featuregate: adds EtcdServer.FeatureEnabled interface.

The interface can be used throughout the etcd server binary to check if
the feature is enabled or not.

pkg/featuregate/featuregate.go

@@ -0,0 +1,35 @@
+// Copyright 2015 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package featuregate implements feature gate for to track/manage new features being added to etcd servers.
+package featuregate
+
+import "fmt"
+
+type Feature string
+
+// defaultFeatureGates tracks Features added (but not yet enabled) to etcd servers.
+// TODO(stackbaek): use VersionedSpecs, once kep-4330 is implemented.
+var defaultFeatureGates = map[Feature]interface{}{}
+
+type FeatureGate struct{}
+
+// Enabled returns true if a Feature is enabled, false otherwise.
+// Note that the error will be returned if the Feature is not added to the etcd servers.
+func (fg FeatureGate) Enabled(f Feature) (bool, error) {
+	if _, ok := defaultFeatureGates[f]; !ok {
+		return false, fmt.Errorf("Unknown feature %s.", f)
+	}
+	return false, nil
+}

server/config/config.go

@@ -28,6 +28,7 @@ import (
 	bolt "go.etcd.io/bbolt"
 	"go.etcd.io/etcd/client/pkg/v3/transport"
 	"go.etcd.io/etcd/client/pkg/v3/types"
+	"go.etcd.io/etcd/pkg/v3/featuregate"
 	"go.etcd.io/etcd/pkg/v3/netutil"
 	"go.etcd.io/etcd/server/v3/etcdserver/api/v3discovery"
 	"go.etcd.io/etcd/server/v3/storage/datadir"
@@ -204,6 +205,9 @@ type ServerConfig struct {
 
 	// V2Deprecation defines a phase of v2store deprecation process.
 	V2Deprecation V2DeprecationEnum `json:"v2-deprecation"`
+
+	// ServerFeatureGate server level feature gate
+	ServerFeatureGate featuregate.FeatureGate
 }
 
 // VerifyBootstrap sanity-checks the initial config for bootstrap case

server/etcdserver/server.go

@@ -39,6 +39,7 @@ import (
 	"go.etcd.io/etcd/client/pkg/v3/fileutil"
 	"go.etcd.io/etcd/client/pkg/v3/types"
 	"go.etcd.io/etcd/client/pkg/v3/verify"
+	"go.etcd.io/etcd/pkg/v3/featuregate"
 	"go.etcd.io/etcd/pkg/v3/idutil"
 	"go.etcd.io/etcd/pkg/v3/notify"
 	"go.etcd.io/etcd/pkg/v3/pbutil"
@@ -456,6 +457,15 @@ func (s *EtcdServer) Config() config.ServerConfig {
 	return s.Cfg
 }
 
+// FeatureEnabled returns true if the feature is enabled by the etcd server, false otherwise.
+func (s *EtcdServer) FeatureEnabled(f featuregate.Feature) bool {
+	enabled, err := s.Cfg.ServerFeatureGate.Enabled(f)
+	if err != nil {
+		s.Logger().Warn(fmt.Sprintf("Failed to check if feature is enabled: %v", err))
+	}
+	return enabled
+}
+
 func tickToDur(ticks int, tickMs uint) string {
 	return fmt.Sprintf("%v", time.Duration(ticks)*time.Duration(tickMs)*time.Millisecond)
 }