LGTM

Found 1 issue.

1. High: cloud/src/recycler/recycler.cpp now skips KV deletion whenever the successor instance key still exists, but I could not find any production path that ever clears successor_instance_id on the deleted source instance after the handoff finishes. SnapshotManager::decouple_instance() only clears source_instance_id / source_snapshot_id on the successor, and the only set_successor_instance_id() usage in this repo is the test setup. That means a deleted source instance will keep returning early here for as long as its successor instance exists, so its meta/versioned KV ranges are never reclaimed. This fixes the immediate data-loss concern by turning it into a permanent recycle leak.

Critical checkpoint conclusions:

• Goal of the task: protect metadata still needed by a rollback successor. Current code does not achieve that safely end-to-end because it blocks recycler progress on successor existence alone, and there is no test proving eventual cleanup.
• Small/clear/focused: the patch is small, but the lifecycle condition is incomplete.
• Concurrency: no new in-memory lock ordering issue found; this path uses txn-kv operations only.
• Lifecycle management: this is the main problem. The deleted-source/successor lifecycle is not closed because successor_instance_id is never cleared in production code.
• Configuration: no new config.
• Compatibility: no FE/BE or storage-format compatibility issue introduced.
• Parallel code paths: no equivalent recycler path appears to handle this cleanup elsewhere.
• Conditional checks: the new guard lacks a proven terminal condition; mere successor existence is too broad.
• Test coverage: insufficient. Existing test coverage only exercises resource-manager refresh behavior, not deleted-instance recycling or eventual cleanup.
• Observability: logging is acceptable for diagnosing this path.
• Transaction/persistence: yes, this changes persistent cleanup behavior, and the new condition can leave persistent KV data behind indefinitely.
• Data write/modification safety: not safe from a cleanup-progress perspective because deleted instances can become unrecyclable forever.
• FE/BE variable propagation: not applicable.
• Performance: repeated recycler scans on permanently skipped deleted instances can accumulate avoidable background work.
• Other issues: none beyond the finding above.

Recommend guarding on the actual remaining dependency, or clearing successor_instance_id when the successor no longer needs the source instance metadata, and adding a recycler test for that lifecycle.