etcdserver: renaming db happens before snapshot persists to wal and s…

…nap files

In the case that follower recieves a snapshot from leader
and crashes before renaming xxx.snap.db to db but after
snapshot has persisted to .wal and .snap, restarting
follower results loading old db, new .wal, and new .snap.
This will causes a index mismatch between snap metadata index
and consistent index from db.

This pr forces an ordering where saving/renaming db must
happen before snapshot is persisted to wal and snap file.
this ensures that db file can never be newer than  wal and snap file.
hence, it guarantees the invariant snapshot.Metadata.Index <= db.ConsistentIndex()
in NewServer() when checking validity of db and snap file.

FIXES etcd-io#7628

etcdserver/raft.go

@@ -81,9 +81,10 @@ type RaftTimer interface {
 // to raft storage concurrently; the application must read
 // raftDone before assuming the raft messages are stable.
 type apply struct {
-	entries  []raftpb.Entry
-	snapshot raftpb.Snapshot
-	raftDone <-chan struct{} // rx {} after raft has persisted messages
+	entries       []raftpb.Entry
+	snapshot      raftpb.Snapshot
+	raftDone      <-chan struct{} // rx {} after raft has persisted messages
+	replaceDBDone chan struct{}   // snapshot db from leader has replaced the current db
 }
 
 type raftNode struct {
@@ -191,10 +192,12 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 				}
 
 				raftDone := make(chan struct{}, 1)
+				replaceDBDone := make(chan struct{}, 1)
 				ap := apply{
-					entries:  rd.CommittedEntries,
-					snapshot: rd.Snapshot,
-					raftDone: raftDone,
+					entries:       rd.CommittedEntries,
+					snapshot:      rd.Snapshot,
+					raftDone:      raftDone,
+					replaceDBDone: replaceDBDone,
 				}
 
 				updateCommittedIndex(&ap, rh)
@@ -223,6 +226,8 @@ func (r *raftNode) start(rh *raftReadyHandler) {
 				// gofail: var raftAfterSave struct{}
 
 				if !raft.IsEmptySnap(rd.Snapshot) {
+					// waits etcd server to finish renaming snap db to db.
+					<-replaceDBDone
 					// gofail: var raftBeforeSaveSnap struct{}
 					if err := r.storage.SaveSnap(rd.Snapshot); err != nil {
 						plog.Fatalf("raft save snapshot error: %v", err)

etcdserver/server.go

@@ -812,6 +812,8 @@ func (s *EtcdServer) applySnapshot(ep *etcdProgress, apply *apply) {
 	if err := os.Rename(snapfn, fn); err != nil {
 		plog.Panicf("rename snapshot file error: %v", err)
 	}
+	// notifies raftNode that db has been replaced.
+	apply.replaceDBDone <- struct{}{}
 
 	newbe := newBackend(fn, s.Cfg.QuotaBackendBytes)