Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14442
In `BackgroundEmptyTrash()`, the bucket counter uses post-decrement
(`iter->second--`), which assigns the pre-decrement value to
`pending_files_in_bucket`. When a bucket transitions from 1 to 0 files,
`pending_files_in_bucket` receives the value 1 (not 0), so the check
`pending_files_in_bucket == 0` fails. This means `WaitForEmptyTrashBucket()`
is never woken up when a specific bucket empties, unless all global pending
files are also zero.
Fix: Use pre-decrement (`--iter->second`) so the decremented value correctly
triggers the condition variable signal.
Reviewed By: xingbowang
Differential Revision: D95303385
fbshipit-source-id: 3c5f0978ff33600acaf406b3f839cf13d9983055
2afb387917