etcdserver: return membership.ErrIDNotFound when the memberID not found

Backport https://github.com/etcd-io/etcd/pull/15095 to 3.4.

When promoting a learner, we need to wait until the leader's applied ID
catches up to the commitId. Afterwards, check whether the learner ID
exist or not, and return `membership.ErrIDNotFound` directly in the API
if the member ID not found, to avoid the request being unnecessarily
delivered to raft.

Signed-off-by: Benjamin Wang <wachao@vmware.com>

etcdserver/server.go

@@ -1798,6 +1798,10 @@ func (s *EtcdServer) mayPromoteMember(id types.ID) error {
 // Note: it will return nil if member is not found in cluster or if member is not learner.
 // These two conditions will be checked before apply phase later.
 func (s *EtcdServer) isLearnerReady(id uint64) error {
+	if err := s.waitAppliedIndex(); err != nil {
+		return err
+	}
+
 	rs := s.raftStatus()
 
 	// leader's raftStatus.Progress is not nil
@@ -1817,13 +1821,17 @@ func (s *EtcdServer) isLearnerReady(id uint64) error {
 		}
 	}
 
-	if isFound {
+	// We should return an error in API directly, to avoid the request
+	// being unnecessarily delivered to raft.
+	if !isFound {
+		return membership.ErrIDNotFound
+	}
+
 	leaderMatch := rs.Progress[leaderID].Match
 	// the learner's Match not caught up with leader yet
 	if float64(learnerMatch) < float64(leaderMatch)*readyPercent {
 		return ErrLearnerNotReady
 	}
-	}
 
 	return nil
 }