raft: add Ready.MustSync

Add Ready.MustSync which indicates that the hard state and raft log
entries in a Ready message must be synchronously written to persistent
storage.

raft/node.go

@@ -83,6 +83,10 @@ type Ready struct {
 	// If it contains a MsgSnap message, the application MUST report back to raft
 	// when the snapshot has been received or has failed by calling ReportSnapshot.
 	Messages []pb.Message
+
+	// MustSync indicates whether the HardState and Entries must be synchronously
+	// written to disk or if an asynchronous write is permissible.
+	MustSync bool
 }
 
 func isHardStateEqual(a, b pb.HardState) bool {
@@ -517,5 +521,17 @@ func newReady(r *raft, prevSoftSt *SoftState, prevHardSt pb.HardState) Ready {
 	if len(r.readStates) != 0 {
 		rd.ReadStates = r.readStates
 	}
+	rd.MustSync = MustSync(rd.HardState, prevHardSt, len(rd.Entries))
 	return rd
 }
+
+// MustSync returns true if the hard state and count of Raft entries indicate
+// that a synchronous write to persistent storage is required.
+func MustSync(st, prevst pb.HardState, entsnum int) bool {
+	// Persistent state on all servers:
+	// (Updated on stable storage before responding to RPCs)
+	// currentTerm
+	// votedFor
+	// log entries[]
+	return entsnum != 0 || st.Vote != prevst.Vote || st.Term != prevst.Term
+}