MongoDB: a TTL enable whose index build lands after DeleteTable dropped the collection recreates it #605

Summary

On MongoDB, UpdateTimeToLive enable runs update_ttl on the catalog document, then create_ttl_index, which reads the document and runs create_index on _ddb_<table_id>. A DeleteTable that marks the document and drops the collection between those two steps lets create_index create the collection again to hold the index; the delete then removes the only document that names it, and nothing sweeps orphaned data collections.

Measured (MongoDB 7, single-node replica set, 2026-10-26)

DeleteTable and UpdateTimeToLive enable raced through the API, twenty rounds per build: an orphaned _ddb_<id> collection with idx_ttl_ttl after 4 of 20 rounds on main. With #599 (no index DDL for a document read as DELETING, and an ACTIVE filter on create_ttl_index's catalog read) the window narrows to a catalog read taken before the mark with the create_index after the drop: 1 of 20 in one run and 0 of 80 in another.

Notes

The remaining case needs the collection drop and the index build to be ordered, or a sweep for data collections with no catalog document (worker_store.rs reconciles CREATING to ACTIVE only). list_collection_names on the data database shows the orphan.