Fix DropColumnFamily data race

Summary:
It should hold db mutex while accessing max_total_in_memory_state_.
Closes https://github.com/facebook/rocksdb/pull/2784

Differential Revision: D5696536

Pulled By: yiwu-arbug

fbshipit-source-id: 45430634d7fe11909b38e42e5f169f618681c4ee

db/db_impl.cc

@@ -1324,6 +1324,11 @@ Status DBImpl::DropColumnFamilyImpl(ColumnFamilyHandle* column_family) {
                                  &edit, &mutex_);
       write_thread_.ExitUnbatched(&w);
     }
+    if (s.ok()) {
+      auto* mutable_cf_options = cfd->GetLatestMutableCFOptions();
+      max_total_in_memory_state_ -= mutable_cf_options->write_buffer_size *
+                                    mutable_cf_options->max_write_buffer_number;
+    }
 
     if (!cf_support_snapshot) {
       // Dropped Column Family doesn't support snapshot. Need to recalculate
@@ -1345,9 +1350,6 @@ Status DBImpl::DropColumnFamilyImpl(ColumnFamilyHandle* column_family) {
     // later inside db_mutex.
     EraseThreadStatusCfInfo(cfd);
     assert(cfd->IsDropped());
-    auto* mutable_cf_options = cfd->GetLatestMutableCFOptions();
-    max_total_in_memory_state_ -= mutable_cf_options->write_buffer_size *
-                                  mutable_cf_options->max_write_buffer_number;
     ROCKS_LOG_INFO(immutable_db_options_.info_log,
                    "Dropped column family with id %u\n", cfd->GetID());
   } else {