From b3841afcc33f25d0f0607e8232326c335553c174 Mon Sep 17 00:00:00 2001
From: Xiang Li <xiang.li@coreos.com>
Date: Tue, 2 Dec 2014 21:34:14 -0800
Subject: [PATCH] raft: do not restore snapshot if local raft has longer
 matching history

Raft should not restore the snapshot if it has longer matching history.
Or restoring snapshot might remove the matched entries.
---
 raft/raft.go      |  7 +++++++
 raft/raft_test.go | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/raft/raft.go b/raft/raft.go
index bd3347384..0100bad15 100644
--- a/raft/raft.go
+++ b/raft/raft.go
@@ -581,6 +581,13 @@ func (r *raft) restore(s pb.Snapshot) bool {
 	if s.Metadata.Index <= r.raftLog.committed {
 		return false
 	}
+	if r.raftLog.matchTerm(s.Metadata.Index, s.Metadata.Term) {
+		log.Printf("raft: %x [commit: %d, lastindex: %d, lastterm: %d] fast-forwarded commit to snapshot [index: %d, term: %d]",
+			r.id, r.Commit, r.raftLog.lastIndex(), r.raftLog.lastTerm(), s.Metadata.Index, s.Metadata.Term)
+		r.raftLog.commitTo(s.Metadata.Index)
+		return false
+	}
+
 	log.Printf("raft: %x [commit: %d, lastindex: %d, lastterm: %d] starts to restore snapshot [index: %d, term: %d]",
 		r.id, r.Commit, r.raftLog.lastIndex(), r.raftLog.lastTerm(), s.Metadata.Index, s.Metadata.Term)
 
diff --git a/raft/raft_test.go b/raft/raft_test.go
index d4ed2ea5d..5cc51e25c 100644
--- a/raft/raft_test.go
+++ b/raft/raft_test.go
@@ -1121,6 +1121,40 @@ func TestRestore(t *testing.T) {
 	}
 }
 
+func TestRestoreIgnoreSnapshot(t *testing.T) {
+	previousEnts := []pb.Entry{{Term: 1, Index: 1}, {Term: 1, Index: 2}, {Term: 1, Index: 3}}
+	commit := uint64(1)
+	storage := NewMemoryStorage()
+	sm := newRaft(1, []uint64{1, 2}, 10, 1, storage)
+	sm.raftLog.append(previousEnts...)
+	sm.raftLog.commitTo(commit)
+
+	s := pb.Snapshot{
+		Metadata: pb.SnapshotMetadata{
+			Index:     commit,
+			Term:      1,
+			ConfState: pb.ConfState{Nodes: []uint64{1, 2}},
+		},
+	}
+
+	// ignore snapshot
+	if ok := sm.restore(s); ok {
+		t.Errorf("restore = %t, want %t", ok, false)
+	}
+	if sm.raftLog.committed != commit {
+		t.Errorf("commit = %d, want %d", sm.raftLog.committed, commit)
+	}
+
+	// ignore snapshot and fast forward commit
+	s.Metadata.Index = commit + 1
+	if ok := sm.restore(s); ok {
+		t.Errorf("restore = %t, want %t", ok, false)
+	}
+	if sm.raftLog.committed != commit+1 {
+		t.Errorf("commit = %d, want %d", sm.raftLog.committed, commit+1)
+	}
+}
+
 func TestProvideSnap(t *testing.T) {
 	// restore the statemachin from a snapshot
 	// so it has a compacted log and a snapshot