Fix a race condition in DumpStats() during iteration of the ColumnFamilySet (#8714)

Summary:
DumpStats() iterates through the ColumnFamilySet. There is a potential
race condition because it does Ref the cfd, and the cfd could get
destroyed during the iteration.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8714

Test Plan: make check

Reviewed By: ltamasi

Differential Revision: D30580199

Pulled By: anand1976

fbshipit-source-id: 60a3443ad0d4f7ac6a977dec780e6d2c1b70b850

HISTORY.md

@@ -3,6 +3,7 @@
 ### Bug Fixes
 * Allow secondary instance to refresh iterator. Assign read seq after referencing SuperVersion.
 * Fixed a bug of secondary instance's last_sequence going backward, and reads on the secondary fail to see recent updates from the primary.
+* Fix a race in DumpStats() with column family destruction due to not taking a Ref on each entry while iterating the ColumnFamilySet.
 
 ### New Features
 * RemoteCompaction's interface now includes `db_name`, `db_id`, `session_id`, which could help the user uniquely identify compaction job between db instances and sessions.

db/db_impl/db_impl.cc

@@ -958,8 +958,13 @@ void DBImpl::DumpStats() {
         // Release DB mutex for gathering cache entry stats. Pass over all
         // column families for this first so that other stats are dumped
         // near-atomically.
-        InstrumentedMutexUnlock u(&mutex_);
+        // Get a ref before unlocking
-        cfd->internal_stats()->CollectCacheEntryStats(/*foreground=*/false);
+        cfd->Ref();
+        {
+          InstrumentedMutexUnlock u(&mutex_);
+          cfd->internal_stats()->CollectCacheEntryStats(/*foreground=*/false);
+        }
+        cfd->UnrefAndTryDelete();
       }
     }