Fix a data race between GetColumnFamilyMetaData and MarkFilesBeingCompacted (#6056)

Summary:
Use db mutex to protect the execution of Version::GetColumnFamilyMetaData()
called in DBImpl::GetColumnFamilyMetaData().
Without mutex, GetColumnFamilyMetaData() races with MarkFilesBeingCompacted()
for access to FileMetaData::being_compacted.
Other than mutex, there are several more alternatives.

- Make FileMetaData::being_compacted an atomic variable. This will make
  FileMetaData non-copy-able.

- Separate being_compacted from FileMetaData. This requires re-organizing data
  structures that are already used in many places.

Test Plan (dev server):
```
make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6056

Differential Revision: D18620488

Pulled By: riversand963

fbshipit-source-id: 87f89660b5d5e2ab4ef7962b7b2a7d00e346aa3b

HISTORY.md

@@ -2,6 +2,7 @@
 ## Unreleased
 ### Bug Fixes
 * Fix data corruption casued by output of intra-L0 compaction on ingested file not being placed in correct order in L0.
+* Fix a data race between Version::GetColumnFamilyMetaData() and Compaction::MarkFilesBeingCompacted() for access to being_compacted (#6056). The current fix acquires the db mutex during Version::GetColumnFamilyMetaData(), which may cause regression.
 
 ### Public API Change
 * TTL Compactions in Level compaction style now initiate successive cascading compactions on a key range so that it reaches the bottom level quickly on TTL expiry. `creation_time` table property for compaction output files is now set to the minimum of the creation times of all compaction inputs.

db/db_impl/db_impl.cc

@@ -3241,7 +3241,19 @@ void DBImpl::GetColumnFamilyMetaData(ColumnFamilyHandle* column_family,
   assert(column_family);
   auto* cfd = reinterpret_cast<ColumnFamilyHandleImpl*>(column_family)->cfd();
   auto* sv = GetAndRefSuperVersion(cfd);
-  sv->current->GetColumnFamilyMetaData(cf_meta);
+  {
+    // Without mutex, Version::GetColumnFamilyMetaData will have data race with
+    // Compaction::MarkFilesBeingCompacted. One solution is to use mutex, but
+    // this may cause regression. An alternative is to make
+    // FileMetaData::being_compacted atomic, but it will make FileMetaData
+    // non-copy-able. Another option is to separate these variables from
+    // original FileMetaData struct, and this requires re-organization of data
+    // structures. For now, we take the easy approach. If
+    // DB::GetColumnFamilyMetaData is not called frequently, the regression
+    // should not be big. We still need to keep an eye on it.
+    InstrumentedMutexLock l(&mutex_);
+    sv->current->GetColumnFamilyMetaData(cf_meta);
+  }
   ReturnAndCleanupSuperVersion(cfd, sv);
 }