Avoid chaos between NotifyOnMemTableSealed and NotifyOnFlushCompleted (…

…#347)

Signed-off-by: SpadeA-Tang <u6748471@anu.edu.au>

db/db_impl/db_impl_write.cc

@@ -2254,11 +2254,16 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
  mutable_cf_options);
 
 #ifndef ROCKSDB_LITE
- mutex_.Unlock();
+ // From above, the memtable has been converted to imm memtable and apended to
+ // memlist. If we unlock here, it has possibility to be picked by a concurrent
+ // flush job. Furthermore, that flush job may even be completed and calls
+ // NotifyOnFlushCompleted before the following NotifyOnMemTableSealed which
+ // can cause troubles.
+ // So, unlike Facebook/Rocksdb, we does not unlock here.
+
  // Notify client that memtable is sealed, now that we have successfully
  // installed a new memtable
  NotifyOnMemTableSealed(cfd, memtable_info);
- mutex_.Lock();
 #endif // ROCKSDB_LITE
  // It is possible that we got here without checking the value of i_os, but
  // that is okay. If we did, it most likely means that s was already an error.