raftexample: Fix recovery from snapshot

* If there is a snapshot, HTTP server won't start.
* Resotring form snapshot occurs after replaying WAL.
* When taking a snapshot, the last change is not applied to the state machine yet.

contrib/raftexample/kvstore.go

@@ -21,6 +21,7 @@ import (
 	"log"
 	"sync"
 
+	"go.etcd.io/etcd/raft/v3/raftpb"
 	"go.etcd.io/etcd/server/v3/etcdserver/api/snap"
 )
 
@@ -39,8 +40,16 @@ type kv struct {
 
 func newKVStore(snapshotter *snap.Snapshotter, proposeC chan<- string, commitC <-chan *string, errorC <-chan error) *kvstore {
 	s := &kvstore{proposeC: proposeC, kvStore: make(map[string]string), snapshotter: snapshotter}
-	// replay log into key-value map
-	s.readCommits(commitC, errorC)
+	snapshot, err := s.loadSnapshot()
+	if err != nil {
+		log.Panic(err)
+	}
+	if snapshot != nil {
+		log.Printf("loading snapshot at term %d and index %d", snapshot.Metadata.Term, snapshot.Metadata.Index)
+		if err := s.recoverFromSnapshot(snapshot.Data); err != nil {
+			log.Panic(err)
+		}
+	}
 	// read commits from raft into kvStore map until error
 	go s.readCommits(commitC, errorC)
 	return s
@@ -64,18 +73,16 @@ func (s *kvstore) Propose(k string, v string) {
 func (s *kvstore) readCommits(commitC <-chan *string, errorC <-chan error) {
 	for data := range commitC {
 		if data == nil {
-			// done replaying log; new data incoming
-			// OR signaled to load snapshot
-			snapshot, err := s.snapshotter.Load()
-			if err == snap.ErrNoSnapshot {
-				return
-			}
+			// signaled to load snapshot
+			snapshot, err := s.loadSnapshot()
 			if err != nil {
 				log.Panic(err)
 			}
-			log.Printf("loading snapshot at term %d and index %d", snapshot.Metadata.Term, snapshot.Metadata.Index)
-			if err := s.recoverFromSnapshot(snapshot.Data); err != nil {
-				log.Panic(err)
+			if snapshot != nil {
+				log.Printf("loading snapshot at term %d and index %d", snapshot.Metadata.Term, snapshot.Metadata.Index)
+				if err := s.recoverFromSnapshot(snapshot.Data); err != nil {
+					log.Panic(err)
+				}
 			}
 			continue
 		}
@@ -100,6 +107,17 @@ func (s *kvstore) getSnapshot() ([]byte, error) {
 	return json.Marshal(s.kvStore)
 }
 
+func (s *kvstore) loadSnapshot() (*raftpb.Snapshot, error) {
+	snapshot, err := s.snapshotter.Load()
+	if err == snap.ErrNoSnapshot {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, err
+	}
+	return snapshot, nil
+}
+
 func (s *kvstore) recoverFromSnapshot(snapshot []byte) error {
 	var store map[string]string
 	if err := json.Unmarshal(snapshot, &store); err != nil {

contrib/raftexample/raft.go

@@ -50,7 +50,6 @@ type raftNode struct {
 	waldir      string   // path to WAL directory
 	snapdir     string   // path to snapshot directory
 	getSnapshot func() ([]byte, error)
-	lastIndex   uint64 // index of log at start
 
 	confState     raftpb.ConfState
 	snapshotIndex uint64
@@ -175,15 +174,6 @@ func (rc *raftNode) publishEntries(ents []raftpb.Entry) bool {
 
 		// after commit, update appliedIndex
 		rc.appliedIndex = ents[i].Index
-
-		// special nil commit to signal replay has finished
-		if ents[i].Index == rc.lastIndex {
-			select {
-			case rc.commitC <- nil:
-			case <-rc.stopc:
-				return false
-			}
-		}
 	}
 	return true
 }
@@ -240,12 +230,7 @@ func (rc *raftNode) replayWAL() *wal.WAL {
 
 	// append to storage so raft starts at the right place in log
 	rc.raftStorage.Append(ents)
-	// send nil once lastIndex is published so client knows commit channel is current
-	if len(ents) > 0 {
-		rc.lastIndex = ents[len(ents)-1].Index
-	} else {
-		rc.commitC <- nil
-	}
+
 	return w
 }
 
@@ -264,11 +249,13 @@ func (rc *raftNode) startRaft() {
 		}
 	}
 	rc.snapshotter = snap.New(zap.NewExample(), rc.snapdir)
-	rc.snapshotterReady <- rc.snapshotter
 
 	oldwal := wal.Exist(rc.waldir)
 	rc.wal = rc.replayWAL()
 
+	// signal replay has finished
+	rc.snapshotterReady <- rc.snapshotter
+
 	rpeers := make([]raft.Peer, len(rc.peers))
 	for i := range rpeers {
 		rpeers[i] = raft.Peer{ID: uint64(i + 1)}

contrib/raftexample/raftexample_test.go

@@ -61,17 +61,6 @@ func newCluster(n int) *cluster {
 	return clus
 }
 
-// sinkReplay reads all commits in each node's local log.
-func (clus *cluster) sinkReplay() {
-	for i := range clus.peers {
-		for s := range clus.commitC[i] {
-			if s == nil {
-				break
-			}
-		}
-	}
-}
-
 // Close closes all cluster nodes and returns an error if any failed.
 func (clus *cluster) Close() (err error) {
 	for i := range clus.peers {
@@ -102,8 +91,6 @@ func TestProposeOnCommit(t *testing.T) {
 	clus := newCluster(3)
 	defer clus.closeNoErrors(t)
 
-	clus.sinkReplay()
-
 	donec := make(chan struct{})
 	for i := range clus.peers {
 		// feedback for "n" committed entries, then update donec
@@ -149,8 +136,6 @@ func TestCloseProposerInflight(t *testing.T) {
 	clus := newCluster(1)
 	defer clus.closeNoErrors(t)
 
-	clus.sinkReplay()
-
 	// some inflight ops
 	go func() {
 		clus.proposeC[0] <- "foo"