featuregate: adds EtcdServer.FeatureEnabled interface.

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

Note that this commit also copies necessary FeatureGate interface from
k8s component-base.

go.mod

@@ -1,6 +1,6 @@
 module go.etcd.io/etcd/v3
 
-go 1.22
+go 1.22.0
 
 toolchain go1.22.4
 
@@ -95,6 +95,6 @@ require (
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20240520151616-dc85e6b867a5 // indirect
 	gopkg.in/natefinch/lumberjack.v2 v2.2.1 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect
-	sigs.k8s.io/json v0.0.0-20211020170558-c049b76a60c6 // indirect
+	sigs.k8s.io/json v0.0.0-20221116044647-bc3834ca7abd // indirect
 	sigs.k8s.io/yaml v1.4.0 // indirect
 )

go.sum

@@ -250,7 +250,7 @@ gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
 gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
 honnef.co/go/tools v0.0.0-20190102054323-c2f93a96b099/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
 honnef.co/go/tools v0.0.0-20190523083050-ea95bdfd59fc/go.mod h1:rf3lG4BRIbNafJWhAfAdb/ePZxsR/4RtNHQocxwk9r4=
-sigs.k8s.io/json v0.0.0-20211020170558-c049b76a60c6 h1:fD1pz4yfdADVNfFmcP2aBEtudwUQ1AlLnRBALr33v3s=
-sigs.k8s.io/json v0.0.0-20211020170558-c049b76a60c6/go.mod h1:p4QtZmO4uMYipTQNzagwnNoseA6OxSUutVw05NhYDRs=
+sigs.k8s.io/json v0.0.0-20221116044647-bc3834ca7abd h1:EDPBXCAspyGV4jQlpZSudPeMmr1bNJefnuqLsRAsHZo=
+sigs.k8s.io/json v0.0.0-20221116044647-bc3834ca7abd/go.mod h1:B8JuhiUyNFVKdsE8h686QcCxMaH6HrOAZj4vswFpcB0=
 sigs.k8s.io/yaml v1.4.0 h1:Mk1wCc2gy/F0THH0TAp1QYyJNzRm2KCLy3o5ASXVI5E=
 sigs.k8s.io/yaml v1.4.0/go.mod h1:Ejl7/uTz7PSA4eKMyQCUTnhZYNmLIl+5c2lQPGR2BPY=

server/config/config.go

@@ -30,6 +30,7 @@ import (
 	"go.etcd.io/etcd/client/pkg/v3/types"
 	"go.etcd.io/etcd/pkg/v3/netutil"
 	"go.etcd.io/etcd/server/v3/etcdserver/api/v3discovery"
+	"go.etcd.io/etcd/server/v3/internal/pkg/featuregate"
 	"go.etcd.io/etcd/server/v3/storage/datadir"
 )
 
@@ -207,6 +208,9 @@ type ServerConfig struct {
 
 	// ExperimentalLocalAddress is the local IP address to use when communicating with a peer.
 	ExperimentalLocalAddress string `json:"experimental-local-address"`
+
+	// ServerFeatureGate is a server level feature gate
+	ServerFeatureGate featuregate.FeatureGate
 }
 
 // VerifyBootstrap sanity-checks the initial config for bootstrap case

server/etcdserver/server.go

@@ -62,6 +62,7 @@ import (
 	"go.etcd.io/etcd/server/v3/etcdserver/errors"
 	"go.etcd.io/etcd/server/v3/etcdserver/txn"
 	serverversion "go.etcd.io/etcd/server/v3/etcdserver/version"
+	"go.etcd.io/etcd/server/v3/internal/pkg/featuregate"
 	"go.etcd.io/etcd/server/v3/lease"
 	"go.etcd.io/etcd/server/v3/lease/leasehttp"
 	serverstorage "go.etcd.io/etcd/server/v3/storage"
@@ -456,6 +457,11 @@ 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 {
+	return s.Cfg.ServerFeatureGate.Enabled(f)
+}
+
 func tickToDur(ticks int, tickMs uint) string {
 	return fmt.Sprintf("%v", time.Duration(ticks)*time.Duration(tickMs)*time.Millisecond)
 }

server/internal/pkg/featuregate/doc.go

@@ -0,0 +1,17 @@
+// Copyright 2024 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.
+
+// featuregate package is copied from k8s.io/[email protected] to avoid any potential circular dependency between k8s and etcd.
+// Note that only necessary portions are copied to support etcd feature gate functionalities.s
+package featuregate

server/internal/pkg/featuregate/feature_gate.go

@@ -0,0 +1,35 @@
+// Copyright 2024 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
+
+type Feature string
+
+// FeatureGate indicates whether a given feature is enabled or not
+type FeatureGate interface {
+	// Enabled returns true if the key is enabled.
+	Enabled(key Feature) bool
+	// KnownFeatures returns a slice of strings describing the FeatureGate's known features.
+	KnownFeatures() []string
+	// DeepCopy returns a deep copy of the FeatureGate object, such that gates can be
+	// set on the copy without mutating the original. This is useful for validating
+	// config against potential feature gate changes before committing those changes.
+	DeepCopy() MutableFeatureGate
+}
+
+// MutableFeatureGate parses and stores flag gates for known features from
+// a string like feature1=true,feature2=false,...
+type MutableFeatureGate interface {
+	FeatureGate
+}