[22.06 backport] api: set default "Builder-Version" to "2" (BuildKit) on Linux

api/server/router/build/build.go

@@ -1,6 +1,8 @@
 package build // import "github.com/docker/docker/api/server/router/build"
 
 import (
+	"runtime"
+
 	"github.com/docker/docker/api/server/router"
 	"github.com/docker/docker/api/types"
 )
@@ -37,17 +39,24 @@ func (r *buildRouter) initRoutes() {
 	}
 }
 
-// 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.
+// BuilderVersion derives the default docker builder version from the config.
+//
+// The default on Linux is version "2" (BuildKit), but the daemon can be
+// configured to recommend version "1" (classic Builder). Windows does not
+// yet support BuildKit for native Windows images, and uses "1" (classic builder)
+// as a default.
+//
+// This value is only a recommendation as advertised by the daemon, and 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
-		}
+	// TODO(thaJeztah) move the default to daemon/config
+	if runtime.GOOS == "windows" {
+		return types.BuilderV1
+	}
+
+	bv := types.BuilderBuildKit
+	if v, ok := features["buildkit"]; ok && !v {
+		bv = types.BuilderV1
 	}
 	return bv
 }

api/swagger.yaml

@@ -8786,7 +8786,17 @@ paths:
               description: "Max API Version the server supports"
             Builder-Version:
               type: "string"
-              description: "Default version of docker image builder"
+              description: |
+                Default version of docker image builder
+
+                The default on Linux is version "2" (BuildKit), but the daemon
+                can be configured to recommend version "1" (classic Builder).
+                Windows does not yet support BuildKit for native Windows images,
+                and uses "1" (classic builder) as a default.
+
+                This value is a recommendation as advertised by the daemon, and
+                it is up to the client to choose which builder to use.
+              default: "2"
             Docker-Experimental:
               type: "boolean"
               description: "If the server is running with experimental mode enabled"

docs/api/version-history.md

@@ -53,6 +53,18 @@ keywords: "API, Docker, rcli, REST, documentation"
   if they are not set.
 * `GET /info` now omits the `KernelMemory` and `KernelMemoryTCP` if they are not
   supported by the host or host's configuration (if cgroups v2 are in use).
+* `GET /_ping` and `HEAD /_ping` now return `Builder-Version` by default.
+  This header contains the default builder to use, and is a recommendation as
+  advertised by the daemon. However, it is up to the client to choose which builder
+  to use.
+
+  The default value on Linux is version "2" (BuildKit), but the daemon can be
+  configured to recommend version "1" (classic Builder). Windows does not yet
+  support BuildKit for native Windows images, and uses "1" (classic builder) as
+  a default.
+
+  This change is not versioned, and affects all API versions if the daemon has
+  this patch. 
 * `GET /_ping` and `HEAD /_ping` now return a `Swarm` header, which allows a
   client to detect if Swarm is enabled on the daemon, without having to call
   additional endpoints.

integration/system/ping_test.go

@@ -3,9 +3,13 @@ package system // import "github.com/docker/docker/integration/system"
 import (
 	"context"
 	"net/http"
+	"os"
+	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 
+	"github.com/docker/docker/api/types"
 	"github.com/docker/docker/api/types/swarm"
 	"github.com/docker/docker/api/types/versions"
 	"github.com/docker/docker/testutil/daemon"
@@ -93,6 +97,44 @@ func TestPingSwarmHeader(t *testing.T) {
 	})
 }
 
+func TestPingBuilderHeader(t *testing.T) {
+	skip.If(t, testEnv.IsRemoteDaemon)
+	skip.If(t, testEnv.DaemonInfo.OSType == "windows", "cannot spin up additional daemons on windows")
+
+	defer setupTest(t)()
+	d := daemon.New(t)
+	client := d.NewClientT(t)
+	defer client.Close()
+	ctx := context.TODO()
+
+	t.Run("default config", func(t *testing.T) {
+		d.Start(t)
+		defer d.Stop(t)
+
+		var expected = types.BuilderBuildKit
+		if runtime.GOOS == "windows" {
+			expected = types.BuilderV1
+		}
+
+		p, err := client.Ping(ctx)
+		assert.NilError(t, err)
+		assert.Equal(t, p.BuilderVersion, expected)
+	})
+
+	t.Run("buildkit disabled", func(t *testing.T) {
+		cfg := filepath.Join(d.RootDir(), "daemon.json")
+		err := os.WriteFile(cfg, []byte(`{"features": { "buildkit": false }}`), 0644)
+		assert.NilError(t, err)
+		d.Start(t, "--config-file", cfg)
+		defer d.Stop(t)
+
+		var expected = types.BuilderV1
+		p, err := client.Ping(ctx)
+		assert.NilError(t, err)
+		assert.Equal(t, p.BuilderVersion, expected)
+	})
+}
+
 func hdr(res *http.Response, name string) string {
 	val, ok := res.Header[http.CanonicalHeaderKey(name)]
 	if !ok || len(val) == 0 {