Summary:
Possible interleaved execution of background compaction thread calling `FindObsoleteFiles (no full scan) / PurgeObsoleteFiles` and user thread calling `FindObsoleteFiles (full scan) / PurgeObsoleteFiles` can lead to race condition on which RocksDB attempts to delete a file twice. The second attempt will fail and return `IO error`. This may occur to other files, but this PR targets sst.
Also add a unit test to verify that this PR fixes the issue.
The newly added unit test `obsolete_files_test` has a test case for this scenario, implemented in `ObsoleteFilesTest#RaceForObsoleteFileDeletion`. `TestSyncPoint`s are used to coordinate the interleaving the `user_thread` and background compaction thread. They execute as follows
```
timeline user_thread background_compaction thread
t1 | FindObsoleteFiles(full_scan=false)
t2 | FindObsoleteFiles(full_scan=true)
t3 | PurgeObsoleteFiles
t4 | PurgeObsoleteFiles
V
```
When `user_thread` invokes `FindObsoleteFiles` with full scan, it collects ALL files in RocksDB directory, including the ones that background compaction thread have collected in its job context. Then `user_thread` will see an IO error when trying to delete these files in `PurgeObsoleteFiles` because background compaction thread has already deleted the file in `PurgeObsoleteFiles`.
To fix this, we make RocksDB remember which (SST) files have been found by threads after calling `FindObsoleteFiles` (see `DBImpl#files_grabbed_for_purge_`). Therefore, when another thread calls `FindObsoleteFiles` with full scan, it will not collect such files.
ajkr could you take a look and comment? Thanks!
Closes https://github.com/facebook/rocksdb/pull/3638
Differential Revision: D7384372
Pulled By: riversand963
fbshipit-source-id: 01489516d60012e722ee65a80e1449e589ce26d3
