Avoid logs_ operation out of DB mutex

Summary:
logs_.back() is called out of DB mutex, which can cause data race. We move the access into the DB mutex protection area.
Closes https://github.com/facebook/rocksdb/pull/1774

Reviewed By: AsyncDBConnMarkedDownDBException

Differential Revision: D4417472

Pulled By: AsyncDBConnMarkedDownDBException

fbshipit-source-id: 2da1f1e

db/db_impl.cc

@@ -4807,6 +4807,7 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
     // and protects against concurrent loggers and concurrent writes
     // into memtables
   }
+  log::Writer* cur_log_writer = logs_.back().writer;
 
   mutex_.Unlock();
 
@@ -4894,7 +4895,7 @@ Status DBImpl::WriteImpl(const WriteOptions& write_options,
       WriteBatchInternal::SetSequence(merged_batch, current_sequence);
 
       Slice log_entry = WriteBatchInternal::Contents(merged_batch);
-      status = logs_.back().writer->AddRecord(log_entry);
+      status = cur_log_writer->AddRecord(log_entry);
       total_log_size_ += log_entry.size();
       alive_log_files_.back().AddSize(log_entry.size());
       log_empty_ = false;