbatim from crates/mesh/src/hash.rs).
• crates/kv_index/src/lib.rs — register the module and re-export the three symbols at the crate root.
• crates/mesh/src/hash.rs — deleted.
• crates/mesh/src/lib.rs — drop the hash module declaration and the three re-exports.
• model_gateway/src/policies/cache_aware.rs — switch 12 call sites from smg_mesh::hash_* to kv_index::hash_*.
• model_gateway/src/mesh/adapters/tree_sync.rs — update one stale doc comment (smg_mesh::tree_ops::GLOBAL_EVICTION_HASH → kv_index::GLOBAL_EVICTION_HASH).

Mesh crate keeps blake3 as a workspace dependency for the unrelated replica_id derivation in kv.rs:536.

Test Plan

• cargo test -p kv-index --lib path_hash — all 4 hash tests pass in their new home.
• cargo test -p smg-mesh --lib — 146 pass (4 fewer than pre-move because the tests moved with the file).
• cargo build -p smg-mesh -p smg — clean.
• cargo clippy --workspace --all-targets --all-features -- -D warnings — clean.

Checklist

• cargo +nightly fmt passes
• cargo clippy --all-targets --all-features -- -D warnings passes
• Documentation updated (tree_sync.rs doc reference)
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

Summary by CodeRabbit

• Refactor
  • Hash computation utilities have been reorganized across internal modules to improve code structure and maintainability.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 92d3dc22-83cd-42af-bb0b-98505bffa292

📥 Commits

Reviewing files that changed from the base of the PR and between fd89557 and 907aead.

📒 Files selected for processing (6)

• crates/kv_index/Cargo.toml
• crates/kv_index/src/lib.rs
• crates/kv_index/src/path_hash.rs
• crates/mesh/src/lib.rs
• model_gateway/src/mesh/adapters/tree_sync.rs
• model_gateway/src/policies/cache_aware.rs

💤 Files with no reviewable changes (1)

• crates/mesh/src/lib.rs

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📝 Walkthrough

Walkthrough

This PR migrates three hashing utilities (hash_node_path, hash_token_path, GLOBAL_EVICTION_HASH) from the smg_mesh crate to kv_index. The blake3 dependency is added to kv_index, and all call sites in model_gateway are updated to use the new module path. Documentation and tests are synchronized accordingly.

Changes

Hash Function Relocation

Layer / File(s)	Summary
kv_index hash module setup crates/kv_index/Cargo.toml, crates/kv_index/src/lib.rs	Blake3 is added as a workspace dependency. The path_hash submodule is declared and its hashing utilities are publicly re-exported from kv_index.
smg_mesh hash module removal crates/mesh/src/lib.rs	The internal hash module and its public re-exports are removed from smg_mesh as functions are now sourced from kv_index.
model_gateway routing and repair logic migration model_gateway/src/policies/cache_aware.rs	All hash function calls are switched from smg_mesh::hash_* to kv_index::hash_* across imbalanced worker selection, cold-start repair, and balanced tree routing paths.
model_gateway documentation and tests model_gateway/src/mesh/adapters/tree_sync.rs, model_gateway/src/policies/cache_aware.rs	Documentation comment and all unit tests are updated to reference kv_index hash functions instead of smg_mesh equivalents.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Possibly related PRs

• lightseekorg/smg#1476: Introduces and publicly re-exports GLOBAL_EVICTION_HASH, hash_node_path, and hash_token_path from crates/mesh/src/hash.rs, which are the exact utilities being relocated by this PR.
• lightseekorg/smg#1364: Modifies cache membership logic in cache_aware.rs and tree_sync.rs using the same hash functions; this PR updates those call sites to use the new module location.

Suggested labels

dependencies, kv-index, model-gateway, mesh, tests

Suggested reviewers

• tonyluj
• llfl
• slin1237

Poem

🐰 Hash functions hop to a new home today,
From mesh to kv_index they make their way,
Blake3 dependencies and tests align,
Migration complete—the logic will shine! ✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'refactor(mesh): move tree path hashing out of mesh into kv-index' directly summarizes the main change—relocating tree path hashing functions from the mesh crate to kv-index.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch refactor/move-tree-path-hash-to-kv-index

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[claude]

Clean refactoring — moves path hashing from mesh into kv_index where it logically belongs. File rename is 100% identical, all call sites updated, no stale references remain. blake3 dependency correctly added to kv_index/Cargo.toml. Doc comment in tree_sync.rs updated to reference the new location. LGTM.

[gemini-code-assist]

Code Review

This pull request relocates hashing utilities, specifically hash_node_path, hash_token_path, and the GLOBAL_EVICTION_HASH constant, from the mesh crate to the kv_index crate. The changes include adding the blake3 dependency to kv_index, creating the path_hash module, and updating all call sites within the model_gateway and mesh crates to point to the new location. I have no feedback to provide.