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rocksdb/unreleased_history/bug_fixes/fix_multiget_sv_cleanup.md
Yu Zhang e1fd348b92 Fix a bug in multiget for cleaning up SuperVersion (#11830) 
Summary:
When `MultiGet` acquires `SuperVersion` via locking the db mutex and get the current `ColumnFamilyData::super_version_`, its corresponding cleanup logic is not correctly done.

It's currently doing this:
`MultiGetColumnFamilyData::cfd->GetSuperVersion().Unref()`

This operates on the most recent `SuperVersion` without locking db mutex , which is not thread safe by itself. And this unref operation is intended for the originally acquired `SuperVersion` instead of the current one. Because a race condition could happen where a new `SuperVersion` is installed in between this `MultiGet`'s ref and unref. When this race condition does happen, it's not sufficient to just unref the `SuperVersion`, `DBImpl::CleanupSuperVersion` should be called instead to properly clean up the `SuperVersion` had this `MultiGet` call be its last reference holder.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11830

Test Plan:
`make all check`
Added a unit test that would originally fail

Reviewed By: ltamasi

Differential Revision: D49287715

Pulled By: jowlyzhang

fbshipit-source-id: 8353636ee11b2e90d85c677a96a92360072644b0
2023-09-15 09:50:39 -07:00

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Fixed a bug in MultiGet for cleaning up SuperVersion acquired with locking db mutex.