etcdserver, pkg: skip needless log entry applying

This commit lets etcdserver skip needless log entry applying. If the
result of log applying isn't required by the node (client that issued
the request isn't talking with the node) and the operation has no side
effects, applying can be skipped.

It would contribute to reduce disk I/O on followers and be useful for
a cluster that processes much serializable get.

etcdserver/apply.go

@@ -784,3 +784,7 @@ func compareInt64(a, b int64) int {
 func isGteRange(rangeEnd []byte) bool {
 	return len(rangeEnd) == 1 && rangeEnd[0] == 0
 }
+
+func noSideEffect(r *pb.InternalRaftRequest) bool {
+	return r.Range != nil || r.AuthUserGet != nil || r.AuthRoleGet != nil
+}

etcdserver/server.go

@@ -1082,8 +1082,16 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
 		id = raftReq.Header.ID
 	}
 
-	ar := s.applyV3.Apply(&raftReq)
+	var ar *applyResult
+	if s.w.IsRegistered(id) || !noSideEffect(&raftReq) {
+		ar = s.applyV3.Apply(&raftReq)
+	}
 	s.setAppliedIndex(e.Index)
+
+	if ar == nil {
+		return
+	}
+
 	if ar.err != ErrNoSpace || len(s.alarmStore.Get(pb.AlarmType_NOSPACE)) > 0 {
 		s.w.Trigger(id, ar)
 		return

pkg/mock/mockwait/wait_recorder.go

@@ -41,3 +41,7 @@ func (w *waitRecorder) Register(id uint64) <-chan interface{} {
 func (w *waitRecorder) Trigger(id uint64, x interface{}) {
 	w.Record(testutil.Action{Name: "Trigger"})
 }
+
+func (w *waitRecorder) IsRegistered(id uint64) bool {
+	panic("waitRecorder.IsRegistered() shouldn't be called")
+}

pkg/wait/wait.go

@@ -24,6 +24,7 @@ import (
 type Wait interface {
 	Register(id uint64) <-chan interface{}
 	Trigger(id uint64, x interface{})
+	IsRegistered(id uint64) bool
 }
 
 type List struct {
@@ -59,6 +60,13 @@ func (w *List) Trigger(id uint64, x interface{}) {
 	}
 }
 
+func (w *List) IsRegistered(id uint64) bool {
+	w.l.Lock()
+	defer w.l.Unlock()
+	_, ok := w.m[id]
+	return ok
+}
+
 type waitWithResponse struct {
 	ch <-chan interface{}
 }
@@ -71,3 +79,6 @@ func (w *waitWithResponse) Register(id uint64) <-chan interface{} {
 	return w.ch
 }
 func (w *waitWithResponse) Trigger(id uint64, x interface{}) {}
+func (w *waitWithResponse) IsRegistered(id uint64) bool {
+	panic("waitWithResponse.IsRegistered() shouldn't be called")
+}

pkg/wait/wait_test.go

@@ -77,3 +77,26 @@ func TestTriggerDupSuppression(t *testing.T) {
 		t.Errorf("unexpected non-nil value: %v (%T)", g, g)
 	}
 }
+
+func TestIsRegistered(t *testing.T) {
+	wt := New()
+
+	wt.Register(0)
+	wt.Register(1)
+	wt.Register(2)
+
+	for i := uint64(0); i < 3; i++ {
+		if !wt.IsRegistered(i) {
+			t.Errorf("event ID %d isn't registered", i)
+		}
+	}
+
+	if wt.IsRegistered(4) {
+		t.Errorf("event ID 4 shouldn't be registered")
+	}
+
+	wt.Trigger(0, "foo")
+	if wt.IsRegistered(0) {
+		t.Errorf("event ID 0 is already triggered, shouldn't be registered")
+	}
+}