@@ -7,15 +7,19 @@ import (

 // buildRouter is a router to talk with the build controller

 type buildRouter struct {

-	backend        Backend

-	daemon         experimentalProvider

-	routes         []router.Route

-	builderVersion types.BuilderVersion

+	backend  Backend

+	daemon   experimentalProvider

+	routes   []router.Route

+	features *map[string]bool

 }

 // NewRouter initializes a new build router

-func NewRouter(b Backend, d experimentalProvider, bv types.BuilderVersion) router.Router {

-	r := &buildRouter{backend: b, daemon: d, builderVersion: bv}

+func NewRouter(b Backend, d experimentalProvider, features *map[string]bool) router.Router {

+	r := &buildRouter{

+		backend:  b,

+		daemon:   d,

+		features: features,

+	}

 	r.initRoutes()

 	return r

 }

@@ -32,3 +36,18 @@ func (r *buildRouter) initRoutes() {

 		router.NewPostRoute("/build/cancel", r.postCancel),

 	}

 }

+

+// BuilderVersion derives the default docker builder version from the config

+// Note: it is valid to have BuilderVersion unset which means it is up to the

+// client to choose which builder to use.

+func BuilderVersion(features map[string]bool) types.BuilderVersion {

+	var bv types.BuilderVersion

+	if v, ok := features["buildkit"]; ok {

+		if v {

+			bv = types.BuilderBuildKit

+		} else {

+			bv = types.BuilderV1

+		}

+	}

+	return bv

+}

@@ -230,8 +230,9 @@ func (br *buildRouter) postBuild(ctx context.Context, w http.ResponseWriter, r *

 		return errdefs.InvalidParameter(errors.New("squash is only supported with experimental mode"))

 	}

+	builderVersion := BuilderVersion(*br.features)

 	// check if the builder feature has been enabled from daemon as well.

-	if buildOptions.Version == types.BuilderBuildKit && br.builderVersion != "" && br.builderVersion != types.BuilderBuildKit {

+	if buildOptions.Version == types.BuilderBuildKit && builderVersion != "" && builderVersion != types.BuilderBuildKit {

 		return errdefs.InvalidParameter(errors.New("buildkit is not enabled on daemon"))

 	}

@@ -2,30 +2,29 @@ package system // import "github.com/docker/docker/api/server/router/system"

 import (

 	"github.com/docker/docker/api/server/router"

-	"github.com/docker/docker/api/types"

-	buildkit "github.com/docker/docker/builder/builder-next"

+	"github.com/docker/docker/builder/builder-next"

 	"github.com/docker/docker/builder/fscache"

 )

 // systemRouter provides information about the Docker system overall.

 // It gathers information about host, daemon and container events.

 type systemRouter struct {

-	backend        Backend

-	cluster        ClusterBackend

-	routes         []router.Route

-	fscache        *fscache.FSCache // legacy

-	builder        *buildkit.Builder

-	builderVersion types.BuilderVersion

+	backend  Backend

+	cluster  ClusterBackend

+	routes   []router.Route

+	fscache  *fscache.FSCache // legacy

+	builder  *buildkit.Builder

+	features *map[string]bool

 }

 // NewRouter initializes a new system router

-func NewRouter(b Backend, c ClusterBackend, fscache *fscache.FSCache, builder *buildkit.Builder, bv types.BuilderVersion) router.Router {

+func NewRouter(b Backend, c ClusterBackend, fscache *fscache.FSCache, builder *buildkit.Builder, features *map[string]bool) router.Router {

 	r := &systemRouter{

-		backend:        b,

-		cluster:        c,

-		fscache:        fscache,

-		builder:        builder,

-		builderVersion: bv,

+		backend:  b,

+		cluster:  c,

+		fscache:  fscache,

+		builder:  builder,

+		features: features,

 	}

 	r.routes = []router.Route{

@@ -8,6 +8,7 @@ import (

 	"time"

 	"github.com/docker/docker/api/server/httputils"

+	"github.com/docker/docker/api/server/router/build"

 	"github.com/docker/docker/api/types"

 	"github.com/docker/docker/api/types/events"

 	"github.com/docker/docker/api/types/filters"

@@ -26,7 +27,8 @@ func optionsHandler(ctx context.Context, w http.ResponseWriter, r *http.Request,

 }

 func (s *systemRouter) pingHandler(ctx context.Context, w http.ResponseWriter, r *http.Request, vars map[string]string) error {

-	if bv := s.builderVersion; bv != "" {

+	builderVersion := build.BuilderVersion(*s.features)

+	if bv := builderVersion; bv != "" {

 		w.Header().Set("Builder-Version", string(bv))

 	}

 	_, err := w.Write([]byte{'O', 'K'})

@@ -27,7 +27,6 @@ import (

 	swarmrouter "github.com/docker/docker/api/server/router/swarm"

 	systemrouter "github.com/docker/docker/api/server/router/system"

 	"github.com/docker/docker/api/server/router/volume"

-	"github.com/docker/docker/api/types"

 	buildkit "github.com/docker/docker/builder/builder-next"

 	"github.com/docker/docker/builder/dockerfile"

 	"github.com/docker/docker/builder/fscache"

@@ -253,8 +252,8 @@ type routerOptions struct {

 	sessionManager *session.Manager

 	buildBackend   *buildbackend.Backend

 	buildCache     *fscache.FSCache // legacy

+	features       *map[string]bool

 	buildkit       *buildkit.Builder

-	builderVersion types.BuilderVersion

 	daemon         *daemon.Daemon

 	api            *apiserver.Server

 	cluster        *cluster.Cluster

@@ -300,20 +299,12 @@ func newRouterOptions(config *config.Config, daemon *daemon.Daemon) (routerOptio

 	if err != nil {

 		return opts, errors.Wrap(err, "failed to create buildmanager")

 	}

-	var bv types.BuilderVersion

-	if v, ok := config.Features["buildkit"]; ok {

-		if v {

-			bv = types.BuilderBuildKit

-		} else {

-			bv = types.BuilderV1

-		}

-	}

 	return routerOptions{

 		sessionManager: sm,

 		buildBackend:   bb,

 		buildCache:     buildCache,

 		buildkit:       bk,

-		builderVersion: bv,

+		features:       daemon.Features(),

 		daemon:         daemon,

 	}, nil

 }

@@ -487,9 +478,9 @@ func initRouter(opts routerOptions) {

 		checkpointrouter.NewRouter(opts.daemon, decoder),

 		container.NewRouter(opts.daemon, decoder),

 		image.NewRouter(opts.daemon.ImageService()),

-		systemrouter.NewRouter(opts.daemon, opts.cluster, opts.buildCache, opts.buildkit, opts.builderVersion),

+		systemrouter.NewRouter(opts.daemon, opts.cluster, opts.buildCache, opts.buildkit, opts.features),

 		volume.NewRouter(opts.daemon.VolumesService()),

-		build.NewRouter(opts.buildBackend, opts.daemon, opts.builderVersion),

+		build.NewRouter(opts.buildBackend, opts.daemon, opts.features),

 		sessionrouter.NewRouter(opts.sessionManager),

 		swarmrouter.NewRouter(opts.cluster),

 		pluginrouter.NewRouter(opts.daemon.PluginManager()),

@@ -136,6 +136,11 @@ func (daemon *Daemon) HasExperimental() bool {

 	return daemon.configStore != nil && daemon.configStore.Experimental

 }

+// Features returns the features map from configStore

+func (daemon *Daemon) Features() *map[string]bool {

+	return &daemon.configStore.Features

+}

+

 func (daemon *Daemon) restore() error {

 	containers := make(map[string]*container.Container)

@@ -45,6 +45,7 @@ func (daemon *Daemon) Reload(conf *config.Config) (err error) {

 	daemon.reloadDebug(conf, attributes)

 	daemon.reloadMaxConcurrentDownloadsAndUploads(conf, attributes)

 	daemon.reloadShutdownTimeout(conf, attributes)

+	daemon.reloadFeatures(conf, attributes)

 	if err := daemon.reloadClusterDiscovery(conf, attributes); err != nil {

 		return err

@@ -322,3 +323,13 @@ func (daemon *Daemon) reloadNetworkDiagnosticPort(conf *config.Config, attribute

 	return nil

 }

+

+// reloadFeatures updates configuration with enabled/disabled features

+func (daemon *Daemon) reloadFeatures(conf *config.Config, attributes map[string]string) {

+	// update corresponding configuration

+	// note that we allow features option to be entirely unset

+	daemon.configStore.Features = conf.Features

+

+	// prepare reload event attributes with updatable configurations

+	attributes["features"] = fmt.Sprintf("%v", daemon.configStore.Features)

+}