server/etcdserver/cluster_util.go - external/github.com/coreos/etcd - Git at Google

 // 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 etcdserver

 import (
 	"context"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
 	"net/http"
 	"sort"
 	"strconv"
 	"strings"
 	"time"

 	"go.etcd.io/etcd/api/v3/version"
 	"go.etcd.io/etcd/client/pkg/v3/types"
 	"go.etcd.io/etcd/server/v3/etcdserver/api/membership"

 	"github.com/coreos/go-semver/semver"
 	"go.uber.org/zap"
 )

 // isMemberBootstrapped tries to check if the given member has been bootstrapped
 // in the given cluster.
 func isMemberBootstrapped(lg *zap.Logger, cl *membership.RaftCluster, member string, rt http.RoundTripper, timeout time.Duration) bool {
 	rcl, err := getClusterFromRemotePeers(lg, getRemotePeerURLs(cl, member), timeout, false, rt)
 	if err != nil {
 		return false
 	}
 	id := cl.MemberByName(member).ID
 	m := rcl.Member(id)
 	if m == nil {
 		return false
 	}
 	if len(m.ClientURLs) > 0 {
 		return true
 	}
 	return false
 }

 // GetClusterFromRemotePeers takes a set of URLs representing etcd peers, and
 // attempts to construct a Cluster by accessing the members endpoint on one of
 // these URLs. The first URL to provide a response is used. If no URLs provide
 // a response, or a Cluster cannot be successfully created from a received
 // response, an error is returned.
 // Each request has a 10-second timeout. Because the upper limit of TTL is 5s,
 // 10 second is enough for building connection and finishing request.
 func GetClusterFromRemotePeers(lg *zap.Logger, urls []string, rt http.RoundTripper) (*membership.RaftCluster, error) {
 	return getClusterFromRemotePeers(lg, urls, 10*time.Second, true, rt)
 }

 // If logerr is true, it prints out more error messages.
 func getClusterFromRemotePeers(lg *zap.Logger, urls []string, timeout time.Duration, logerr bool, rt http.RoundTripper) (*membership.RaftCluster, error) {
 	if lg == nil {
 		lg = zap.NewNop()
 	}
 	cc := &http.Client{
 		Transport: rt,
 		Timeout:   timeout,
 	}
 	for _, u := range urls {
 		addr := u + "/members"
 		resp, err := cc.Get(addr)
 		if err != nil {
 			if logerr {
 				lg.Warn("failed to get cluster response", zap.String("address", addr), zap.Error(err))
 			}
 			continue
 		}
 		b, err := ioutil.ReadAll(resp.Body)
 		resp.Body.Close()
 		if err != nil {
 			if logerr {
 				lg.Warn("failed to read body of cluster response", zap.String("address", addr), zap.Error(err))
 			}
 			continue
 		}
 		var membs []*membership.Member
 		if err = json.Unmarshal(b, &membs); err != nil {
 			if logerr {
 				lg.Warn("failed to unmarshal cluster response", zap.String("address", addr), zap.Error(err))
 			}
 			continue
 		}
 		id, err := types.IDFromString(resp.Header.Get("X-Etcd-Cluster-ID"))
 		if err != nil {
 			if logerr {
 				lg.Warn(
 					"failed to parse cluster ID",
 					zap.String("address", addr),
 					zap.String("header", resp.Header.Get("X-Etcd-Cluster-ID")),
 					zap.Error(err),
 				)
 			}
 			continue
 		}

 		// check the length of membership members
 		// if the membership members are present then prepare and return raft cluster
 		// if membership members are not present then the raft cluster formed will be
 		// an invalid empty cluster hence return failed to get raft cluster member(s) from the given urls error
 		if len(membs) > 0 {
 			return membership.NewClusterFromMembers(lg, id, membs), nil
 		}
 		return nil, fmt.Errorf("failed to get raft cluster member(s) from the given URLs")
 	}
 	return nil, fmt.Errorf("could not retrieve cluster information from the given URLs")
 }

 // getRemotePeerURLs returns peer urls of remote members in the cluster. The
 // returned list is sorted in ascending lexicographical order.
 func getRemotePeerURLs(cl *membership.RaftCluster, local string) []string {
 	us := make([]string, 0)
 	for _, m := range cl.Members() {
 		if m.Name == local {
 			continue
 		}
 		us = append(us, m.PeerURLs...)
 	}
 	sort.Strings(us)
 	return us
 }

 // getVersions returns the versions of the members in the given cluster.
 // The key of the returned map is the member's ID. The value of the returned map
 // is the semver versions string, including server and cluster.
 // If it fails to get the version of a member, the key will be nil.
 func getVersions(lg *zap.Logger, cl *membership.RaftCluster, local types.ID, rt http.RoundTripper) map[string]*version.Versions {
 	members := cl.Members()
 	vers := make(map[string]*version.Versions)
 	for _, m := range members {
 		if m.ID == local {
 			cv := "not_decided"
 			if cl.Version() != nil {
 				cv = cl.Version().String()
 			}
 			vers[m.ID.String()] = &version.Versions{Server: version.Version, Cluster: cv}
 			continue
 		}
 		ver, err := getVersion(lg, m, rt)
 		if err != nil {
 			lg.Warn("failed to get version", zap.String("remote-member-id", m.ID.String()), zap.Error(err))
 			vers[m.ID.String()] = nil
 		} else {
 			vers[m.ID.String()] = ver
 		}
 	}
 	return vers
 }

 // allowedVersionRange decides the available version range of the cluster that local server can join in;
 // if the downgrade enabled status is true, the version window is [oneMinorHigher, oneMinorHigher]
 // if the downgrade is not enabled, the version window is [MinClusterVersion, localVersion]
 func allowedVersionRange(downgradeEnabled bool) (minV *semver.Version, maxV *semver.Version) {
 	minV = semver.Must(semver.NewVersion(version.MinClusterVersion))
 	maxV = semver.Must(semver.NewVersion(version.Version))
 	maxV = &semver.Version{Major: maxV.Major, Minor: maxV.Minor}

 	if downgradeEnabled {
 		// Todo: handle the case that downgrading from higher major version(e.g. downgrade from v4.0 to v3.x)
 		maxV.Minor = maxV.Minor + 1
 		minV = &semver.Version{Major: maxV.Major, Minor: maxV.Minor}
 	}
 	return minV, maxV
 }

 // isCompatibleWithCluster return true if the local member has a compatible version with
 // the current running cluster.
 // The version is considered as compatible when at least one of the other members in the cluster has a
 // cluster version in the range of [MinV, MaxV] and no known members has a cluster version
 // out of the range.
 // We set this rule since when the local member joins, another member might be offline.
 func isCompatibleWithCluster(lg *zap.Logger, cl *membership.RaftCluster, local types.ID, rt http.RoundTripper) bool {
 	vers := getVersions(lg, cl, local, rt)
 	minV, maxV := allowedVersionRange(getDowngradeEnabledFromRemotePeers(lg, cl, local, rt))
 	return isCompatibleWithVers(lg, vers, local, minV, maxV)
 }

 func isCompatibleWithVers(lg *zap.Logger, vers map[string]*version.Versions, local types.ID, minV, maxV *semver.Version) bool {
 	var ok bool
 	for id, v := range vers {
 		// ignore comparison with local version
 		if id == local.String() {
 			continue
 		}
 		if v == nil {
 			continue
 		}
 		clusterv, err := semver.NewVersion(v.Cluster)
 		if err != nil {
 			lg.Warn(
 				"failed to parse cluster version of remote member",
 				zap.String("remote-member-id", id),
 				zap.String("remote-member-cluster-version", v.Cluster),
 				zap.Error(err),
 			)
 			continue
 		}
 		if clusterv.LessThan(*minV) {
 			lg.Warn(
 				"cluster version of remote member is not compatible; too low",
 				zap.String("remote-member-id", id),
 				zap.String("remote-member-cluster-version", clusterv.String()),
 				zap.String("minimum-cluster-version-supported", minV.String()),
 			)
 			return false
 		}
 		if maxV.LessThan(*clusterv) {
 			lg.Warn(
 				"cluster version of remote member is not compatible; too high",
 				zap.String("remote-member-id", id),
 				zap.String("remote-member-cluster-version", clusterv.String()),
 				zap.String("minimum-cluster-version-supported", minV.String()),
 			)
 			return false
 		}
 		ok = true
 	}
 	return ok
 }

 // getVersion returns the Versions of the given member via its
 // peerURLs. Returns the last error if it fails to get the version.
 func getVersion(lg *zap.Logger, m *membership.Member, rt http.RoundTripper) (*version.Versions, error) {
 	cc := &http.Client{
 		Transport: rt,
 	}
 	var (
 		err  error
 		resp *http.Response
 	)

 	for _, u := range m.PeerURLs {
 		addr := u + "/version"
 		resp, err = cc.Get(addr)
 		if err != nil {
 			lg.Warn(
 				"failed to reach the peer URL",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		var b []byte
 		b, err = ioutil.ReadAll(resp.Body)
 		resp.Body.Close()
 		if err != nil {
 			lg.Warn(
 				"failed to read body of response",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		var vers version.Versions
 		if err = json.Unmarshal(b, &vers); err != nil {
 			lg.Warn(
 				"failed to unmarshal response",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		return &vers, nil
 	}
 	return nil, err
 }

 func promoteMemberHTTP(ctx context.Context, url string, id uint64, peerRt http.RoundTripper) ([]*membership.Member, error) {
 	cc := &http.Client{Transport: peerRt}
 	// TODO: refactor member http handler code
 	// cannot import etcdhttp, so manually construct url
 	requestUrl := url + "/members/promote/" + fmt.Sprintf("%d", id)
 	req, err := http.NewRequest("POST", requestUrl, nil)
 	if err != nil {
 		return nil, err
 	}
 	req = req.WithContext(ctx)
 	resp, err := cc.Do(req)
 	if err != nil {
 		return nil, err
 	}
 	defer resp.Body.Close()
 	b, err := ioutil.ReadAll(resp.Body)
 	if err != nil {
 		return nil, err
 	}

 	if resp.StatusCode == http.StatusRequestTimeout {
 		return nil, ErrTimeout
 	}
 	if resp.StatusCode == http.StatusPreconditionFailed {
 		// both ErrMemberNotLearner and ErrLearnerNotReady have same http status code
 		if strings.Contains(string(b), ErrLearnerNotReady.Error()) {
 			return nil, ErrLearnerNotReady
 		}
 		if strings.Contains(string(b), membership.ErrMemberNotLearner.Error()) {
 			return nil, membership.ErrMemberNotLearner
 		}
 		return nil, fmt.Errorf("member promote: unknown error(%s)", string(b))
 	}
 	if resp.StatusCode == http.StatusNotFound {
 		return nil, membership.ErrIDNotFound
 	}

 	if resp.StatusCode != http.StatusOK { // all other types of errors
 		return nil, fmt.Errorf("member promote: unknown error(%s)", string(b))
 	}

 	var membs []*membership.Member
 	if err := json.Unmarshal(b, &membs); err != nil {
 		return nil, err
 	}
 	return membs, nil
 }

 // getDowngradeEnabledFromRemotePeers will get the downgrade enabled status of the cluster.
 func getDowngradeEnabledFromRemotePeers(lg *zap.Logger, cl *membership.RaftCluster, local types.ID, rt http.RoundTripper) bool {
 	members := cl.Members()

 	for _, m := range members {
 		if m.ID == local {
 			continue
 		}
 		enable, err := getDowngradeEnabled(lg, m, rt)
 		if err != nil {
 			lg.Warn("failed to get downgrade enabled status", zap.String("remote-member-id", m.ID.String()), zap.Error(err))
 		} else {
 			// Since the "/downgrade/enabled" serves linearized data,
 			// this function can return once it gets a non-error response from the endpoint.
 			return enable
 		}
 	}
 	return false
 }

 // getDowngradeEnabled returns the downgrade enabled status of the given member
 // via its peerURLs. Returns the last error if it fails to get it.
 func getDowngradeEnabled(lg *zap.Logger, m *membership.Member, rt http.RoundTripper) (bool, error) {
 	cc := &http.Client{
 		Transport: rt,
 	}
 	var (
 		err  error
 		resp *http.Response
 	)

 	for _, u := range m.PeerURLs {
 		addr := u + DowngradeEnabledPath
 		resp, err = cc.Get(addr)
 		if err != nil {
 			lg.Warn(
 				"failed to reach the peer URL",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		var b []byte
 		b, err = ioutil.ReadAll(resp.Body)
 		resp.Body.Close()
 		if err != nil {
 			lg.Warn(
 				"failed to read body of response",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		var enable bool
 		if enable, err = strconv.ParseBool(string(b)); err != nil {
 			lg.Warn(
 				"failed to convert response",
 				zap.String("address", addr),
 				zap.String("remote-member-id", m.ID.String()),
 				zap.Error(err),
 			)
 			continue
 		}
 		return enable, nil
 	}
 	return false, err
 }

 func convertToClusterVersion(v string) (*semver.Version, error) {
 	ver, err := semver.NewVersion(v)
 	if err != nil {
 		// allow input version format Major.Minor
 		ver, err = semver.NewVersion(v + ".0")
 		if err != nil {
 			return nil, ErrWrongDowngradeVersionFormat
 		}
 	}
 	// cluster version only keeps major.minor, remove patch version
 	ver = &semver.Version{Major: ver.Major, Minor: ver.Minor}
 	return ver, nil
 }
	// 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 etcdserver

	import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"net/http"
	"sort"
	"strconv"
	"strings"
	"time"

	"go.etcd.io/etcd/api/v3/version"
	"go.etcd.io/etcd/client/pkg/v3/types"
	"go.etcd.io/etcd/server/v3/etcdserver/api/membership"

	"github.com/coreos/go-semver/semver"
	"go.uber.org/zap"
	)

	// isMemberBootstrapped tries to check if the given member has been bootstrapped
	// in the given cluster.
	func isMemberBootstrapped(lg zap.Logger, cl membership.RaftCluster, member string, rt http.RoundTripper, timeout time.Duration) bool {
	rcl, err := getClusterFromRemotePeers(lg, getRemotePeerURLs(cl, member), timeout, false, rt)
	if err != nil {
	return false
	}
	id := cl.MemberByName(member).ID
	m := rcl.Member(id)
	if m == nil {
	return false
	}
	if len(m.ClientURLs) > 0 {
	return true
	}
	return false
	}

	// GetClusterFromRemotePeers takes a set of URLs representing etcd peers, and
	// attempts to construct a Cluster by accessing the members endpoint on one of
	// these URLs. The first URL to provide a response is used. If no URLs provide
	// a response, or a Cluster cannot be successfully created from a received
	// response, an error is returned.
	// Each request has a 10-second timeout. Because the upper limit of TTL is 5s,
	// 10 second is enough for building connection and finishing request.
	func GetClusterFromRemotePeers(lg zap.Logger, urls []string, rt http.RoundTripper) (membership.RaftCluster, error) {
	return getClusterFromRemotePeers(lg, urls, 10*time.Second, true, rt)
	}

	// If logerr is true, it prints out more error messages.
	func getClusterFromRemotePeers(lg zap.Logger, urls []string, timeout time.Duration, logerr bool, rt http.RoundTripper) (membership.RaftCluster, error) {
	if lg == nil {
	lg = zap.NewNop()
	}
	cc := &http.Client{
	Transport: rt,
	Timeout: timeout,
	}
	for _, u := range urls {
	addr := u + "/members"
	resp, err := cc.Get(addr)
	if err != nil {
	if logerr {
	lg.Warn("failed to get cluster response", zap.String("address", addr), zap.Error(err))
	}
	continue
	}
	b, err := ioutil.ReadAll(resp.Body)
	resp.Body.Close()
	if err != nil {
	if logerr {
	lg.Warn("failed to read body of cluster response", zap.String("address", addr), zap.Error(err))
	}
	continue
	}
	var membs []*membership.Member
	if err = json.Unmarshal(b, &membs); err != nil {
	if logerr {
	lg.Warn("failed to unmarshal cluster response", zap.String("address", addr), zap.Error(err))
	}
	continue
	}
	id, err := types.IDFromString(resp.Header.Get("X-Etcd-Cluster-ID"))
	if err != nil {
	if logerr {
	lg.Warn(
	"failed to parse cluster ID",
	zap.String("address", addr),
	zap.String("header", resp.Header.Get("X-Etcd-Cluster-ID")),
	zap.Error(err),
	)
	}
	continue
	}

	// check the length of membership members
	// if the membership members are present then prepare and return raft cluster
	// if membership members are not present then the raft cluster formed will be
	// an invalid empty cluster hence return failed to get raft cluster member(s) from the given urls error
	if len(membs) > 0 {
	return membership.NewClusterFromMembers(lg, id, membs), nil
	}
	return nil, fmt.Errorf("failed to get raft cluster member(s) from the given URLs")
	}
	return nil, fmt.Errorf("could not retrieve cluster information from the given URLs")
	}

	// getRemotePeerURLs returns peer urls of remote members in the cluster. The
	// returned list is sorted in ascending lexicographical order.
	func getRemotePeerURLs(cl *membership.RaftCluster, local string) []string {
	us := make([]string, 0)
	for _, m := range cl.Members() {
	if m.Name == local {
	continue
	}
	us = append(us, m.PeerURLs...)
	}
	sort.Strings(us)
	return us
	}

	// getVersions returns the versions of the members in the given cluster.
	// The key of the returned map is the member's ID. The value of the returned map
	// is the semver versions string, including server and cluster.
	// If it fails to get the version of a member, the key will be nil.
	func getVersions(lg zap.Logger, cl membership.RaftCluster, local types.ID, rt http.RoundTripper) map[string]*version.Versions {
	members := cl.Members()
	vers := make(map[string]*version.Versions)
	for _, m := range members {
	if m.ID == local {
	cv := "not_decided"
	if cl.Version() != nil {
	cv = cl.Version().String()
	}
	vers[m.ID.String()] = &version.Versions{Server: version.Version, Cluster: cv}
	continue
	}
	ver, err := getVersion(lg, m, rt)
	if err != nil {
	lg.Warn("failed to get version", zap.String("remote-member-id", m.ID.String()), zap.Error(err))
	vers[m.ID.String()] = nil
	} else {
	vers[m.ID.String()] = ver
	}
	}
	return vers
	}

	// allowedVersionRange decides the available version range of the cluster that local server can join in;
	// if the downgrade enabled status is true, the version window is [oneMinorHigher, oneMinorHigher]
	// if the downgrade is not enabled, the version window is [MinClusterVersion, localVersion]
	func allowedVersionRange(downgradeEnabled bool) (minV semver.Version, maxV semver.Version) {
	minV = semver.Must(semver.NewVersion(version.MinClusterVersion))
	maxV = semver.Must(semver.NewVersion(version.Version))
	maxV = &semver.Version{Major: maxV.Major, Minor: maxV.Minor}

	if downgradeEnabled {
	// Todo: handle the case that downgrading from higher major version(e.g. downgrade from v4.0 to v3.x)
	maxV.Minor = maxV.Minor + 1
	minV = &semver.Version{Major: maxV.Major, Minor: maxV.Minor}
	}
	return minV, maxV
	}

	// isCompatibleWithCluster return true if the local member has a compatible version with
	// the current running cluster.
	// The version is considered as compatible when at least one of the other members in the cluster has a
	// cluster version in the range of [MinV, MaxV] and no known members has a cluster version
	// out of the range.
	// We set this rule since when the local member joins, another member might be offline.
	func isCompatibleWithCluster(lg zap.Logger, cl membership.RaftCluster, local types.ID, rt http.RoundTripper) bool {
	vers := getVersions(lg, cl, local, rt)
	minV, maxV := allowedVersionRange(getDowngradeEnabledFromRemotePeers(lg, cl, local, rt))
	return isCompatibleWithVers(lg, vers, local, minV, maxV)
	}

	func isCompatibleWithVers(lg zap.Logger, vers map[string]version.Versions, local types.ID, minV, maxV *semver.Version) bool {
	var ok bool
	for id, v := range vers {
	// ignore comparison with local version
	if id == local.String() {
	continue
	}
	if v == nil {
	continue
	}
	clusterv, err := semver.NewVersion(v.Cluster)
	if err != nil {
	lg.Warn(
	"failed to parse cluster version of remote member",
	zap.String("remote-member-id", id),
	zap.String("remote-member-cluster-version", v.Cluster),
	zap.Error(err),
	)
	continue
	}
	if clusterv.LessThan(*minV) {
	lg.Warn(
	"cluster version of remote member is not compatible; too low",
	zap.String("remote-member-id", id),
	zap.String("remote-member-cluster-version", clusterv.String()),
	zap.String("minimum-cluster-version-supported", minV.String()),
	)
	return false
	}
	if maxV.LessThan(*clusterv) {
	lg.Warn(
	"cluster version of remote member is not compatible; too high",
	zap.String("remote-member-id", id),
	zap.String("remote-member-cluster-version", clusterv.String()),
	zap.String("minimum-cluster-version-supported", minV.String()),
	)
	return false
	}
	ok = true
	}
	return ok
	}

	// getVersion returns the Versions of the given member via its
	// peerURLs. Returns the last error if it fails to get the version.
	func getVersion(lg zap.Logger, m membership.Member, rt http.RoundTripper) (*version.Versions, error) {
	cc := &http.Client{
	Transport: rt,
	}
	var (
	err error
	resp *http.Response
	)

	for _, u := range m.PeerURLs {
	addr := u + "/version"
	resp, err = cc.Get(addr)
	if err != nil {
	lg.Warn(
	"failed to reach the peer URL",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	var b []byte
	b, err = ioutil.ReadAll(resp.Body)
	resp.Body.Close()
	if err != nil {
	lg.Warn(
	"failed to read body of response",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	var vers version.Versions
	if err = json.Unmarshal(b, &vers); err != nil {
	lg.Warn(
	"failed to unmarshal response",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	return &vers, nil
	}
	return nil, err
	}

	func promoteMemberHTTP(ctx context.Context, url string, id uint64, peerRt http.RoundTripper) ([]*membership.Member, error) {
	cc := &http.Client{Transport: peerRt}
	// TODO: refactor member http handler code
	// cannot import etcdhttp, so manually construct url
	requestUrl := url + "/members/promote/" + fmt.Sprintf("%d", id)
	req, err := http.NewRequest("POST", requestUrl, nil)
	if err != nil {
	return nil, err
	}
	req = req.WithContext(ctx)
	resp, err := cc.Do(req)
	if err != nil {
	return nil, err
	}
	defer resp.Body.Close()
	b, err := ioutil.ReadAll(resp.Body)
	if err != nil {
	return nil, err
	}

	if resp.StatusCode == http.StatusRequestTimeout {
	return nil, ErrTimeout
	}
	if resp.StatusCode == http.StatusPreconditionFailed {
	// both ErrMemberNotLearner and ErrLearnerNotReady have same http status code
	if strings.Contains(string(b), ErrLearnerNotReady.Error()) {
	return nil, ErrLearnerNotReady
	}
	if strings.Contains(string(b), membership.ErrMemberNotLearner.Error()) {
	return nil, membership.ErrMemberNotLearner
	}
	return nil, fmt.Errorf("member promote: unknown error(%s)", string(b))
	}
	if resp.StatusCode == http.StatusNotFound {
	return nil, membership.ErrIDNotFound
	}

	if resp.StatusCode != http.StatusOK { // all other types of errors
	return nil, fmt.Errorf("member promote: unknown error(%s)", string(b))
	}

	var membs []*membership.Member
	if err := json.Unmarshal(b, &membs); err != nil {
	return nil, err
	}
	return membs, nil
	}

	// getDowngradeEnabledFromRemotePeers will get the downgrade enabled status of the cluster.
	func getDowngradeEnabledFromRemotePeers(lg zap.Logger, cl membership.RaftCluster, local types.ID, rt http.RoundTripper) bool {
	members := cl.Members()

	for _, m := range members {
	if m.ID == local {
	continue
	}
	enable, err := getDowngradeEnabled(lg, m, rt)
	if err != nil {
	lg.Warn("failed to get downgrade enabled status", zap.String("remote-member-id", m.ID.String()), zap.Error(err))
	} else {
	// Since the "/downgrade/enabled" serves linearized data,
	// this function can return once it gets a non-error response from the endpoint.
	return enable
	}
	}
	return false
	}

	// getDowngradeEnabled returns the downgrade enabled status of the given member
	// via its peerURLs. Returns the last error if it fails to get it.
	func getDowngradeEnabled(lg zap.Logger, m membership.Member, rt http.RoundTripper) (bool, error) {
	cc := &http.Client{
	Transport: rt,
	}
	var (
	err error
	resp *http.Response
	)

	for _, u := range m.PeerURLs {
	addr := u + DowngradeEnabledPath
	resp, err = cc.Get(addr)
	if err != nil {
	lg.Warn(
	"failed to reach the peer URL",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	var b []byte
	b, err = ioutil.ReadAll(resp.Body)
	resp.Body.Close()
	if err != nil {
	lg.Warn(
	"failed to read body of response",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	var enable bool
	if enable, err = strconv.ParseBool(string(b)); err != nil {
	lg.Warn(
	"failed to convert response",
	zap.String("address", addr),
	zap.String("remote-member-id", m.ID.String()),
	zap.Error(err),
	)
	continue
	}
	return enable, nil
	}
	return false, err
	}

	func convertToClusterVersion(v string) (*semver.Version, error) {
	ver, err := semver.NewVersion(v)
	if err != nil {
	// allow input version format Major.Minor
	ver, err = semver.NewVersion(v + ".0")
	if err != nil {
	return nil, ErrWrongDowngradeVersionFormat
	}
	}
	// cluster version only keeps major.minor, remove patch version
	ver = &semver.Version{Major: ver.Major, Minor: ver.Minor}
	return ver, nil
	}