overwrite the member if already exist

When promoting a learner, the member is definitely already
exist.

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

Author: ahrtr

server/etcdserver/api/membership/cluster_test.go

@@ -1048,12 +1048,6 @@ func TestAddMemberSyncsBackendAndStoreV2(t *testing.T) {
 			name:           "Adding member should succeed if it was only in backend",
 			backendMembers: []*Member{alice},
 		},
-		{
-			name:           "Adding member should fail if it exists in both",
-			storeV2Members: []*Member{alice},
-			backendMembers: []*Member{alice},
-			expectPanics:   true,
-		},
 		{
 			name:           "Adding member should fail if it exists in storeV2 and backend is nil",
 			storeV2Members: []*Member{alice},

server/etcdserver/api/membership/store.go

@@ -54,9 +54,6 @@ func unsafeSaveMemberToBackend(lg *zap.Logger, be backend.Backend, m *Member) er
 	tx := be.BatchTx()
 	tx.LockInsideApply()
 	defer tx.Unlock()
-	if unsafeMemberExists(tx, mkey) {
-		return errMemberAlreadyExist
-	}
 	tx.UnsafePut(buckets.Members, mkey, mvalue)
 	return nil
 }

server/etcdserver/server.go

@@ -2470,9 +2470,9 @@ func (s *EtcdServer) verifyV3StoreInSyncWithV2Store() {
 		v3Data, v3Err := json.Marshal(v3Members)
 
 		if v2Err != nil || v3Err != nil {
-			panic("members in v2store doesn't members in v3store")
+			panic("members in v2store doesn't match v3store")
 		}
-		panic(fmt.Sprintf("members in v2store doesn't v3store, v2store: %s, v3store: %s", string(v2Data), string(v3Data)))
+		panic(fmt.Sprintf("members in v2store doesn't match v3store, v2store: %s, v3store: %s", string(v2Data), string(v3Data)))
 	}
 
 	if match := reflect.DeepEqual(v2RemovedMembers, v3RemovedMembers); !match {