The pending drop ident reaper does not notify the MongoDB layer cursor caches to release any cursors on tables that we need to drop. This then results in the pending drop ident reaper consistently failing to drop these idents until the cursors are cleared from the cache the normal way which happens when sessions are released for the standard hybrid cursor caching approach. If the queries running are very slow this can then take a while before free space is reclaimed.

The problem is worse if the wiredTigerCursorCacheSize setting is set to a positive value where these cursors can then remain in the cache for a long time.

Before SERVER-69363 we had a separate drop pending system that did clear cursors from cached sessions. But even this system has issues as only cursors in cached (non-active) sessions would be cleared. So an aggregation that has created a temporary table would not be able to have this table dropped until the whole aggregation is completed and the session is released (and the cursors cleared).