support multi-thread compaction

Author: caijieming

src/leveldb/db/db_impl.cc

@@ -949,17 +949,15 @@ Status DBImpl::RecoverInsertMem(WriteBatch* batch, VersionEdit* edit) {
 Status DBImpl::RecoverLastDumpToLevel0(VersionEdit* edit) {
   MutexLock lock(&mutex_);
   Status s;
-  if (recover_mem_ == NULL) {
-      DeleteObsoleteFiles();
-      MaybeScheduleCompaction();
-      return s;
-  }
-  if (recover_mem_->GetLastSequence() > 0) {
+  if (recover_mem_ != NULL) {
+    if (recover_mem_->GetLastSequence() > 0) {
       edit->SetLastSequence(recover_mem_->GetLastSequence());
       s = WriteLevel0Table(recover_mem_, edit, NULL);
+    }
+    recover_mem_->Unref();
+    recover_mem_ = NULL;
   }
-  recover_mem_->Unref();
-  recover_mem_ = NULL;
+  assert(recover_mem_ == NULL);
 
   // LogAndApply to lg's manifest
   if (s.ok()) {