etcdserver: commit v3 storage before releasing WAL

This ensures that v3 storage could always find the following log entries
when restart.

etcdserver/server.go

@@ -1029,6 +1029,13 @@ func (s *EtcdServer) snapshot(snapi uint64, confState raftpb.ConfState) {
 			}
 			plog.Panicf("unexpected create snapshot error %v", err)
 		}
+		if s.cfg.V3demo {
+			// commit v3 storage because WAL file before snapshot index
+			// could be removed after SaveSnap.
+			s.kv.Commit()
+		}
+		// SaveSnap saves the snapshot and releases the locked wal files
+		// to the snapshot index.
 		if err := s.r.storage.SaveSnap(snap); err != nil {
 			plog.Fatalf("save snapshot error: %v", err)
 		}

etcdserver/snapshot_store_test.go

@@ -186,6 +186,7 @@ func (kv *nopKV) TxnDeleteRange(txnID int64, key, end []byte) (n, rev int64, err
 func (kv *nopKV) Compact(rev int64) error     { return nil }
 func (kv *nopKV) Hash() (uint32, error)       { return 0, nil }
 func (kv *nopKV) Snapshot() dstorage.Snapshot { return &fakeSnapshot{} }
+func (kv *nopKV) Commit()                     {}
 func (kv *nopKV) Restore() error              { return nil }
 func (kv *nopKV) Close() error                { return nil }
 

storage/kv.go

@@ -69,6 +69,9 @@ type KV interface {
 	// Snapshot snapshots the full KV store.
 	Snapshot() Snapshot
 
+	// Commit commits txns into the underlying backend.
+	Commit()
+
 	Restore() error
 	Close() error
 }

storage/kvstore.go

@@ -294,6 +294,8 @@ func (s *store) Snapshot() Snapshot {
 	return s.b.Snapshot()
 }
 
+func (s *store) Commit() { s.b.ForceCommit() }
+
 func (s *store) Restore() error {
 	s.mu.Lock()
 	defer s.mu.Unlock()