build: set default builder name if not specified

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

cmd/docker/builder.go

@@ -2,13 +2,16 @@ package main
 
 import (
 	"fmt"
+	"io"
 	"os"
 	"strconv"
+	"strings"
 
 	pluginmanager "github.com/docker/cli/cli-plugins/manager"
 	"github.com/docker/cli/cli/command"
 	"github.com/pkg/errors"
 	"github.com/spf13/cobra"
+	"github.com/spf13/pflag"
 )
 
 const (
@@ -127,3 +130,22 @@ func forwardBuilder(alias string, args, osargs []string) ([]string, []string, bo
 	}
 	return args, osargs, false
 }
+
+// hasBuilderName checks if a builder name is defined in args or env vars
+func hasBuilderName(args []string, envs []string) bool {
+	var builder string
+	flagset := pflag.NewFlagSet("buildx", pflag.ContinueOnError)
+	flagset.Usage = func() {}
+	flagset.SetOutput(io.Discard)
+	flagset.StringVar(&builder, "builder", "", "")
+	_ = flagset.Parse(args)
+	if builder != "" {
+		return true
+	}
+	for _, e := range envs {
+		if strings.HasPrefix(e, "BUILDX_BUILDER=") {
+			return true
+		}
+	}
+	return false
+}

cmd/docker/builder_test.go

@@ -129,3 +129,43 @@ func TestBuilderBrokenEnforced(t *testing.T) {
 		2: output.Suffix("ERROR: BuildKit is enabled but the buildx component is missing or broken."),
 	})
 }
+
+func TestHasBuilderName(t *testing.T) {
+	cases := []struct {
+		name     string
+		args     []string
+		envs     []string
+		expected bool
+	}{
+		{
+			name:     "no args",
+			args:     []string{"docker", "build", "."},
+			envs:     []string{"FOO=bar"},
+			expected: false,
+		},
+		{
+			name:     "env var",
+			args:     []string{"docker", "build", "."},
+			envs:     []string{"BUILDX_BUILDER=foo"},
+			expected: true,
+		},
+		{
+			name:     "flag",
+			args:     []string{"docker", "build", "--builder", "foo", "."},
+			envs:     []string{"FOO=bar"},
+			expected: true,
+		},
+		{
+			name:     "both",
+			args:     []string{"docker", "build", "--builder", "foo", "."},
+			envs:     []string{"BUILDX_BUILDER=foo"},
+			expected: true,
+		},
+	}
+	for _, tt := range cases {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.expected, hasBuilderName(tt.args, tt.envs))
+		})
+	}
+}

cmd/docker/docker.go

@@ -179,12 +179,23 @@ func setValidateArgs(dockerCli command.Cli, cmd *cobra.Command) {
 	})
 }
 
-func tryPluginRun(dockerCli command.Cli, cmd *cobra.Command, subcommand string) error {
+func tryPluginRun(dockerCli command.Cli, cmd *cobra.Command, orargs []string, subcommand string) error {
 	plugincmd, err := pluginmanager.PluginRunCommand(dockerCli, subcommand, cmd)
 	if err != nil {
 		return err
 	}
 
+	// if build subcommand has changed to a builder alias (buildx),
+	// user would expect "docker build" to always create a local docker
+	// image (default builder). this is for better backward compatibility in
+	// case where user could switch to a docker container builder with
+	// "docker buildx --use foo" which does not --load by default. also makes
+	// sure that an arbitrary builder name is not being set in the command
+	// line or in the environment before setting the default one.
+	if orargs[0] == "build" && orargs[0] != subcommand && !hasBuilderName(plugincmd.Args, plugincmd.Env) {
+		plugincmd.Env = append([]string{"BUILDX_BUILDER=default"}, plugincmd.Env...)
+	}
+
 	go func() {
 		// override SIGTERM handler so we let the plugin shut down first
 		<-appcontext.Context().Done()
@@ -223,15 +234,18 @@ func runDocker(dockerCli *command.DockerCli) error {
 		return err
 	}
 
+	// makes a copy of the original args that can be modified by processAliases
+	orargs := args
+
 	args, os.Args, err = processAliases(dockerCli, cmd, args, os.Args)
 	if err != nil {
 		return err
 	}
 
 	if len(args) > 0 {
-		command, _, err := cmd.Find(args)
-		if err != nil || command.Annotations[pluginmanager.CommandAnnotationPlugin] == "true" {
-			err := tryPluginRun(dockerCli, cmd, args[0])
+		pcmd, _, err := cmd.Find(args)
+		if err != nil || pcmd.Annotations[pluginmanager.CommandAnnotationPlugin] == "true" {
+			err := tryPluginRun(dockerCli, cmd, orargs, args[0])
 			if !pluginmanager.IsNotFound(err) {
 				return err
 			}