@greptileai review

Greptile Summary

This PR refactors the tag event handlers to support partial recovery by adopting a graph-last deletion order, a check_set_member guard to prevent double-decrements on retry, and an Updated-branch retry path for PUT that re-runs idempotent Redis writes.

• P1: The Updated retry branch in put_sync_post (and put_sync_user) re-runs only the three idempotent Redis writes; all counter increments (UserCounts, PostCounts) and score updates (TagPost, PostsByTagSearch, PostStream) are silently skipped. If the original PUT's graph write succeeded but every Redis op failed, the retry leaves counts and scores permanently incorrect with no auto-healing path.
• P2: TagUser::del_from_graph is called as the graph-deletion step for both user-tag and post-tag paths, which is functionally correct but reads as a type mismatch for the post-tag case; a clarifying comment is needed.

Confidence Score: 3/5

Safe to merge for the DEL-side improvements; the PUT Updated retry path leaves a known correctness gap in counter/score recovery that should be addressed before this is relied upon in production.

DEL handler changes are well-reasoned and tested. The PUT Updated branch is intentionally partial but could silently leave Redis counts wrong in a graph-wrote/Redis-never-ran failure scenario, which may be hard to detect without monitoring.

nexus-watcher/src/events/handlers/tag.rs — Updated branches in put_sync_post and put_sync_user

Important Files Changed

Sequence Diagram

sequenceDiagram
    participant H as tag::del handler
    participant G as Neo4j Graph
    participant R as Redis

    Note over H: DEL flow (graph-last)
    H->>G: get_tag_target(user_id, tag_id)
    G-->>H: Some(tagged_id, post_id, author_id, label) OR None

    alt Edge found
        H->>R: check_set_member(tagger_in_index?)
        R-->>H: bool
        alt tagger_in_index == true
            H->>R: decrement counters + scores
            H->>R: send untag notification
        end
        H->>R: SREM tagger (idempotent)
        H->>R: del_from_index / PostsByTagSearch cleanup (idempotent)
        H->>G: del_from_graph (graph-last)
    else Edge already gone (idempotent replay)
        H-->>H: return Ok(()) silently
    end

    Note over H: PUT flow (Updated = prior attempt)
    H->>G: put_to_graph(...)
    G-->>H: Updated (edge existed)
    H->>R: add_tagger_to_index (idempotent set-add)
    H->>R: PostsByTagSearch::put_to_index (idempotent)
    H->>R: TagSearch::put_to_index (idempotent)
    Note over H,R: CounterIncrements / ScoreUpdates NOT retried

_{Reviews (1): Last reviewed commit: "review" | Re-trigger Greptile}